Age-related aneuploidy through cohesion exhaustion

RNF212B E3 ligase is essential for crossover designation and maturation during male and female meiosis in the mouse

Meiosis, a reductional cell division, relies on precise initiation, maturation, and resolution of crossovers (COs) during prophase I to ensure the accurate segregation of homologous chromosomes during metaphase I. This process is regulated by the interplay of RING-E3 ligases such as RNF212 and HEI10 in mammals. In this study, we functionally characterized a recently identified RING-E3 ligase, RNF212B. RNF212B colocalizes and interacts with RNF212, forming foci along chromosomes from zygonema onward in a synapsis-dependent and DSB-independent manner. These consolidate into larger foci at maturing COs, colocalizing with HEI10, CNTD1, and MLH1 by late pachynema. Genetically, RNF212B foci formation depends on Rnf212 but not on Msh4, Hei10, and Cntd1, while the unloading of RNF212B at the end of pachynema is dependent on Hei10 and Cntd1. Mice lacking RNF212B, or expressing an inactive RNF212B protein, exhibit modest synapsis defects, a reduction in the localization of pro-CO factors (MSH4, TEX11, RPA, MZIP2) and absence of late CO-intermediates (MLH1). This loss of most COs by diakinesis results in mostly univalent chromosomes. Double mutants for Rnf212b and Rnf212 exhibit an identical phenotype to that of Rnf212b single mutants, while double heterozygous demonstrate a dosage-dependent reduction in CO number, indicating a functional interplay between paralogs. SUMOylome analysis of testes from Rnf212b mutants and pull-down analysis of Sumo- and Ubiquitin-tagged HeLa cells, suggest that RNF212B is an E3-ligase with Ubiquitin activity, serving as a crucial factor for CO maturation. Thus, RNF212 and RNF212B play vital, yet overlapping roles, in ensuring CO homeostasis through their distinct E3 ligase activities.

Chromatin-associated cohesin turns over extensively and forms new cohesive linkages in Drosophila oocytes during meiotic prophase

In dividing cells, accurate chromosome segregation depends on sister chromatid cohesion, protein linkages that are established during DNA replication. Faithful chromosome segregation in oocytes requires that cohesion, first established in S phase, remain intact for days to decades, depending on the organism. Premature loss of meiotic cohesion in oocytes leads to the production of aneuploid gametes and contributes to the increased incidence of meiotic segregation errors as women age (maternal age effect). The prevailing model is that cohesive linkages do not turn over in mammalian oocytes. However, we have previously reported that cohesion-related defects arise in Drosophila oocytes when individual cohesin subunits or cohesin regulators are knocked down after meiotic S phase. Here, we use two strategies to express a tagged cohesin subunit exclusively during mid-prophase in Drosophila oocytes and demonstrate that newly expressed cohesin is used to form de novo linkages after meiotic S phase. Cohesin along the arms of oocyte chromosomes appears to completely turn over within a 2-day window during prophase, whereas replacement is less extensive at centromeres. Unlike S-phase cohesion establishment, the formation of new cohesive linkages during meiotic prophase does not require acetylation of conserved lysines within the Smc3 head. Our findings indicate that maintenance of cohesion between S phase and chromosome segregation in Drosophila oocytes requires an active cohesion rejuvenation program that generates new cohesive linkages during meiotic prophase.

The impact of maternal age on aneuploidy in oocytes: Reproductive consequences, molecular mechanisms, and future directions

Age-related aneuploidy in human oocytes is a major factor contributing to decreased fertility and adverse reproductive outcomes. As females age, their oocytes are more prone to meiotic chromosome segregation errors, leading primarily to aneuploidy. Elevated aneuploidy rates have also been observed in oocytes from very young, prepubertal conceptions. A key barrier to developing effective treatments for age-related oocyte aneuploidy is our incomplete understanding of the molecular mechanisms involved. The challenge is becoming increasingly critical as more people choose to delay childbearing, a trend that has significant societal implications. In this review, we summarize current knowledge regarding the process of oocyte meiosis and folliculogenesis, highlighting the relationship between age and chromosomal aberrations in oocytes and embryos, and integrate proposed mechanisms of age-related meiotic disturbances across structural, protein, and genomic levels. Our goal is to spur new research directions and therapeutic avenues.

Epigenetic aging of mammalian gametes

The process of aging refers to physiological changes that occur to an organism as time progresses and involves changes to DNA, proteins, metabolism, cells, and organs. Like the rest of the cells in the body, gametes age, and it is well established that there is a decline in reproductive capabilities in females and males with aging. One of the major pathways known to be involved in aging is epigenetic changes. The epigenome is the multitude of chemical modifications performed on DNA and chromatin that affect the ability of chromatin to be transcribed. In this review, we explore the effects of aging on female and male gametes with a focus on the epigenetic changes that occur in gametes throughout aging. Quality decline in oocytes occurs at a relatively early age. Epigenetic changes constitute an important part of oocyte aging. DNA methylation is reduced with age, along with reduced expression of DNA methyltransferases (DNMTs). Histone deacetylases (HDAC) expression is also reduced, and a loss of heterochromatin marks occurs with age. As a consequence of heterochromatin loss, retrotransposon expression is elevated, and aged oocytes suffer from DNA damage. In sperm, aging affects sperm number, motility and fecundity, and epigenetic changes may constitute a part of this process. 5 methyl-cytosine (5mC) methylation is elevated in sperm from aged men, but methylation on Long interspersed nuclear elements (LINE) elements is reduced. Di and trimethylation of histone 3 lysine 9 (H3K9me2/3) is reduced in sperm from aged men and trimethylation of histone 3 lysine 27 (H3K27me3) is elevated. The protamine makeup of sperm from aged men is also changed, with reduced protamine expression and a misbalanced ratio between protamine proteins protamine P1 and protamine P2. The study of epigenetic reproductive aging is recently gaining interest. The current status of the field suggests that many aspects of gamete epigenetic aging are still open for investigation. The clinical applications of these investigations have far-reaching consequences for fertility and sociological human behavior.

Chromatin-associated cohesin turns over extensively and forms new cohesive linkages during meiotic prophase

In dividing cells, accurate chromosome segregation depends on sister chromatid cohesion, protein linkages that are established during DNA replication. Faithful chromosome segregation in oocytes requires that cohesion, first established in S phase, remain intact for days to decades, depending on the organism. Premature loss of meiotic cohesion in oocytes leads to the production of aneuploid gametes and contributes to the increased incidence of meiotic segregation errors as women age (maternal age effect). The prevailing model is that cohesive linkages do not turn over in mammalian oocytes. However, we have previously reported that cohesion-related defects arise in Drosophila oocytes when individual cohesin subunits or cohesin regulators are knocked down after meiotic S phase. Here we use two strategies to express a tagged cohesin subunit exclusively during mid-prophase in Drosophila oocytes and demonstrate that newly expressed cohesin is used to form de novo linkages after meiotic S phase. Moreover, nearly complete turnover of chromosome-associated cohesin occurs during meiotic prophase, with faster replacement on the arms than at the centromeres. Unlike S-phase cohesion establishment, the formation of new cohesive linkages during meiotic prophase does not require acetylation of conserved lysines within the Smc3 head. Our findings indicate that maintenance of cohesion between S phase and chromosome segregation in Drosophila oocytes requires an active cohesion rejuvenation program that generates new cohesive linkages during meiotic prophase.

