Age-Related Loss of Cohesion: Causes and Effects

Reproductive Ageing: Metabolic contribution to age-related chromosome missegregation in mammalian oocytes

In brief

Chromosome missegregation and declining energy metabolism are considered to be unrelated features of oocyte ageing that contribute to poor reproductive outcomes. Given the bioenergetic cost of chromosome segregation, we propose here that altered energy metabolism during ageing may be an underlying cause of age-related chromosome missegregation and aneuploidy.

Abstract

Advanced reproductive age in women is a major cause of infertility, miscarriage and congenital abnormalities. This is principally caused by a decrease in oocyte quality and developmental competence with age. Oocyte ageing is characterised by an increase in chromosome missegregation and aneuploidy. However, the underlying mechanisms of age-related aneuploidy have not been fully elucidated and are still under active investigation. In addition to chromosome missegregation, oocyte ageing is also accompanied by metabolic dysfunction. In this review, we integrate old and new perspectives on oocyte ageing, chromosome segregation and metabolism in mammalian oocytes and make direct links between these processes. We consider age-related alterations to chromosome segregation machinery, including the loss of cohesion, microtubule stability and the integrity of the spindle assembly checkpoint. We focus on how metabolic dysfunction in the ageing oocyte disrupts chromosome segregation machinery to contribute to and exacerbate age-related aneuploidy. More specifically, we discuss how mitochondrial function, ATP production and the generation of free radicals are altered during ageing. We also explore recent developments in oocyte metabolic ageing, including altered redox reactions (NAD+ metabolism) and the interactions between oocytes and their somatic nurse cells. Throughout the review, we integrate the mechanisms by which changes in oocyte metabolism influence age-related chromosome missegregation.

The role of declining ataxia-telangiectasia-mutated (ATM) function in oocyte aging

Despite the advances in the understanding of reproductive physiology, the mechanisms underlying ovarian aging are still not deciphered. Recent research found an association between impaired ATM-mediated DNA double-strand break (DSB) repair mechanisms and oocyte aging. However, direct evidence connecting ATM-mediated pathway function decline and impaired oocyte quality is lacking. The objective of this study was to determine the role of ATM-mediated DNA DSB repair in the maintenance of oocyte quality in a mouse oocyte knockdown model. Gene interference, in vitro culture, parthenogenesis coupled with genotoxicity assay approaches, as well as molecular cytogenetic analyses based upon next-generation sequencing, were used to test the hypothesis that intact ATM function is critical in the maintenance of oocyte quality. We found that ATM knockdown impaired oocyte quality, resulting in poor embryo development. ATM knockdown significantly lowered or blocked the progression of meiosis in vitro, as well as retarding and reducing embryo cleavage after parthenogenesis. After ATM knockdown, all embryos were of poor quality, and none reached the blastocyst stage. ATM knockdown was also associated with an increased aneuploidy rate compared to controls. Finally, ATM knockdown increased the sensitivity of the oocytes to a genotoxic active metabolite of cyclophosphamide, with increased formation of DNA DSBs, reduced survival, and earlier apoptotic death compared to controls. These findings suggest a key role for ATM in maintaining oocyte quality and resistance to genotoxic stress, and that the previously observed age-induced decline in oocyte ATM function may be a prime factor contributing to oocyte aging.

Influx of zwitterionic buffer after intracytoplasmic sperm injection (ICSI) membrane piercing alters the transcriptome of human oocytes

PURPOSE/STUDY QUESTION

Does piercing oocyte membranes during ICSI allow the influx of surrounding zwitterionic buffer into human oocytes and result in altered developmental competence?

METHODS

Human oocytes directed to IRB-approved research were used to determine the unrestricted influx of surrounding buffer into the oocyte after piercing of membranes via confocal fluorescence microscopy (n = 80 human MII oocytes) and the influence of the select buffer influx of HEPES, MOPS, and bicarbonate buffer on the oocyte transcriptome using ultra-low input RNA sequencing (n = 40 human MII oocytes).

RESULTS

Piercing membranes of human MII oocytes during sham-ICSI resulted in the unrestricted influx of surrounding culture buffer into the oocyte that was beyond technician control. Transcriptome analysis revealed statistically significant decreased cytoskeletal transcripts in the pierced buffer cohorts, higher levels of embryo competency transcripts (IGF2 and G6PD) in the bicarbonate buffer cohort, higher levels of stress-induced transcriptional repressor transcripts (MAF1) in the HEPES and MOPS cohorts, and decreased levels of numerous chromosomal maintenance transcripts (SMC3) in the HEPES buffer cohort. The HEPES buffer cohort also revealed higher levels of transcripts suggesting increased oxidative (GPX1) and lysosomal stress (LAMP1).

CONCLUSION

The influence of zwitterionic buffer on intrinsic cellular mechanisms provides numerous concerns for their use in IVF clinical applications. The primary concern is the ICSI procedure, in which the surrounding buffer is allowed influx into the oocytes after membrane piercing. Selecting a physiological bicarbonate buffer may reduce imposed stress on oocytes, resulting in improved embryo development and clinical results because intracellular MOPS, and especially HEPES, may negatively impact intrinsic biological mechanisms, as revealed by transcriptome changes. These findings further support the utilization of bicarbonate buffer as the oocyte-holding medium during ICSI.

Predicting Infertility: How Genetic Variants in Oocyte Spindle Genes Affect Egg Quality

Successful reproduction relies on the union of a single chromosomally normal egg and sperm. Chromosomally normal eggs develop from precursor cells, called oocytes, that have undergone accurate chromosome segregation. The process of chromosome segregation is governed by the oocyte spindle, a unique cytoskeletal machine that splits chromatin content of the meiotically dividing oocyte. The oocyte spindle develops and functions in an idiosyncratic process, which is vulnerable to genetic variation in spindle-associated proteins. Human genetic variants in several spindle-associated proteins are associated with poor clinical fertility outcomes, suggesting that heritable etiologies for oocyte dysfunction leading to infertility exist and that the spindle is a crux for female fertility. This chapter examines the mammalian oocyte spindle through the lens of human genetic variation, covering the genes TUBB8, TACC3, CEP120, AURKA, AURKC, AURKB, BUB1B, and CDC20. Specifically, it explores how patient-identified variants perturb spindle development and function, and it links these molecular changes in the oocyte to their cognate clinical consequences, such as oocyte maturation arrest, elevated egg aneuploidy, primary ovarian insufficiency, and recurrent pregnancy loss. This discussion demonstrates that small genetic errors in oocyte meiosis can result in remarkably far-ranging embryonic consequences, and thus reveals the importance of the oocyte's fine machinery in sustaining life.

Metabolic Mechanisms and Potential Therapeutic Targets for Prevention of Ovarian Aging: Data from Up-to-Date Experimental Studies

Female infertility and reproduction is an ongoing and rising healthcare issue, resulting in delaying the decision to start a family. Therefore, in this review, we examine potential novel metabolic mechanisms involved in ovarian aging according to recent data and how these mechanisms may be addressed through new potential medical treatments. We examine novel medical treatments currently available based mostly on experimental stem cell procedures as well as caloric restriction (CR), hyperbaric oxygen treatment and mitochondrial transfer. Understanding the connection between metabolic and reproductive pathways has the potential to offer a significant scientific breakthrough in preventing ovarian aging and prolonging female fertility. Overall, the field of ovarian aging is an emerging field that may expand the female fertility window and perhaps even reduce the need for artificial reproductive techniques.

Reproductive aging: biological pathways and potential interventive strategies

Reproductive aging is a natural process conserved across species and is well-known in females. It shows age-related follicle depletion and reduction of oocyte quality, eventually causing reproductive senescence and menopause. Although reproductive aging in males is not well noticed as in females, it also causes infertility and has deleterious consequences on the offspring. Various factors have been suggested to contribute to reproductive aging, including oxidative stress, mitochondrial defects, telomere shortening, meiotic chromosome segregation errors and genetic alterations. With the increasing trend of pregnancy age, it is particularly crucial to find interventions to preserve or extend human fertility. Studies in humans and model organisms have provided insights into the biological pathways associated with reproductive aging, and a series of potential interventive strategies have been tested. Here, we review factors affecting reproductive aging in females and males and summarize interventive strategies that may help delay or rescue the aging phenotypes of reproduction.

Genetic aetiology of Down syndrome birth: novel variants of maternal DNMT3B and RFC1 genes increase risk of meiosis II nondisjunction in the oocyte

The aim of the present work was to explore the intriguing association of maternal folate regulator gene polymorphisms and mutations with the incidence of chromosome 21 nondisjunction and Down syndrome birth. We tested polymorphisms/mutations of DNMT3B and RFC1 genes for their association with meiotic errors in oocyte among the 1215 Down syndrome child-bearing women and 900 controls. We observed that 23 out of 31 variants of DNMT3B and RFC1 exhibited an association with meiosis II nondisjunction in maternal age-independent manner. Additionally, we have reported 17 novel mutations and 1 novel polymorphic variant that are unique to the Indian Bengali speaking cohort and increased odds in favour of meiosis II nondisjunction. We hypothesize that the risk variants and mutations of DNMT3B and RFC1 genes may cause reduction in two or more recombination events and also cause peri-centromeric single exchange that increases the risk of nondisjunction at any age of women. In silico analyses predicted the probable damages of the transcripts or proteins from the respective genes owing to the said polymorphisms. These findings from the largest population sample tested ever revealed that mutations/polymorphisms of the genes DNMT3B and RFC1 impair recombination that leads to chromosome 21 nondisjunction in the oocyte at meiosis II stage and bring us a significant step closer towards understanding the aetiology of chromosome 21 nondisjunction and birth of a child with Down syndrome to women at any age.

Oocyte Development and Quality in Young and Old Mice following Exposure to Atrazine

BACKGROUND

Egg development has unique features that render it vulnerable to environmental perturbation. The herbicide atrazine is an endocrine disruptor shown to have detrimental effects on reproduction across several vertebrate species.

OBJECTIVES

This study was designed to determine whether exposure to low levels of atrazine impairs meiosis in female mammals, using a mouse model; in particular, the study's researchers sought to determine whether and how the fidelity of oocyte chromosome segregation may be affected and whether aging-related aneuploidy is exacerbated.