Cohesin SMC1β promotes closed chromatin and controls TERRA expression at spermatocyte telomeres

Previous data showed that meiotic cohesin SMC1β protects spermatocyte telomeres from damage. The underlying reason, however, remained unknown as the expressions of telomerase and shelterin components were normal in Smc1β -/- spermatocytes. Here. we report that SMC1β restricts expression of the long noncoding RNA TERRA (telomeric repeat containing RNA) in spermatocytes. In somatic cell lines increased TERRA was reported to cause telomere damage through altering telomere chromatin structure. In Smc1β -/- spermatocytes, we observed strongly increased levels of TERRA which accumulate on damaged chromosomal ends, where enhanced R-loop formation was found. This suggested a more open chromatin configuration near telomeres in Smc1β -/- spermatocytes, which was confirmed by ATAC-seq. Telomere-distal regions were not affected by the absence of SMC1β but RNA-seq revealed increased transcriptional activity in telomere-proximal regions. Thus, SMC1β promotes closed chromatin specifically near telomeres and limits TERRA expression in spermatocytes.

Rolf Jessberger: cohesin, telomeres, & germ cells

Rolf Jessberger is Professor and Chairman at the Institute of Physiological Chemistry, and Faculty of Medicine at the Technische Universität Dresden. We asked him about his recent article published in Life Science Alliance (LSA) and his experience in science thus far.

Aneuploidy in mammalian oocytes and the impact of maternal ageing

During fertilization, the egg and the sperm are supposed to contribute precisely one copy of each chromosome to the embryo. However, human eggs frequently contain an incorrect number of chromosomes - a condition termed aneuploidy, which is much more prevalent in eggs than in either sperm or in most somatic cells. In turn, aneuploidy in eggs is a leading cause of infertility, miscarriage and congenital syndromes. Aneuploidy arises as a consequence of aberrant meiosis during egg development from its progenitor cell, the oocyte. In human oocytes, chromosomes often segregate incorrectly. Chromosome segregation errors increase in women from their mid-thirties, leading to even higher levels of aneuploidy in eggs from women of advanced maternal age, ultimately causing age-related infertility. Here, we cover the two main areas that contribute to aneuploidy: (1) factors that influence the fidelity of chromosome segregation in eggs of women from all ages and (2) factors that change in response to reproductive ageing. Recent discoveries reveal new error-causing pathways and present a framework for therapeutic strategies to extend the span of female fertility.

OXIDATIVE STRESS AND REPRODUCTIVE FUNCTION: Oxidative stress and in vitro ageing of the post-ovulatory oocyte: an update on recent advances in the field

In brief

Post-ovulatory ageing of oocytes leads to poor oocyte and embryo quality as well as abnormalities in offspring. This review provides an update on the contributions of oxidative stress to this process and discusses the current literature surrounding the use of antioxidant media to delay post-ovulatory oocyte ageing.

Abstract

Following ovulation, the metaphase II stage oocyte has a limited functional lifespan before succumbing to a process known as post-ovulatory oocyte ageing. This progressive demise occurs both in vivo and in vitro and is accompanied by a deterioration in oocyte quality, leading to a well-defined sequelae of reduced fertilisation rates, poor embryo quality, post-implantation errors, and abnormalities in the offspring. Although the physiological consequences of post-ovulatory oocyte ageing have largely been characterised, less is known regarding the molecular mechanisms that drive this process. This review presents an update on the established relationships between the biochemical changes exhibited by the ageing oocyte and the myriad of symptoms associated with the ageing phenotype. In doing so, we consider the molecular events that are potentially involved in orchestrating post-ovulatory ageing with a particular focus on the role of oxidative stress. We highlight the mounting evidence that oxidative stress acts as an initiator for a cascade of events that create the aged oocyte phenotype. Specifically, oxidative stress has the capacity to disrupt mitochondrial function and directly damage multiple intracellular components of the oocyte such as lipids, proteins, and DNA. Finally, this review addresses emerging strategies for delaying post-ovulatory oocyte ageing with emphasis placed on the promise afforded by the use of selected antioxidants to guide the development of media tailored for the preservation of oocyte integrity during in vitro fertilisation procedures.

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.

Age-dependent aneuploidy in mammalian oocytes instigated at the second meiotic division

Ageing severely affects the chromosome segregation process in human oocytes resulting in aneuploidy, infertility and developmental disorders. A considerable amount of segregation errors in humans are introduced at the second meiotic division. We have here compared the chromosome segregation process in young adult and aged female mice during the second meiotic division. More than half of the oocytes in aged mice displayed chromosome segregation irregularities at anaphase II, resulting in dramatically increased level of aneuploidy in haploid gametes, from 4% in young adult mice to 30% in aged mice. We find that the post‐metaphase II process that efficiently corrects aberrant kinetochore‐microtubule attachments in oocytes in young adult mice is approximately 10‐fold less efficient in aged mice, in particular affecting chromosomes that show small inter‐centromere distances at the metaphase II stage in aged mice. Our results reveal that post‐metaphase II processes have critical impact on age‐dependent aneuploidy in mammalian eggs.

Ribosomal DNA methylation in human and mouse oocytes increases with age

An age-dependent increase in ribosomal DNA (rDNA) methylation has been observed across a broad spectrum of somatic tissues and the male mammalian germline. Bisulfite pyrosequencing (BPS) was used to determine the methylation levels of the rDNA core promoter and the rDNA upstream control element (UCE) along with two oppositely genomically imprinted control genes (PEG3 and GTL2) in individual human germinal vesicle (GV) oocytes from 90 consenting women undergoing fertility treatment because of male infertility. Apart from a few (4%) oocytes with single imprinting defects (in either PEG3 or GTL2), the analyzed GV oocytes displayed correct imprinting patterns. In 95 GV oocytes from 42 younger women (26-32 years), the mean methylation levels of the rDNA core promoter and UCE were 7.4±4.0% and 9.3±6.1%, respectively. In 79 GV oocytes from 48 older women (33-39 years), methylation levels increased to 9.3±5.3% (P = 0.014) and 11.6±7.4% (P = 0.039), respectively. An age-related increase in oocyte rDNA methylation was also observed in 123 mouse GV oocytes from 29 4-16-months-old animals. Similar to the continuously mitotically dividing male germline, ovarian aging is associated with a gain of rDNA methylation in meiotically arrested oocytes. Oocytes from the same woman can exhibit varying rDNA methylation levels and, by extrapolation, different epigenetic ages.