METHODS

Female C57BL/6J mice were exposed to two levels of atrazine in drinking water: The higher level equaled aqueous saturation, and the lower level corresponded to detected environmental contamination. To model developmental exposure, atrazine was ingested by pregnant females at 0.5 d post coitum and continued until pups were weaned at 21 d postpartum. For adult exposure, 2-month-old females ingested atrazine for 3 months. Following exposure, various indicators of oocyte development and quality were determined, including: a) chromosome synapsis and crossing over in fetal oocytes using immunofluorescence staining of prophase-I chromosome preparations; b) sizes of follicle pools in sectioned ovaries; c) efficiencies of in vitro fertilization and early embryogenesis; d) chromosome alignment and segregation in cultured oocytes; e) chromosomal errors in metaphase-I and -II (MI and MII) preparations; and f) sister-chromatid cohesion via immunofluorescence intensity of cohesin subunit REC8 on MI-chromosome preparations, and measurement of interkinetochore distances in MII preparations.

RESULTS

Mice exposed to atrazine during development showed slightly higher levels of defects in chromosome synapsis, but sizes of initial follicle pools were indistinguishable from controls. However, although more eggs were ovulated, oocyte quality was lower. At the chromosome level, frequencies of spindle misalignment and numerical and structural abnormalities were greater at both meiotic divisions. In vitro fertilization was less efficient, and there were more apoptotic cells in blastocysts derived from eggs of atrazine-exposed females. Similar levels of chromosomal defects were seen in oocytes following both developmental and adult exposure regimens, suggesting quiescent primordial follicles may be a consequential target of atrazine. An important finding was that defects were observed long after exposure was terminated. Moreover, chromosomally abnormal eggs were very frequent in older mice, implying that atrazine exposure during development exacerbates effects of maternal aging on oocyte quality. Indeed, analogous to the effects of maternal age, weaker cohesion between sister chromatids was observed in oocytes from atrazine-exposed animals.

CONCLUSION

Low-level atrazine exposure caused persistent changes to the female mammalian germline in mice, with potential consequences for reproductive lifespan and congenital disease. https://doi.org/10.1289/EHP11343.

The cell cycle, cancer development and therapy

The process of cell division plays a vital role in cancer progression. Cell proliferation and error-free chromosomes segregation during mitosis are central events in life cycle. Mistakes during cell division generate changes in chromosome content and alter the balances of chromosomes number. Any defects in expression of TIF1 family proteins, SAC proteins network, mitotic checkpoint proteins involved in chromosome mis-segregation and cancer development. Here we discuss the function of organelles deal with the chromosome segregation machinery, proteins and correction mechanisms involved in the accurate chromosome segregation during mitosis.

Meiotic genes in premature ovarian insufficiency: variants in HROB and REC8 as likely genetic causes

Premature ovarian insufficiency (POI), affecting 1 in 100 women, is characterised by loss of ovarian function associated with elevated gonadotropin, before the age of 40. In addition to infertility, patients face increased risk of comorbidities such as heart disease, osteoporosis, cancer and/or early mortality. We used whole exome sequencing to identify the genetic cause of POI in seven women. Each had biallelic candidate variants in genes with a primary role in DNA damage repair and/or meiosis. This includes two genes, REC8 and HROB, not previously associated with autosomal recessive POI. REC8 encodes a component of the cohesin complex and HROB encodes a factor that recruits MCM8/9 for DNA damage repair. In silico analyses, combined with concordant mouse model phenotypes support these as new genetic causes of POI. We also identified novel variants in MCM8, NUP107, STAG3 and HFM1 and a known variant in POF1B. Our study highlights the pivotal role of meiosis in ovarian function. We identify novel variants, consolidate the pathogenicity of variants previously considered of unknown significance, and propose HROB and REC8 variants as new genetic causes while exploring their link to pathogenesis.

CDC26 is a key factor in human oocyte aging

STUDY QUESTION

Is CDC26 a key factor in human oocyte aging?

SUMMARY ANSWER

The lack of CDC26 disrupts the oocytes maturation process, leading to oocyte aging, but these defects could be partially rescued by overexpression of the CDC26 protein.

WHAT IS KNOWN ALREADY

Age-related oocyte aging is the main cause of female fertility decline. In mammalian oocytes, aberrant meiosis can cause chromosomal abnormalities that might lead to infertility and developmental disorders. CDC26 participates in the meiosis process.

STUDY DESIGN, SIZE, DURATION

Differential gene expression in young and old women oocytes were screened by single-cell RNA-seq technology, and the functions of differentially genes were verified on mouse oocytes. Finally, transfection technology was used to evaluate the effect of a differentially expressed gene in rescuing human oocyte from aging.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Discarded human oocytes were collected for single-cell RNA-seq, q-PCR and immunocytochemical analyses to screen for and identify differential gene expression. Female KM mice oocytes were collected for IVM of oocytes, q-PCR and immunocytochemical analyses to delineate the relationships between oocyte aging and differential gene expression. Additionally, recombinant lentiviral vectors encoding CDC26 were transfected into the germinal vesicle oocytes of older women, to investigate the effects of the CDC26 gene expression on oocyte development.

MAIN RESULTS AND THE ROLE OF CHANCE

Many genes were found to be differentially expressed in the oocytes of young versus old patients via RNA-seq technology. CDC26 mRNA and protein levels in aged oocytes were severely decreased, when compared with the levels observed in young oocytes. Moreover, aged oocytes lacking CDC26 were more prone to aneuploidy. These defects in aged oocytes could be partially rescued by overexpression of the CDC26 protein.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Our study delineated key steps in the oocyte aging process by identifying the key role of CDC26 in the progression of oocyte maturation. Future studies are required to address whether other signaling pathways play a role in regulating oocyte maturation via CDC26 and which genes are the direct molecular targets of CDC26.

WIDER IMPLICATIONS OF THE FINDINGS

Our results using in vitro systems for both mouse and human oocyte maturation provide a proof of principle that CDC26 may represent a novel therapeutic approach against maternal aging-related spindle and chromosomal abnormalities.

STUDY FUNDING/COMPETING INTEREST(S)

This work was supported by grants from the National Natural Science Foundation of China (81571442 and 81170571), the outstanding Talent Project of Shanghai Municipal Commission of Health (XBR2011067) and Clinical Research and Cultivation Project in Shanghai Municipal Hospitals (SHDC12019X32). The authors declare no conflict of interest.

The etiology of Down syndrome: Maternal MCM9 polymorphisms increase risk of reduced recombination and nondisjunction of chromosome 21 during meiosis I within oocyte

Altered patterns of recombination on 21q have long been associated with the nondisjunction chromosome 21 within oocytes and the increased risk of having a child with Down syndrome. Unfortunately the genetic etiology of these altered patterns of recombination have yet to be elucidated. We for the first time genotyped the gene MCM9, a candidate gene for recombination regulation and DNA repair in mothers with or without children with Down syndrome. In our approach, we identified the location of recombination on the maternal chromosome 21 using short tandem repeat markers, then stratified our population by the origin of meiotic error and age at conception. We observed that twenty-five out of forty-one single nucleotide polymorphic sites within MCM9 exhibited an association with meiosis I error (N = 700), but not with meiosis II error (N = 125). This association was maternal age-independent. Several variants exhibited aprotective association with MI error, some were neutral. Maternal age stratified characterization of cases revealed that MCM9 risk variants were associated with an increased chance of reduced recombination on 21q within oocytes. The spatial distribution of single observed recombination events revealed no significant change in the location of recombination among women harbouring MCM9 risk, protective, or neutral variant. Additionally, we identified a total of six novel polymorphic variants and two novel alleles that were either risk imparting or protective against meiosis I nondisjunction. In silico analyses using five different programs suggest the risk variants either cause a change in protein function or may alter the splicing pattern of transcripts and disrupt the proportion of different isoforms of MCM9 products within oocytes. These observations bring us a significant step closer to understanding the molecular basis of recombination errors in chromosome 21 nondisjunction within oocytes that leads to birth of child with Down syndrome.

We studied MCM9 variations in the genome of women with a Down syndrome child by stratifying the women based on MCM9 genotypes, meiotic error group, and their age of conception. We identified polymorphisms are associated with reduced recombination and nondisjunction of chromosome 21 at the meiosis I stage of oogenesis in a maternal age-independent manner. But these variants do not affect the position of chiasma formation. In Silico analyses revealed the presence of MCM9 variants that may cause alteration in protein function due to amino acid substitution. We also identified splice variants in MCM9. We hypothesize that the polymorphisms in MCM9 predispose women to experience reduced recombination on chromosome 21 in oocytes at meiosis I, which ultimately leads to the birth of a child with Down syndrome.

Crossover patterns under meiotic chromosome program

Repairing DNA double-strand breaks (DSBs) with homologous chromosomes as templates is the hallmark of meiosis. The critical outcome of meiotic homologous recombination is crossovers, which ensure faithful chromosome segregation and promote genetic diversity of progenies. Crossover patterns are tightly controlled and exhibit three characteristics: obligatory crossover, crossover interference, and crossover homeostasis. Aberrant crossover patterns are the leading cause of infertility, miscarriage, and congenital disease. Crossover recombination occurs in the context of meiotic chromosomes, and it is tightly integrated with and regulated by meiotic chromosome structure both locally and globally. Meiotic chromosomes are organized in a loop-axis architecture. Diverse evidence shows that chromosome axis length determines crossover frequency. Interestingly, short chromosomes show different crossover patterns compared to long chromosomes. A high frequency of human embryos are aneuploid, primarily derived from female meiosis errors. Dramatically increased aneuploidy in older women is the well-known “maternal age effect.” However, a high frequency of aneuploidy also occurs in young women, derived from crossover maturation inefficiency in human females. In addition, frequency of human aneuploidy also shows other age-dependent alterations. Here, current advances in the understanding of these issues are reviewed, regulation of crossover patterns by meiotic chromosomes are discussed, and issues that remain to be investigated are suggested.