A fertilin-derived peptide improves in vitro maturation and ploidy of human oocytes

OBJECTIVE

To analyze the effect of a cyclic fertilin-derived peptide (cFEE) on in vitro maturation of human oocytes.

DESIGN

Randomized study.

SETTING

Fertility center in an academic hospital.

PATIENT(S)

Not applicable.

INTERVENTION(S)

Human immature germinal vesicle-stage oocytes (n = 1,629) donated for research according to French bioethics laws were randomly allocated to groups treated with 1 or 100 μM of cFEE or to a control group. They were incubated at 37 °C in 6% CO₂ and 5% O₂, and their maturation was assessed using time-lapse microscopy over 24 hours. In vitro maturated metaphase II oocytes were analyzed for chromosomal content using microarray comparative genomic hybridization, and their transcriptomes were analyzed using Affymetrix Clariom D microarrays.

MAIN OUTCOME MEASURE(S)

The percentage of oocytes undergoing maturation in vitro was observed. Aneuploidy and euploidy were assessed for all chromosomes, and differential gene expression was analyzed in oocytes treated with cFEE compared with the control to obtain insights into its mechanism of action.

RESULT(S)

cFEE significantly increased the percentage of oocytes that matured in vitro and improved euploidy in meiosis II oocytes by the up-regulation of FMN1 and FLNA genes, both of which encode proteins involved in spindle structure.

CONCLUSION(S)

cFEE improves human oocyte maturation in vitro and reduces aneuploidy. It may prove useful for treating oocytes before fertilization in assisted reproductive technology and for in vitro maturation in fertility preservation programs to improve oocyte quality and the chances for infertile couples to conceive.

Analysis of clinical outcomes and meiotic segregation modes following preimplantation genetic testing for structural rearrangements using aCGH/NGS in couples with balanced chromosome rearrangement

Purpose

To retrospectively evaluate the effectiveness of PGT‐SR by array comparative genomic hybridization (aCGH) or next‐generation sequencing (NGS) in preventing recurrent miscarriages.

Methods

Thirty one couples with balanced translocation who underwent 68 PGT‐SR cycles between 2012 and 2020 were evaluated. A total of 242 blastocysts were biopsied for aCGH or NGS. The genetically transferable blastocysts were transferred in the subsequent frozen‐thawed single embryo transfer cycle.

Results

The genetically transferable blastocyst rate was 21.2% (51/241). Thirty five genetically transferable blastocysts were transferred into the uterine cavity. The clinical pregnancy rate was 57.1% (20/35), and the ongoing pregnancy rate was 100.0% (20/20). The incidence of interchromosomal effect (ICE) was influenced by ovarian stimulation protocol, female age, and carrier's gender, but dependent on the types of balanced translocation carriers. Furthermore, there was no significant difference in meiotic segregation modes in ovarian stimulation protocols and carrier's gender. Interestingly, the incidence of adjacent‐1 segregation in ≧40 years group increased significantly compared with <35 years group.

Conclusions

For the first time in Japan, we show the effectiveness of PGT‐SR using aCGH or NGS, which enables comprehensive analysis of chromosomes, in the prevention of recurrent miscarriages. Furthermore, our results may support better genetic counseling of balanced translocation carriers for PGT‐SR cycles.

Effects of post‐ovulatory aging on centromeric cohesin protection in murine MII oocytes

Purpose

Methods

Results

Conclusions

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.

CENP-V is required for proper chromosome segregation through interaction with spindle microtubules in mouse oocytes

Proper chromosome segregation is essential to avoid aneuploidy, yet this process fails with increasing age in mammalian oocytes. Here we report a role for the scarcely described protein CENP-V in oocyte spindle formation and chromosome segregation. We show that depending on the oocyte maturation state, CENP-V localizes to centromeres, to microtubule organizing centers, and to spindle microtubules. We find that Cenp-V-/- oocytes feature severe deficiencies, including metaphase I arrest, strongly reduced polar body extrusion, increased numbers of mis-aligned chromosomes and aneuploidy, multipolar spindles, unfocused spindle poles and loss of kinetochore spindle fibres. We also show that CENP-V protein binds, diffuses along, and bundles microtubules in vitro. The spindle assembly checkpoint arrests about half of metaphase I Cenp-V-/- oocytes from young adults only. This finding suggests checkpoint weakening in ageing oocytes, which mature despite carrying mis-aligned chromosomes. Thus, CENP-V is a microtubule bundling protein crucial to faithful oocyte meiosis, and Cenp-V-/- oocytes reveal age-dependent weakening of the spindle assembly checkpoint.

TRF1 Depletion Reveals Mutual Regulation Between Telomeres, Kinetochores, and Inner Centromeres in Mouse Oocytes

In eukaryotic chromosomes, the centromere and telomere are two specialized structures that are essential for chromosome stability and segregation. Although centromeres and telomeres often are located in close proximity to form telocentric chromosomes in mice, it remained unclear whether these two structures influence each other. Here we show that TRF1 is required for inner centromere and kinetochore assembly in addition to its role in telomere protection in mouse oocytes. TRF1 depletion caused premature chromosome segregation by abrogating the spindle assembly checkpoint (SAC) and impairing kinetochore-microtubule (kMT) attachment, which increased the incidence of aneuploidy. Notably, TRF1 depletion disturbed the localization of Survivin and Ndc80/Hec1 at inner centromeres and kinetochores, respectively. Moreover, SMC3 and SMC4 levels significantly decreased after TRF1 depletion, suggesting that TRF1 is involved in chromosome cohesion and condensation. Importantly, inhibition of inner centromere or kinetochore function led to a significant decrease in TRF1 level and telomere shortening. Therefore, our results suggest that telomere integrity is required to preserve inner centromere and kinetochore architectures, and vice versa, suggesting mutual regulation between telomeres and centromeres.

Erzhi Tiangui Granules Improve In Vitro Fertilization Outcomes in Infertile Women with Advanced Age

Background

The fertility of females with advanced age declines with aging. Therefore, for medical and social reasons, it is important to establish mechanisms to protect and improve the fertility of such populations. With widespread use of traditional Chinese medicine (TCM) in in vitro fertilization (IVF), studies have evaluated their impact on improving the fertility of females with advanced age. In this study, we performed proteomic analysis of follicular fluid to reveal mechanisms of the Erzhi Tiangui (EZTG) granule (Chinese herbs for replenishing vital essence to tonify the kidney) in improving the outcomes of IVF in infertile women with advanced age.

Methods

This was a randomized, double-blind, and placebo-controlled trial in which 100 patients with advanced age were divided into the EZTG group and the placebo group by the random number table plus envelope method. Both groups were subjected to controlled ovarian stimulation with a GnRH antagonist regimen. Differences between the two groups were evaluated, including the TCM syndrome score after treatment, gonadotrophin (Gn) days and Gn doses, the number of retrieved oocytes, 2 pronucleus (PN) fertilization, 2PN cleavage, and high-quality embryos. Differentially expressed proteins were identified using the LC-MS/MS method, and their functions were determined through bioinformatics analyses.