Small Noncoding RNA Signatures for Determining the Developmental Potential of an Embryo at the Morula Stage

As part of the optimization of assisted reproductive technology programs, the aim of the study was to identify key small noncoding RNA (sncRNA) molecules that participate in maternal-to-zygotic transition and determine development potential and competence to form a healthy fetus. Small RNA deep sequencing followed by quantitative real-time RT-PCR was used to profile sncRNAs in 50 samples of spent culture medium from morula with different development potentials (no potential (degradation/developmental arrest), low potential (poor-quality blastocyst), and high potential (good/excellent quality blastocyst capable of implanting and leading to live birth)) obtained from 27 subfertile couples who underwent in vitro fertilization. We have shown that the quality of embryos at the morula stage is determined by secretion/uptake rates of certain sets of piRNAs and miRNAs, namely hsa_piR_011291, hsa_piR_019122, hsa_piR_001311, hsa_piR_015026, hsa_piR_015462, hsa_piR_016735, hsa_piR_019675, hsa_piR_020381, hsa_piR_020485, hsa_piR_004880, hsa_piR_000807, hsa-let-7b-5p, and hsa-let-7i-5p. Predicted gene targets of these sncRNAs included those globally decreased at the 8-cell–morula–blastocyst stage and critical to early embryo development. We show new original data on sncRNA profiling in spent culture medium from morula with different development potential. Our findings provide a view of a more complex network that controls human embryogenesis at the pre-implantation stage. Further research is required using reporter analysis to experimentally confirm interactions between identified sncRNA/gene target pairs.

Aging mechanisms-A perspective mostly from Drosophila

A mechanistic understanding of the natural aging process, which is distinct from aging-related disease mechanisms, is essential for developing interventions to extend lifespan or healthspan. Here, we discuss current trends in aging research and address conceptual and experimental challenges in the field. We examine various molecular markers implicated in aging with an emphasis on the role of heterochromatin and epigenetic changes. Studies in model organisms have been advantageous in elucidating conserved genetic and epigenetic mechanisms and assessing interventions that affect aging. We highlight the use of Drosophila, which allows controlled studies for evaluating genetic and environmental contributors to aging conveniently. Finally, we propose the use of novel methodologies and future strategies using Drosophila in aging research.

The horse as a natural model to study reproductive aging-induced aneuploidy and weakened centromeric cohesion in oocytes

Aneuploidy of meiotic origin is a major contributor to age-related subfertility and an increased risk of miscarriage in women. Although age-related aneuploidy has been studied in rodents, the mare may be a more appropriate animal model to study reproductive aging. Similar to women, aged mares show reduced fertility and an increased incidence of early pregnancy loss; however, it is not known whether aging predisposes to aneuploidy in equine oocytes. We evaluated the effect of advanced mare age on (1) gene expression for cohesin components, (2) incidence of aneuploidy and (3) chromosome centromere cohesion (measured as the distance between sister kinetochores) in oocytes matured in vitro. Oocytes from aged mares showed reduced gene expression for the centromere cohesion stabilizing protein, Shugoshin 1. Moreover, in vitro matured oocytes from aged mares showed a higher incidence of aneuploidy and premature sister chromatid separation, and weakened centromeric cohesion. We therefore propose the mare as a valid model for studying effects of aging on centromeric cohesion; cohesion loss predisposes to disintegration of bivalents and premature separation of sister chromatids during the first meiotic division, leading to embryonic aneuploidy; this probably contributes to the reduced fertility and increased incidence of pregnancy loss observed in aged mares.

Antioxidant Intervention Attenuates Aging-Related Changes in the Murine Ovary and Oocyte

Advanced maternal age (AMA) is associated with reduced fertility due in part to diminished ovarian follicle quantity, inferior oocyte quality, chromosome aneuploidy, and lower implantation rates. Ovarian aging is accompanied by increased oxidative stress and blunted antioxidant signaling, such that antioxidant intervention could improve reproductive potential. The first aim of this study was to determine the molecular effects of antioxidant intervention in the ovaries and oocytes of aged mice, utilizing a supplement containing only naturally occurring açaí (Euterpe oleracea) with an oxygen radical absorbance capacity of 208,628 μmol Trolox equivalent (TE)/100 g indicating high antioxidant activity. Nine month old female CF-1 mice were administered 80 mg/day antioxidants (n = 12) or standard diet (n = 12) for 12 weeks. In the ovary, antioxidant treatment upregulated β-adrenergic signaling, downregulated apoptosis and proinflammatory signaling, and variably affected cell growth and antioxidant pathways (p < 0.05). Exogenous antioxidants also increased the oocyte expression of antioxidant genes GPX1, SOD2, and GSR (p < 0.05). A feasibility analysis was then conducted on female AMA infertility patients as a proof-of-principle investigation. Patients (n = 121; <45 years old) consented to receiving 600 mg antioxidants three times daily for ≥8 weeks preceding infertility treatment. Preliminary results indicate promising outcomes for AMA patients, warranting further investigation.

Molecular basis of reproductive senescence: insights from model organisms

PURPOSE

Reproductive decline due to parental age has become a major barrier to fertility as couples have delayed having offspring into their thirties and forties. Advanced parental age is also associated with increased incidence of neurological and cardiovascular disease in offspring. Thus, elucidating the etiology of reproductive decline is of clinical importance.

METHODS

Deciphering the underlying processes that drive reproductive decline is particularly challenging in women in whom a discrete oocyte pool is established during embryogenesis and may remain dormant for tens of years. Instead, our understanding of the processes that drive reproductive senescence has emerged from studies in model organisms, both vertebrate and invertebrate, that are the focus of this literature review.

CONCLUSIONS

Studies of reproductive aging in model organisms not only have revealed the detrimental cellular changes that occur with age but also are helping identify major regulator proteins controlling them. Here, we discuss what we have learned from model organisms with respect to the molecular mechanisms that maintain both genome integrity and oocyte quality.

Salpingectomy may decrease antral follicle count but not live birth rate for IVF-ET patients aged 35-39 years: a retrospective study

Purpose

Problems with fallopian tubes are one of the main reasons for women to undergo in vitro fertilization-embryo transfer (IVF-ET). A large proportion of women with ectopic pregnancy, fallopian tube obstruction and hydrosalpinx have had one or both fallopian tubes removed by salpingectomy. With increasing age, ovarian reserve deteriorates, the numbers of retrieved oocytes, available embryos and high-quality embryos are reduced, and the live birth rate for women treated with IVF treatment is affected. Thus, it is important to understand how salpingectomy affects live birth rates for IVF patients of different ages. This study analyzed how patients’ age and salpingectomy influenced ovarian reserve, ovarian response and pregnancy outcomes for infertile women undergoing IVF-ET.

Methods

A total of 1922 patients that underwent IVF-ET treatment from January 1, 2012, to December 31, 2018, were included in this retrospective study. The patients were divided into two groups according to whether or not they had a previous history of salpingectomy. The salpingectomy (group A, 534 patients) and control groups (group B, 1388 patients) were then further divided into two subgroups according to patient age (age<35 years, and age 35–39 years). Ovarian reserve, ovarian response, and IVF outcomes were investigated for each subgroup. Logistic regression model was used to estimate the relationship between clinical pregnancy and live births and patients’ baseline characteristics.

Results

In the salpingectomy group, antral follicle counts (AFC) were significantly lower for the subgroup aged 35 to 39 years compared with the control group. But this difference did not appear in women younger than 35 years. In addition, there were no significant differences in levels of basal follicle stimulation hormone (FSH), basal luteinizing hormone (LH), basal estradiol (E₂), total gonadotropins (Gn) dose, duration of Gn, numbers of retrieved oocytes, fertilization rates, numbers of available embryos, live birth rates, clinical pregnancy rates, miscarriage rates, ectopic pregnancy rates, or multiple pregnancy rates between the salpingectomy group and the control group (P > 0.05). Age is a risk factor for the clinical pregnancy and live birth.

Conclusion

Salpingectomy may decrease antral follicle count but not live birth rate for IVF-ET patients aged 35–39 years. The increased female age was negative related with clinical pregnancy and live birth.

Expression of lamin C2 in mammalian oocytes

Lamin C2 (LMN C2) is a short product of the lamin a gene. It is a germ cell-specific lamin and has been extensively studied in male germ cells. In this study, we focussed on the expression and localization of LMN C2 in fully-grown germinal vesicle (GV) oocytes. We detected LMN C2 in the fully-grown germinal vesicle oocytes of various mammalian species with confirmation done by immunoblotting the wild type and Lmnc2 gene deleted testes. Expression of LMN C2 tagged with GFP showed localization of LMN C2 to the nuclear membrane of the oocyte. Moreover, the LMN C2 protein notably disappeared after nuclear envelope breakdown (NEBD) and the expression of LMN C2 was significantly reduced in the oocytes from aged females and ceased altogether during meiotic maturation. These results provide new insights regarding LMN C2 expression in the oocytes of various mammalian species.

The CDK Pef1 and protein phosphatase 4 oppose each other for regulating cohesin binding to fission yeast chromosomes

Cohesin has essential roles in chromosome structure, segregation and repair. Cohesin binding to chromosomes is catalyzed by the cohesin loader, Mis4 in fission yeast. How cells fine tune cohesin deposition is largely unknown. Here, we provide evidence that Mis4 activity is regulated by phosphorylation of its cohesin substrate. A genetic screen for negative regulators of Mis4 yielded a CDK called Pef1, whose closest human homologue is CDK5. Inhibition of Pef1 kinase activity rescued cohesin loader deficiencies. In an otherwise wild-type background, Pef1 ablation stimulated cohesin binding to its regular sites along chromosomes while ablating Protein Phosphatase 4 had the opposite effect. Pef1 and PP4 control the phosphorylation state of the cohesin kleisin Rad21. The CDK phosphorylates Rad21 on Threonine 262. Pef1 ablation, non-phosphorylatable Rad21-T262 or mutations within a Rad21 binding domain of Mis4 alleviated the effect of PP4 deficiency. Such a CDK/PP4-based regulation of cohesin loader activity could provide an efficient mechanism for translating cellular cues into a fast and accurate cohesin response.

Crossover Interference, Crossover Maturation, and Human Aneuploidy

A striking feature of human female sexual reproduction is the high level of gametes that exhibit an aberrant number of chromosomes (aneuploidy). A high baseline observed in women of prime reproductive age is followed by a dramatic increase in older women. Proper chromosome segregation requires one or more DNA crossovers (COs) between homologous maternal and paternal chromosomes, in combination with cohesion between sister chromatid arms. In human females, CO designations occur normally, according to the dictates of CO interference, giving early CO-fated intermediates. However, ≈25% of these intermediates fail to mature to final CO products. This effect explains the high baseline of aneuploidy and is predicted to synergize with age-dependent cohesion loss to explain the maternal age effect. Here, modern advances in the understanding of crossing over and CO interference are reviewed, the implications of human female CO maturation inefficiency are further discussed, and areas of interest for future studies are suggested.