Results

The number of high-quality embryos in the placebo group was significantly lower than that in the EZTG group (2.88 ± 1.85 vs. 4.13 ± 2.83, p=0.011). Eleven differentially expressed proteins were identified between the two groups. Four proteins were highly expressed, whereas seven were suppressed in the control group, compared to the EZTG group. The overall trend suggested that the apoptotic effect in the follicular fluid of the EZTG group was downregulated.

Conclusion

Treatment with the EZTG granule can improve embryo quality in IVF of advanced age females with both kidney Qi and Yin deficiency syndromes. The mechanism is attributed to downregulation of apoptotic-effector protein expressions in the follicular fluid. This trial is registered with ChiCTR1900025139.

Meiotic Cohesin and Variants Associated With Human Reproductive Aging and Disease

Successful human reproduction relies on the well-orchestrated development of competent gametes through the process of meiosis. The loading of cohesin, a multi-protein complex, is a key event in the initiation of mammalian meiosis. Establishment of sister chromatid cohesion via cohesin rings is essential for ensuring homologous recombination-mediated DNA repair and future proper chromosome segregation. Cohesin proteins loaded during female fetal life are not replenished over time, and therefore are a potential etiology of age-related aneuploidy in oocytes resulting in decreased fecundity and increased infertility and miscarriage rates with advancing maternal age. Herein, we provide a brief overview of meiotic cohesin and summarize the human genetic studies which have identified genetic variants of cohesin proteins and the associated reproductive phenotypes including primary ovarian insufficiency, trisomy in offspring, and non-obstructive azoospermia. The association of cohesion defects with cancer predisposition and potential impact on aging are also described. Expansion of genetic testing within clinical medicine, with a focus on cohesin protein-related genes, may provide additional insight to previously unknown etiologies of disorders contributing to gamete exhaustion in females, and infertility and reproductive aging in both men and women.

A prometaphase mechanism of securin destruction is essential for meiotic progression in mouse oocytes

Successful cell division relies on the timely removal of key cell cycle proteins such as securin. Securin inhibits separase, which cleaves the cohesin rings holding chromosomes together. Securin must be depleted before anaphase to ensure chromosome segregation occurs with anaphase. Here we find that in meiosis I, mouse oocytes contain an excess of securin over separase. We reveal a mechanism that promotes excess securin destruction in prometaphase I. Importantly, this mechanism relies on two phenylalanine residues within the separase-interacting segment (SIS) of securin that are only exposed when securin is not bound to separase. We suggest that these residues facilitate the removal of non-separase-bound securin ahead of metaphase, as inhibiting this period of destruction by mutating both residues causes the majority of oocytes to arrest in meiosis I. We further propose that cellular securin levels exceed the amount an oocyte is capable of removing in metaphase alone, such that the prometaphase destruction mechanism identified here is essential for correct meiotic progression in mouse oocytes.

Aneuploidy in human eggs: contributions of the meiotic spindle

Human eggs frequently contain an incorrect number of chromosomes, a condition termed aneuploidy. Aneuploidy affects ∼10-25% of eggs in women in their early 30s, and more than 50% of eggs from women over 40. Most aneuploid eggs cannot develop to term upon fertilization, making aneuploidy in eggs a leading cause of miscarriages and infertility. The cellular origins of aneuploidy in human eggs are incompletely understood. Aneuploidy arises from chromosome segregation errors during the two meiotic divisions of the oocyte, the progenitor cell of the egg. Chromosome segregation is driven by a microtubule spindle, which captures and separates the paired chromosomes during meiosis I, and sister chromatids during meiosis II. Recent studies reveal that defects in the organization of the acentrosomal meiotic spindle contribute to human egg aneuploidy. The microtubules of the human oocyte spindle are very frequently incorrectly attached to meiotic kinetochores, the multi-protein complexes on chromosomes to which microtubules bind. Multiple features of human oocyte spindles favour incorrect attachments. These include spindle instability and many age-related changes in chromosome and kinetochore architecture. Here, we review how the unusual spindle assembly mechanism in human oocytes contributes to the remarkably high levels of aneuploidy in young human eggs, and how age-related changes in chromosome and kinetochore architecture cause aneuploidy levels to rise even higher as women approach their forties.

Mechanisms of oocyte aneuploidy associated with advanced maternal age

It is well established that maternal age is associated with a rapid decline in the production of healthy and high-quality oocytes resulting in reduced fertility in women older than 35 years of age. In particular, chromosome segregation errors during meiotic divisions are increasingly common and lead to the production of oocytes with an incorrect number of chromosomes, a condition known as aneuploidy. When an aneuploid oocyte is fertilized by a sperm it gives rise to an aneuploid embryo that, except in rare situations, will result in a spontaneous abortion. As females advance in age, they are at higher risk of infertility, miscarriage, or having a pregnancy affected by congenital birth defects such as Down syndrome (trisomy 21), Edwards syndrome (trisomy 18), and Turner syndrome (monosomy X). Here, we review the potential molecular mechanisms associated with increased chromosome segregation errors during meiosis as a function of maternal age. Our review shows that multiple exogenous and endogenous factors contribute to the age-related increase in oocyte aneuploidy. Specifically, the weight of evidence indicates that recombination failure, cohesin deterioration, spindle assembly checkpoint (SAC) disregulation, abnormalities in post-translational modification of histones and tubulin, and mitochondrial dysfunction are the leading causes of oocyte aneuploidy associated with maternal aging. There is also growing evidence that dietary and other bioactive interventions may mitigate the effect of maternal aging on oocyte quality and oocyte aneuploidy, thereby improving fertility outcomes. Maternal age is a major concern for aneuploidy and genetic disorders in the offspring in the context of an increasing proportion of mothers having children at increasingly older ages. A better understanding of the mechanisms associated with maternal aging leading to aneuploidy and of intervention strategies that may mitigate these detrimental effects and reduce its occurrence are essential for preventing abnormal reproductive outcomes in the human population.

CHTF18 ensures the quantity and quality of the ovarian reserve†

The number and quality of oocytes, as well as the decline in both of these parameters with age, determines reproductive potential in women. However, the underlying mechanisms of this diminution are incompletely understood. Previously, we identified novel roles for CHTF18 (Chromosome Transmission Fidelity Factor 18), a component of the conserved Replication Factor C-like complex, in male fertility and gametogenesis. Currently, we reveal crucial roles for CHTF18 in female meiosis and oocyte development. Chtf18-/- female mice are subfertile and have fewer offspring beginning at 6 months of age. Consistent with age-dependent subfertility, Chtf18-/- ovaries contain fewer follicles at all stages of folliculogenesis than wild type ovaries, but the decreases are more significant at 3 and 6 months of age. By 6 months of age, both primordial and growing ovarian follicle pools are markedly reduced to near depletion. Chromosomal synapsis in Chtf18-/- oocytes is complete, but meiotic recombination is impaired resulting in persistent DNA double-strand breaks, fewer crossovers, and early homolog disjunction during meiosis I. Consistent with poor oocyte quality, the majority of Chtf18-/- oocytes fail to progress to metaphase II following meiotic resumption and a significant percentage of those that do progress are aneuploid. Collectively, our findings indicate critical functions for CHTF18 in ensuring both the quantity and quality of the mammalian oocyte pool.

PLK1 is required for chromosome compaction and microtubule organization in mouse oocytes

Errors during meiotic resumption in oocytes can result in chromosome missegregation and infertility. Several cell cycle kinases have been linked with roles in coordinating events during meiotic resumption, including polo-like kinases (PLKs). Mammals express four kinase-proficient PLKs (PLK1-4). Previous studies assessing the role of PLK1 have relied on RNA knockdown and kinase inhibition approaches, as Plk1 null mutations are embryonically lethal. To further assess the roles of PLK1 during meiotic resumption, we developed a Plk1 conditional knockout (cKO) mouse to specifically mutate Plk1 in oocytes. Despite normal oocyte numbers and follicle maturation, Plk1 cKO mice were infertile. From analysis of meiotic resumption, Plk1 cKO oocytes underwent nuclear envelope breakdown with the same timing as control oocytes. However, Plk1 cKO oocytes failed to form compact bivalent chromosomes, and localization of cohesin and condensin were defective. Furthermore, Plk1 cKO oocytes either failed to organize α-tubulin or developed an abnormally small bipolar spindle. These abnormalities were attributed to aberrant release of the microtubule organizing center (MTOC) linker protein, C-NAP1, and the failure to recruit MTOC components and liquid-like spindle domain (LISD) factors. Ultimately, these defects result in meiosis I arrest before homologous chromosome segregation.

The repair of endo/exogenous DNA double-strand breaks and its effects on meiotic chromosome segregation in oocytes

Germ cell-derived genomic structure variants not only drive the evolution of species but also induce developmental defects in offspring. The genomic structure variants have different types, but most of them are originated from DNA double-strand breaks (DSBs). It is still not well known whether DNA DSBs exist in adult mammalian oocytes and how the growing and fully grown oocytes repair their DNA DSBs induced by endogenous or exogenous factors. In this study, we detected the endogenous DNA DSBs in the growing and fully grown mouse oocytes and found that the DNA DSBs mainly localized at the centromere-adjacent regions, which are also copy number variation hotspots. When the exogenous DNA DSBs were introduced by Etoposide, we found that Rad51-mediated homologous recombination (HR) was used to repair the broken DNA. However, the HR repair caused the chromatin intertwined and impaired the homologous chromosome segregation in oocytes. Although we had not detected the indication about HR repair of endogenous centromere-adjacent DNA DSBs, we found that Rad52 and RNA:DNA hybrids colocalized with these DNA DSBs, indicating that a Rad52-dependent DNA repair might exist in oocytes. In summary, our results not only demonstrated an association between endogenous DNA DSBs with genomic structure variants but also revealed one specific DNA DSB repair manner in oocytes.

Meiotic sex chromosome cohesion and autosomal synapsis are supported by Esco2

In mitotic cells, establishment of sister chromatid cohesion requires acetylation of the cohesin subunit SMC3 (acSMC3) by ESCO1 and/or ESCO2. Meiotic cohesin plays additional but poorly understood roles in the formation of chromosome axial elements (AEs) and synaptonemal complexes. Here, we show that levels of ESCO2, acSMC3, and the pro-cohesion factor sororin increase on meiotic chromosomes as homologs synapse. These proteins are less abundant on the largely unsynapsed sex chromosomes, whose sister chromatid cohesion appears weaker throughout the meiotic prophase. Using three distinct conditional Esco2 knockout mouse strains, we demonstrate that ESCO2 is essential for male gametogenesis. Partial depletion of ESCO2 in prophase I spermatocytes delays chromosome synapsis and further weakens cohesion along sex chromosomes, which show extensive separation of AEs into single chromatids. Unsynapsed regions of autosomes are associated with the sex chromatin and also display split AEs. This study provides the first evidence for a specific role of ESCO2 in mammalian meiosis, identifies a particular ESCO2 dependence of sex chromosome cohesion and suggests support of autosomal synapsis by acSMC3-stabilized cohesion.

A PP2A-B56-Centered View on Metaphase-to-Anaphase Transition in Mouse Oocyte Meiosis I

Meiosis is required to reduce to haploid the diploid genome content of a cell, generating gametes-oocytes and sperm-with the correct number of chromosomes. To achieve this goal, two specialized cell divisions without intermediate S-phase are executed in a time-controlled manner. In mammalian female meiosis, these divisions are error-prone. Human oocytes have an exceptionally high error rate that further increases with age, with significant consequences for human fertility. To understand why errors in chromosome segregation occur at such high rates in oocytes, it is essential to understand the molecular players at work controlling these divisions. In this review, we look at the interplay of kinase and phosphatase activities at the transition from metaphase-to-anaphase for correct segregation of chromosomes. We focus on the activity of PP2A-B56, a key phosphatase for anaphase onset in both mitosis and meiosis. We start by introducing multiple roles PP2A-B56 occupies for progression through mitosis, before laying out whether or not the same principles may apply to the first meiotic division in oocytes, and describing the known meiosis-specific roles of PP2A-B56 and discrepancies with mitotic cell cycle regulation.

Meiotic Kinetochores Fragment into Multiple Lobes upon Cohesin Loss in Aging Eggs

Chromosome segregation errors during female meiosis are a leading cause of pregnancy loss and human infertility. The segregation of chromosomes is driven by interactions between spindle microtubules and kinetochores. Kinetochores in mammalian oocytes are subjected to special challenges: they need to withstand microtubule pulling forces over multiple hours and are built on centromeric chromatin that in humans is decades old. In meiosis I, sister kinetochores are paired and oriented toward the same spindle pole. It is well established that they progressively separate from each other with advancing female age. However, whether aging also affects the internal architecture of centromeres and kinetochores is currently unclear. Here, we used super-resolution microscopy to study meiotic centromere and kinetochore organization in metaphase-II-arrested eggs from three mammalian species, including humans. We found that centromeric chromatin decompacts with advancing maternal age. Kinetochores built on decompacted centromeres frequently lost their integrity and fragmented into multiple lobes. Fragmentation extended across inner and outer kinetochore regions and affected over 30% of metaphase-II-arrested (MII) kinetochores in aged women and mice, making the lobular architecture a prominent feature of the female meiotic kinetochore. We demonstrate that a partial cohesin loss, as is known to occur in oocytes with advancing maternal age, is sufficient to trigger centromere decompaction and kinetochore fragmentation. Microtubule pulling forces further enhanced the fragmentation and shaped the arrangement of kinetochore lobes. Fragmented kinetochores were frequently abnormally attached to spindle microtubules, suggesting that kinetochore fragmentation could contribute to the maternal age effect in mammalian eggs.