Distinct roles of cohesin acetyltransferases Esco1 and Esco2 in porcine oocyte meiosis I

In mammalian cells, cohesin acetyltransferases Esco1 and Esco2 acetylate cohesin subunit Smc3 to establish chromosome cohesion, ensuring the accurate chromosome segregation. However, we have previously documented that both Esco1 and Esco2 have unique substrates and roles in mouse oocyte meiosis I to orchestrate the meiotic progression, but whether these functions are conserved among species is still not determined. Here, we used porcine oocytes as a model to illustrate that Esco1 and Esco2 exerted conserved functions during oocyte meiosis. We observed that Esco1 and Esco2 exhibited different localization patterns in porcine oocytes. Esco1 was localized to the spindle apparatus while Esco2 was distributed on the chromosomes. Depletion of Esco1 by siRNA microinjection caused the meiotic arrest by showing the reduced frequency of first polar body extrusion and defective spindle/chromosome structure. In addition, Esco1 bound to α-tubulin and was required for its acetylation level to maintain the microtubule dynamics. By contrast, depletion of Esco2 by siRNA microinjection resulted in the accelerated meiotic progression by displaying the precocious polar body extrusion and inactivation of spindle assembly checkpoint. Notably, Esco2 was shown to be associated with histone H4 for the acetylation of H4K16 to modulate the kinetochore function. Collectively, our data reveal that Esco1 and Esco2 perform distinct and conserved functions in oocytes to drive the meiotic progression beyond their canonical roles in the cohesion establishment.

Meiosis: the chromosomal foundation of reproduction

Meiosis is the chromosomal foundation of reproduction, with errors in this important process leading to aneuploidy and/or infertility. In this review celebrating the 50th anniversary of the founding of the Society for the Study of Reproduction, the important chromosomal structures and dynamics contributing to genomic integrity across generations are highlighted. Critical unsolved biological problems are identified, and the advances that will lead to their ultimate resolution are predicted.

Maternal obesity enhances oocyte chromosome abnormalities associated with aging

Obesity is increasing globally, and maternal obesity has adverse effects on pregnancy outcomes and the long-term health of offspring. Maternal obesity has been associated with pregnancy failure through impaired oogenesis and embryogenesis. However, whether maternal obesity causes chromosome abnormalities in oocytes has remained unclear. Here we show that chromosome abnormalities are increased in the oocytes of obese mice fed a high-fat diet and identify weakened sister-chromatid cohesion as the likely cause. Numbers of full-grown follicles retrieved from obese mice were the same as controls and the efficiency of in vitro oocyte maturation remained high. However, chromosome abnormalities presenting in both metaphase-I and metaphase-II were elevated, most prominently the premature separation of sister chromatids. Weakened sister-chromatid cohesion in oocytes from obese mice was manifested both as the terminalization of chiasmata in metaphase-I and as increased separation of sister centromeres in metaphase II. Obesity-associated abnormalities were elevated in older mice implying that maternal obesity exacerbates the deterioration of cohesion seen with advancing age.

Tissue stem cells: the new actors in the aneuploidy field

The development of multicellular organisms and the maintenance of its tissues relies on mitosis. However, this process represents a major challenge for genomic stability as each time a cell division occurs there are multiple steps where errors can lead to an abnormal chromosomal content in daughter cells - aneuploidy. Aneuploidy was first postulated to act as a tumour promoting agent over one century ago. Since then, we have learned to appreciate the complexity involving the cellular responses to aneuploidy and to value the importance of models where aneuploidy is induced in vivo and in a cell-type specific manner. Recent data suggests that stem cells evolved a distinct response to aneuploidy, being able to survive and proliferate as aneuploid. Since stem cells are the main cells responsible for tissue renewal, it is of the utmost importance to place the spotlight on stem cells within the aneuploidy field. Here, we briefly review some of the biological mechanisms implicated in aneuploidy, the relationship between aneuploidy and tissue pathologies, and summarize the most recent findings in Drosophila on how tissue stem cells respond to aneuploidy. Once we understand how stem cell behavior is impacted by aneuploidy, we might be able to better describe the complicated link between aneuploidy and tumourigenesis.

Increased levels of superoxide dismutase suppress meiotic segregation errors in aging oocytes

The risk of meiotic segregation errors increases dramatically during a woman’s thirties, a phenomenon known as the maternal age effect. In addition, several lines of evidence indicate that meiotic cohesion deteriorates as oocytes age. One mechanism that may contribute to age-induced loss of cohesion is oxidative damage. In support of this model, we recently reported (Perkins et al. in Proc Natl Acad Sci U S A 113(44):E6823–E6830, 2016) that the knockdown of the reactive oxygen species (ROS)–scavenging enzyme, superoxide dismutase (SOD), during meiotic prophase causes premature loss of arm cohesion and segregation errors in Drosophila oocytes. If age-dependent oxidative damage causes meiotic segregation errors, then the expression of extra SOD1 (cytosolic/nuclear) or SOD2 (mitochondrial) in oocytes may attenuate this effect. To test this hypothesis, we generated flies that contain a UAS-controlled EMPTY, SOD1, or SOD2 cassette and induced expression using a Gal4 driver that turns on during meiotic prophase. We then compared the fidelity of chromosome segregation in aged and non-aged Drosophila oocytes for all three genotypes. As expected, p{EMPTY} oocytes subjected to aging exhibited a significant increase in nondisjunction (NDJ) compared with non-aged oocytes. In contrast, the magnitude of age-dependent NDJ was significantly reduced when expression of extra SOD1 or SOD2 was induced during prophase. Our findings support the hypothesis that a major factor underlying the maternal age effect in humans is age-induced oxidative damage that results in premature loss of meiotic cohesion. Moreover, our work raises the exciting possibility that antioxidant supplementation may provide a preventative strategy to reduce the risk of meiotic segregation errors in older women.