Chromosome errors in human eggs shape natural fertility over reproductive life span

Chromosome errors, or aneuploidy, affect an exceptionally high number of human conceptions, causing pregnancy loss and congenital disorders. Here, we have followed chromosome segregation in human oocytes from females aged 9 to 43 years and report that aneuploidy follows a U-curve. Specific segregation error types show different age dependencies, providing a quantitative explanation for the U-curve. Whole-chromosome nondisjunction events are preferentially associated with increased aneuploidy in young girls, whereas centromeric and more extensive cohesion loss limit fertility as women age. Our findings suggest that chromosomal errors originating in oocytes determine the curve of natural fertility in humans.

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.

Vitrifying immature equine oocytes impairs their ability to correctly align the chromosomes on the MII spindle

Vitrified-warmed immature equine oocytes are able to complete the first meiotic division, but their subsequent developmental competence is compromised. Therefore, the present study investigated the effects of vitrifying immature horse oocytes on the chromosome and spindle configuration after IVM. Cumulus-oocytes complexes (COCs) were collected and divided into two groups based on mare age (young ≤14 years; old ≥16 years). COCs were then either directly matured invitro or vitrified and warmed before IVM. Spindle morphology and chromosome alignment within MII stage oocytes were assessed using immunofluorescent staining, confocal microscopy and three-dimensional image analysis. Vitrification reduced the ability of oocytes to reach MII and resulted in ultrastructural changes to the meiotic spindle, including shortening of its long axis, and an increased incidence of chromosomes failing to align properly at the metaphase plate. We hypothesise that aberrant chromosome alignment is an important contributor to the reduced developmental competence of vitrified equine oocytes. Contrary to expectation, oocytes from young mares were more severely affected than oocytes from older mares; we propose that the reduced effect of vitrification on oocytes from older mares is related to pre-existing compromise of spindle assembly checkpoint control mechanisms in these mares.

Correlations between embryo morphokinetic development and maternal age: Results from an intracytoplasmic sperm injection program

Objective

It is widely accepted that aging decreases women’s fertility capacity. The aim of this study was to assess correlations between maternal age and the morphokinetic parameters and cleavage pattern of embryos.

Methods

The morphokinetics of embryos derived from women <30, 30–35, 36–40, and >40 years of age were compared retrospectively in terms of time of second polar body extrusion, time of pronuclei appearance, time of pronuclei fading, and time of two to eight discrete cells (t2–t8). Furthermore, abnormal cleavage patterns such as uneven blastomeres at the two-cell stage, cell fusion (Fu), and trichotomous mitoses (TM) were assessed.

Results

Only t5 occurred later in women aged 36–40 and >40 years when compared with those aged <30 and 30–35 years (p<0.001). Other morphokinetic timing parameters, as well the presence of uneven blastomeres, were comparable between the groups (p>0.05). However, Fu and TM were more common in women aged >40 years than in younger women (p<0.001).

Conclusion

Maternal age was correlated with the cleavage pattern of embryos. Therefore, evaluating embryo morphokinetics may contribute to optimal embryo selection, thereby increasing fertility in patients with advanced maternal age.

Depletion of Limiting rDNA Structural Complexes Triggers Chromosomal Instability and Replicative Aging of Saccharomyces cerevisiae

Sir2 is a highly conserved NAD+-dependent histone deacetylase that functions in heterochromatin formation and promotes replicative life span (RLS) in the budding yeast, Saccharomyces cerevisiae Within the yeast rDNA locus, Sir2 is required for efficient cohesin recruitment and maintaining the stability of the tandem array. In addition to the previously reported depletion of Sir2 in replicatively aged cells, we discovered that subunits of the Sir2-containing complexes silent information regulator (SIR) and regulator of nucleolar silencing and telophase (RENT) were depleted. Several other rDNA structural protein complexes also exhibited age-related depletion, most notably the cohesin complex. We hypothesized that mitotic chromosome instability (CIN) due to cohesin depletion could be a driver of replicative aging. Chromatin immunoprecipitation assays of the residual cohesin (Mcd1-Myc) in moderately aged cells showed strong depletion from the rDNA and initial redistribution to the point centromeres, which was then lost in older cells. Despite the shift in cohesin distribution, sister chromatid cohesion was partially attenuated in aged cells and the frequency of chromosome loss was increased. This age-induced CIN was exacerbated in strains lacking Sir2 and its paralog, Hst1, but suppressed in strains that stabilize the rDNA array due to deletion of FOB1 or through caloric restriction. Furthermore, ectopic expression of MCD1 from a doxycycline-inducible promoter was sufficient to suppress rDNA instability in aged cells and to extend RLS. Taken together, we conclude that age-induced depletion of cohesin and multiple other nucleolar chromatin factors destabilize the rDNA locus, which then results in general CIN and aneuploidy that shortens RLS.

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.

Meiotic spindle assembly checkpoint and aneuploidy in males versus females

The production of gametes (sperm and eggs in mammals) involves two sequential cell divisions, meiosis I and meiosis II. In meiosis I, homologous chromosomes segregate to different daughter cells, and meiosis II resembles mitotic divisions in that sister chromatids separate. While in principle the process is identical in males and females, the time frame and susceptibility to chromosomal defects, including achiasmy and cohesion weakening, and the response to mis-segregating chromosomes are not. In this review, we compare and contrast meiotic spindle assembly checkpoint function and aneuploidy in the two sexes.

Destabilization of spindle assembly checkpoint causes aneuploidy during meiosis II in murine post-ovulatory aged oocytes

Mammalian oocyte quality degrades over time after ovulation in vitro, which can cause fatal defects such as chromosomal aneuploidy. As various oocyte manipulations employed in assisted reproductive technology are time consuming, post-ovulatory aging is a serious problem to overcome in reproductive medicine or ova research. In this study, we investigated the effects of postovulatory aging on the incidence of chromosome aneuploidy during meiosis II, with a focus on the expression of functional proteins from the spindle assembly checkpoint (SAC). Chromosome analysis was used to assess the rate of aneuploidy in in vitro aged oocytes, or in early embryos derived from aged oocytes. Immunofluorescent staining was used to detect the localization of MAD2, which is a SAC signal that monitors the correct segregation of sister chromatids. Immunoblotting was used to quantify cohesin subunits, which are adhesion factors connecting sister chromatids at the metaphase II (MII) centromere. It was shown that post-ovulatory oocyte aging inhibits MAD2 localization to the sister kinetochore. Furthermore, oocyte aging prevented cohesin subunits from being maintained or degraded at the appropriate time. These data suggest that the destabilization of SAC signaling causes sister chromatid segregation errors in MII oocytes, and consequently increases the incidence of aneuploidy in early embryos. Our findings have provided distinct evidence that the post-ovulatory aging of oocytes might also be a risk factor for aneuploidy, irrespective of maternal age.

Fluconazole induces ROS in Cryptococcus neoformans and contributes to DNA damage in vitro

Pathogenic basidiomycetous yeast, Cryptococcus neoformans, causes fatal meningitis in immunocompromised individuals. Fluconazole (FLC) is a fungistatic drug commonly administered to treat cryptococcosis. Unfortunately, FLC-resistant strains characterized by various degree of chromosomal instability were isolated from clinical patients. Importantly, the underlying mechanisms that lead to chromosomal instability in FLC-treated C. neoformans remain elusive. Previous studies in fungal and mammalian cells link chromosomal instability to the reactive oxygen species (ROS). This study provides the evidence that exposure of C. neoformans to FLC induces accumulation of intracellular ROS, which correlates with plasma membrane damage. FLC caused transcription changes of oxidative stress related genes encoding superoxide dismutase (SOD1), catalase (CAT3), and thioredoxin reductase (TRR1). Strikingly, FLC contributed to an increase of the DNA damage in vitro, when complexed with iron or copper in the presence of hydrogen peroxide. Strains with isogenic deletion of copper response protein metallothionein were more susceptible to FLC. Addition of ascorbic acid (AA), an anti-oxidant at 10 mM, reduced the inhibitory effects of FLC. Consistent with potential effects of FLC on DNA integrity and chromosomal segregation, FLC treatment led to elevated transcription of RAD54 and repression of cohesin-encoding gene SCC1. We propose that FLC forms complexes with metals and contributes to elevated ROS, which may lead to chromosomal instability in C. neoformans.

Effects of Heyan Kuntai Capsule () on Follicular Development and Oocyte Cohesin Levels in Aged Mice

OBJECTIVE

To evaluate the effect of Heyan Kuntai Capsule (, HYKT) on the ovarian function of aged mice and expressions of cohesion complexes in oocytes.

METHODS

Twenty-five 9-month-old female C57BL/6J mice were randomly divided into 5 groups by block randomization method (n=5 per group), including the control group (saline), 17β-estradiol group [E2, 100 μg/(kg•d)], and low-, medium-, and highdose of HYKT groups [0.3, 0.9, 2.7 g/(kg•d), respectively]. All mice were treated by intragastric administration for 4 weeks. Hematoxylin and eosin staining and anti-VASA staining were used to detect the amounts of follicles. The apoptosis of follicles was measured by anti-gamma H2A histone family member X (γH2AX) staining and TdT-mediated dUTP Nick-End Labeling (TUNEL) assay. The density of cohesin subunits, REC8 meiotic recombination protein (REC8), structural maintenance of chromosome (SMC) 1β and SMC3 in oocytes were evaluated by immunofluorescent staining.

RESULTS

After administration of E2 and high-dose of HYKT, the total number of follicles as well as the number of primordial and primary follicles were significantly increased (P<0.05). Anti-γH2AX staining and TUNEL assay demonstrated that high-dose of HYKT and E2 partly suppressed the apoptosis of follicles (P<0.05). Furthermore, it showed an increased trend in the levels of REC8 and SMC1β, after administration with E2 and HYKT, and no obvious change in the level of SMC3.

CONCLUSION

HYKT could enhance the number of follicles, suppress apoptosis of oocytes and have a trend to elevate the meiotic-specific cohesin subunits (REC8 and SMC1β) in oocytes of aged mice, indicating a beneficial effect on the ovarian function in terms of the quantity and quality of follicles.

Allele-specific methylation of imprinted genes in fetal cord blood is influenced by cis-acting genetic variants and parental factors

Aim:

To examine the effects of genetic variation, parental age and BMI on parental allele-specific methylation of imprinted genes in fetal cord blood samples.

Methodology:

We have developed SNP genotyping and deep bisulphite sequencing assays for six imprinted genes to determine parental allele-specific methylation patterns in diploid somatic tissues.

Results:

Multivariate linear regression analyses revealed a negative correlation of paternal age with paternal MEG3 allele methylation in fetal cord blood. Methylation of the maternal PEG3 allele showed a positive correlation with maternal age. Paternal BMI was positively correlated with paternal MEST allele methylation. In addition to parental origin, allele-specific methylation of most imprinted genes was largely dependent on the underlying SNP haplotype.

Conclusion:

Our study supports the idea that parental factors can have an impact, although of small effect size, on the epigenome of the next generation, providing an additional layer of complexity to phenotypic diversity.

SA1/SA2 cohesion proteins and SIRT1-NAD+ deacetylase modulate telomere homeostasis in cumulus cells and are eligible biomarkers of ovarian aging

STUDY QUESTION

Are cohesins SA1/SA2 and the NAD-dependent deacetylase SIRT1 involved in telomere homeostasis of cumulus cells and thus eligible as biomarkers of follicular physiology and ovarian aging?

SUMMARY ANSWER

SA1/SA2 cohesins and SIRT1 are associated with telomere length in cumulus cells and may be eligible biomarkers of follicular physiology and ovarian aging.

WHAT IS KNOWN ALREADY

In somatic cells, cohesins SA1/SA2 mediate sister chromatid cohesion at the telomere termini (for SA1) and along chromatid arms (for SA2). The NAD+-dependent protein deacetylase Sirtuin 1 (SIRT1), which preserves DNA integrity from oxidative stress, may also modulate genome stability and telomere length.

STUDY DESIGN, SIZE, DURATION

Collectively 280 cumulus/oocyte complex samples were recovered from a total of 50 women undergoing in vitro fertilization.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Cumulus cells were separated from the oocyte-cumulus complex. DNA and total mRNA were extracted from cumulus cells and assayed for telomere length and for SA1, SA2 and SIRT1 gene expression profiling. Telomere length was determined by quantitave PCR and analyzed relative to the single copy of the housekeeping gene (albumin) to generate a T/S ratio (Telomere/single copy gene). Gene expression levels of SA1, SA2 and SIRT1 mRNA were assayed by quantitative RT-PCR and confirmed by western blotting and immunofluorescent studies (SIRT1). SA1/SA2 and SIRT1 gene expression levels and telomere length analysis of patients/samples were ranked in relation to their clinical setting parameters (BMI, age) and to the number of oocyte retrieved.

MAIN RESULTS AND THE ROLE OF CHANCE

SA1 and SA2 transcripts were both detected in all cumulus cells analyzed and the relative amount showed a clear decreasing trend according to the age of patients. A significant increase in SA1 and SA2 was disclosed in high responder women (>6 oocytes retrieved) compared to poor responders (<4 oocytes) (P < 0.05). Furthermore, statistically significant positive correlations were also recorded between the transcripts levels of the two cohesin molecules (r = 0.89; P < 0.05) and, to a lesser extent, between telomere length and SA1 (r = 0.42; P < 0.001) and SA2 (r = 0.36; P < 0.001) mRNA levels. SIRT1 expression was also significantly increased in high responders (>6 oocytes) compared to poor responders. Significant correlations were found between SIRT1 and SA1 (r = 0.69; P < 0.001), between SIRT1 and SA2 (r = 0.78; P < 0.001), and between SIRT1 and telomere length (r = 0.36; P < 0.001). However, in the older patient group (>38 years), SIRT1 mRNA levels were twice as high as the levels recorded in the younger patient cohort (<34 years). Western blot analysis and immunofluorescent studies confirmed the increments in SIRT1 protein levels in patients over 38 years old.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Cumulus/oocyte complexes were retrieved by patients undergoing ovarian stimulation protocol for IVF. We cannot exclude the possibility that different stimulation protocols affect the correlations highlighted in this study. Future investigations should shed light on cumulus cells molecular profile according to different stimulation protocols.