Advanced Maternal Age in IVF: Still a Challenge? The Present and the Future of Its Treatment

Advanced maternal age (AMA; >35 year) is associated with a decline in both ovarian reserve and oocyte competence. At present, no remedies are available to counteract the aging-related fertility decay, however different therapeutic approaches can be offered to women older than 35 year undergoing IVF. This review summarizes the main current strategies proposed for the treatment of AMA: (i) oocyte cryopreservation to conduct fertility preservation for medical reasons or "social freezing" for non-medical reasons, (ii) personalized controlled ovarian stimulation to maximize the exploitation of the ovarian reserve in each patient, (iii) enhancement of embryo selection via blastocyst-stage preimplantation genetic testing for aneuploidies and frozen single embryo transfer, or (iv) oocyte donation in case of minimal/null residual chance of pregnancy. Future strategies and tools are in the pipeline that might minimize the risks of AMA through non-invasive approaches for embryo selection (e.g., molecular analyses of leftover products of IVF, such as spent culture media). These are yet challenging but potentially ground-breaking perspectives promising a lower clinical workload with a higher cost-effectiveness. We also reviewed emerging experimental therapeutic approaches to attempt at restoring maternal reproductive potential, e.g., spindle-chromosomal complex, pronuclear or mitochondrial transfer, and chromosome therapy. In vitro generation of gametes is also an intriguing challenge for the future. Lastly, since infertility is a social issue, social campaigns, and education among future generations are desirable to promote the awareness of the impact of age and lifestyle habits upon fertility. This should be a duty of the clinical operators in this field.

Ovarian ageing and the impact on female fertility

Female fertility decreases with increasing age, a reflection of declining oocyte quantity and quality. The menopausal transition occurs when the oocyte quantity falls below a threshold level. The pattern of follicular depletion as well as the factors, timing and mechanisms surrounding both declining oocyte number and oocyte quality remain incompletely understood. Further studies are needed to examine the factors involved and develop predictive models and biomarkers to assist in the management of age-related subfertility. This review summarises the current knowledge addressing the ageing ovary and its impact on fertility.

Low anti-Müllerian hormone concentration is associated with increased risk of embryonic aneuploidy in women of advanced age

RESEARCH QUESTION

Does an association exist between serum anti-Müllerian hormone (AMH) level, the marker of biological ovarian age, and embryonic aneuploidy risk in recurrent spontaneous miscarriage (RSM) patients of reproductive age?

DESIGN

This retrospective study included a total of 422 IVF cycles of 394 unexplained RSM patients undergoing preimplantation genetic testing for aneuploidy (PGT-A), enrolled from January 2014 to December 2016. Subjects were divided into three groups according to the 25th (1.50 ng/ml) and 75th (5.60 ng/ml) percentiles of AMH level (Group 1: low AMH <1.50 ng/ml [N = 107], Group 2: normal AMH 1.50- < 5.60 ng/ml [N = 210] and Group 3: high AMH ≥ 5.60 ng/ml [N = 105]).

RESULTS

There was a significant difference in embryonic aneuploid rate between AMH groups (66.7% versus 42.9% versus 50.0%, Groups 1 to 3, respectively, P = 0.006). It was significantly higher in the low AMH group (Group 1) compared with that in the normal AMH group (Group 2, P_1vs2 = 0.002) and high AMH group (Group 3, P_1vs3 = 0.015). After age stratification, embryonic aneuploidy rate was still significantly different among AMH groups with a similar trend in women ≥35 years old (68.2% versus 54.4% versus 51.0%, P = 0.038, P_1vs2 = 0.025, P_1vs3 = 0.035), but not in young subjects.

CONCLUSIONS

These findings indicate that low AMH level was associated with increased risk of embryo aneuploidy only in women of advanced age. Maternal diminished ovarian reserve along with oocyte ageing may contribute to impaired chromosomal competence of the embryo.

Impact of Maternal Age on Oocyte and Embryo Competence

The overall success of human reproduction, either spontaneously or after IVF, is highly dependent upon maternal age. The main reasons for age-related infertility include reduced ovarian reserve and decreased oocyte/embryo competence due to aging insults, especially concerning an increased incidence of aneuploidies and possibly decreased mitochondrial activity. Age-related chromosomal abnormalities mainly arise because of meiotic impairments during oogenesis, following flawed chromosome segregation patterns such as non-disjunction, premature separation of sister chromatids, or the recent reverse segregation. In this review, we briefly discuss the main mechanisms putatively impaired by aging in the oocytes and the deriving embryos. We also report the main strategies proposed to improve the management of advanced maternal age women in IVF: fertility preservation through oocyte cryopreservation to prevent aging; optimization of the ovarian stimulation and enhancement of embryo selection to limit its effects; and oocyte donation to circumvent its consequences.