WIDER IMPLICATIONS OF THE FINDINGS

The overall results of our study point to the involvement of cohesins SA1/SA2 and SIRT1 deacetylase in telomere homeostasis in cumulus cells and highlight their possible eligibility as biomarkers of follicular physiology and ovarian aging.

STUDY FUNDING/COMPETING INTEREST(S)

Merck Serono S.P.A Italy sponsored the study with financial support. There are no competing interests to declare.

Multiple Duties for Spindle Assembly Checkpoint Kinases in Meiosis

Cell division in mitosis and meiosis is governed by evolutionary highly conserved protein kinases and phosphatases, controlling the timely execution of key events such as nuclear envelope breakdown, spindle assembly, chromosome attachment to the spindle and chromosome segregation, and cell cycle exit. In mitosis, the spindle assembly checkpoint (SAC) controls the proper attachment to and alignment of chromosomes on the spindle. The SAC detects errors and induces a cell cycle arrest in metaphase, preventing chromatid separation. Once all chromosomes are properly attached, the SAC-dependent arrest is relieved and chromatids separate evenly into daughter cells. The signaling cascade leading to checkpoint arrest depends on several protein kinases that are conserved from yeast to man. In meiosis, haploid cells containing new genetic combinations are generated from a diploid cell through two specialized cell divisions. Though apparently less robust, SAC control also exists in meiosis. Recently, it has emerged that SAC kinases have additional roles in executing accurate chromosome segregation during the meiotic divisions. Here, we summarize the main differences between mitotic and meiotic cell divisions, and explain why meiotic divisions pose special challenges for correct chromosome segregation. The less-known meiotic roles of the SAC kinases are described, with a focus on two model systems: yeast and mouse oocytes. The meiotic roles of the canonical checkpoint kinases Bub1, Mps1, the pseudokinase BubR1 (Mad3), and Aurora B and C (Ipl1) will be discussed. Insights into the molecular signaling pathways that bring about the special chromosome segregation pattern during meiosis will help us understand why human oocytes are so frequently aneuploid.

Age-Related Loss of Cohesion: Causes and Effects

Aneuploidy is a leading genetic cause of birth defects and lower implantation rates in humans. Most errors in chromosome number originate from oocytes. Aneuploidy in oocytes increases with advanced maternal age. Recent studies support the hypothesis that cohesion deterioration with advanced maternal age represents a leading cause of age-related aneuploidy. Cohesin generates cohesion, and is established only during the premeiotic S phase of fetal development without any replenishment throughout a female's period of fertility. Cohesion holds sister chromatids together until meiosis resumes at puberty, and then chromosome segregation requires the release of sister chromatid cohesion from chromosome arms and centromeres at anaphase I and anaphase II, respectively. The time of cohesion cleavage plays an important role in correct chromosome segregation. This review focuses specifically on the causes and effects of age-related cohesion deterioration in female meiosis.

Merotelic kinetochore attachment in oocyte meiosis II causes sister chromatids segregation errors in aged mice

Mammalian oocyte chromosomes undergo 2 meiotic divisions to generate haploid gametes. The frequency of chromosome segregation errors during meiosis I increase with age. However, little attention has been paid to the question of how aging affects sister chromatid segregation during oocyte meiosis II. More importantly, how aneuploid metaphase II (MII) oocytes from aged mice evade the spindle assembly checkpoint (SAC) mechanism to complete later meiosis II to form aneuploid embryos remains unknown. Here, we report that MII oocytes from naturally aged mice exhibited substantial errors in chromosome arrangement and configuration compared with young MII oocytes. Interestingly, these errors in aged oocytes had no impact on anaphase II onset and completion as well as 2-cell formation after parthenogenetic activation. Further study found that merotelic kinetochore attachment occurred more frequently and could stabilize the kinetochore-microtubule interaction to ensure SAC inactivation and anaphase II onset in aged MII oocytes. This orientation could persist largely during anaphase II in aged oocytes, leading to severe chromosome lagging and trailing as well as delay of anaphase II completion. Therefore, merotelic kinetochore attachment in oocyte meiosis II exacerbates age-related genetic instability and is a key source of age-dependent embryo aneuploidy and dysplasia.

RNA-binding proteins in human oogenesis: Balancing differentiation and self-renewal in the female fetal germline

Primordial germ cells undergo three significant processes on their path to becoming primary oocytes: the initiation of meiosis, the formation and breakdown of germ cell nests, and the assembly of single oocytes into primordial follicles. However at the onset of meiosis, the germ cell becomes transcriptionally silenced. Consequently translational control of pre-stored mRNAs plays a central role in coordinating gene expression throughout the remainder of oogenesis; RNA binding proteins are key to this regulation. In this review we examine the role of exemplars of such proteins, namely LIN28, DAZL, BOLL and FMRP, and highlight how their roles during germ cell development are critical to oogenesis and the establishment of the primordial follicle pool.

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RNA-binding proteins (RBPs) are key regulators of gene expression during oogenesis.

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RBPs LIN28, DAZL, BOLL and FMRP display stage-specific expression in fetal oocytes.

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LIN28 and DAZL may regulate self-renewal and progression into meiosis respectively.

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BOLL and FMRP may be involved in the later stages of prophase I and oocyte growth.

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RBPs may have critical roles in establishing the ovarian reserve during fetal life.

Inefficient Crossover Maturation Underlies Elevated Aneuploidy in Human Female Meiosis

Meiosis is the cellular program that underlies gamete formation. For this program, crossovers between homologous chromosomes play an essential mechanical role to ensure regular segregation. We present a detailed study of crossover formation in human male and female meiosis, enabled by modeling analysis. Results suggest that recombination in the two sexes proceeds analogously and efficiently through most stages. However, specifically in female (but not male), ∼25% of the intermediates that should mature into crossover products actually fail to do so. Further, this "female-specific crossover maturation inefficiency" is inferred to make major contributions to the high level of chromosome mis-segregation and resultant aneuploidy that uniquely afflicts human female oocytes (e.g., giving Down syndrome). Additionally, crossover levels on different chromosomes in the same nucleus tend to co-vary, an effect attributable to global per-nucleus modulation of chromatin loop size. Maturation inefficiency could potentially reflect an evolutionary advantage of increased aneuploidy for human females.