The primordial follicle reserve is not renewed after chemical or γ-irradiation mediated depletion

An overview of different methods to establish a murine premature ovarian failure model

Premature ovarian failure (POF)is defined as the loss of normal ovarian function before the age of 40 and is characterized by increased gonadotropin levels and decreased estradiol levels and ovarian reserve, often leading to infertility. The incomplete understanding of the pathogenesis of POF is a major impediment to the development of effective treatments for this disease, so the use of animal models is a promising option for investigating and identifying the molecular mechanisms involved in POF patients and developing therapeutic agents. As mice and rats are the most commonly used models in animal research, this review article considers studies that used murine POF models. In this review based on the most recent studies, first, we introduce 10 different methods for inducing murine POF models, then we demonstrate the advantages and disadvantages of each one, and finally, we suggest the most practical method for inducing a POF model in these animals. This may help researchers find the method of creating a POF model that is most appropriate for their type of study and suits the purpose of their research.

The DNA double-strand break repair proteins γH2AX, RAD51, BRCA1, RPA70, KU80, and XRCC4 exhibit follicle-specific expression differences in the postnatal mouse ovaries from early to older ages

PURPOSE

Ovarian aging is closely related to a decrease in follicular reserve and oocyte quality. The precise molecular mechanisms underlying these reductions have yet to be fully elucidated. Herein, we examine spatiotemporal distribution of key proteins responsible for DNA double-strand break (DSB) repair in ovaries from early to older ages. Functional studies have shown that the γH2AX, RAD51, BRCA1, and RPA70 proteins play indispensable roles in HR-based repair pathway, while the KU80 and XRCC4 proteins are essential for successfully operating cNHEJ pathway.

METHODS

Female Balb/C mice were divided into five groups as follows: Prepuberty (3 weeks old; n = 6), puberty (7 weeks old; n = 7), postpuberty (18 weeks old; n = 7), early aged (52 weeks old; n = 7), and late aged (60 weeks old; n = 7). The expression of DSB repair proteins, cellular senescence (β-GAL) and apoptosis (cCASP3) markers was evaluated in the ovaries using immunohistochemistry.

RESULT

β-GAL and cCASP3 levels progressively increased from prepuberty to aged groups (P < 0.05). Notably, γH2AX levels varied in preantral and antral follicles among the groups (P < 0.05). In aged groups, RAD51, BRCA1, KU80, and XRCC4 levels increased (P < 0.05), while RPA70 levels decreased (P < 0.05) compared to the other groups.

CONCLUSIONS

The observed alterations were primarily attributed to altered expression in oocytes and granulosa cells of the follicles and other ovarian cells. As a result, the findings indicate that these DSB repair proteins may play a role in the repair processes and even other related cellular events in ovarian cells from early to older ages.

Immune checkpoint inhibitor treatment does not impair ovarian or endocrine function in a mouse model of triple negative breast cancer

Background

Representing 15-20% of all breast cancer cases, triple negative breast cancer (TNBC) is diagnosed more frequently in reproductive-age women and exhibits higher rates of disease metastasis and recurrence when compared with other subtypes. Few targeted treatments exist for TNBC, and many patients experience infertility and endocrine disruption as a result of frontline chemotherapy treatment. While they are a promising option for less toxic therapeutic approaches, little is known about the effects of immune checkpoint inhibitors on reproductive and endocrine function.

Results

Our findings in a syngeneic TNBC mouse model revealed that therapeutically relevant immunotherapies targeting PD-1, LAG-3, and TIM-3 had no effect on the quality and abundance of ovarian follicles, estrus cyclicity, or hormonal homeostasis. Similarly, in a tumor-free mouse model, we found that ovarian architecture, follicle abundance, estrus cyclicity, and ovulatory efficiency remain unchanged by PD-1 blockade.

Conclusions

Taken together, our results suggest that immunotherapy may be a promising component of fertility-sparing therapeutic regimens for patients that wish to retain ovarian and endocrine function after cancer treatment.

Genome Instability and DNA Repair in Somatic and Reproductive Aging

Genetic material is constantly subjected to genotoxic insults and is critically dependent on DNA repair. Genome maintenance mechanisms differ in somatic and germ cells as the soma only requires maintenance during an individual's lifespan, while the germline indefinitely perpetuates its genetic information. DNA lesions are recognized and repaired by mechanistically highly diverse repair machineries. The DNA damage response impinges on a vast array of homeostatic processes and can ultimately result in cell fate changes such as apoptosis or cellular senescence. DNA damage causally contributes to the aging process and aging-associated diseases, most prominently cancer. By causing mutations, DNA damage in germ cells can lead to genetic diseases and impact the evolutionary trajectory of a species. The mechanisms ensuring tight control of germline DNA repair could be highly instructive in defining strategies for improved somatic DNA repair. They may provide future interventions to maintain health and prevent disease during aging.

Low dose rate radiation impairs early follicles in young mice

Low-dose radiation is generally considered less harmful than high-dose radiation. However, its impact on ovaries remains debated. Since previous reports predominantly employed low-dose radiation delivered at a high dose rate on the ovary, the effect of low-dose radiation at a low dose rate on the ovary remains unknown. We investigated the effect of low-dose ionizing radiation delivered at a low dose rate on murine ovaries. Three- and ten-week-old mice were exposed to 0.1 and 0.5 Gy of radiation at a rate of 6 mGy/h and monitored after 3 and 30 days. While neither body weight nor ovarian area showed significant changes, ovarian cells were damaged, showing apoptosis and a decrease in cell proliferation after exposure to 0.1 and 0.5 Gy radiation. Follicle numbers decreased over time in both age groups proportionally to the radiation dose. Younger mice were more susceptible to radiation damage, as evidenced by decreased follicles in 3-week-old mice after 30 days of 0.1 Gy exposure, while 10-week-old mice showed reduced follicles only following 0.5 Gy exposure. Primordial or primary follicles were the most vulnerable to radiation. These findings suggest that even low-dose radiation, delivered at a low dose rate, can adversely affect ovarian function, particularly in the early follicles of younger mice.

Beyond apoptosis: evidence of other regulated cell death pathways in the ovary throughout development and life

BACKGROUND

Regulated cell death is a fundamental component of numerous physiological processes; spanning from organogenesis in utero, to normal cell turnover during adulthood, as well as the elimination of infected or damaged cells throughout life. Quality control through regulation of cell death pathways is particularly important in the germline, which is responsible for the generation of offspring. Women are born with their entire supply of germ cells, housed in functional units known as follicles. Follicles contain an oocyte, as well as specialized somatic granulosa cells essential for oocyte survival. Follicle loss-via regulated cell death-occurs throughout follicle development and life, and can be accelerated following exposure to various environmental and lifestyle factors. It is thought that the elimination of damaged follicles is necessary to ensure that only the best quality oocytes are available for reproduction.

OBJECTIVE AND RATIONALE

Understanding the precise factors involved in triggering and executing follicle death is crucial to uncovering how follicle endowment is initially determined, as well as how follicle number is maintained throughout puberty, reproductive life, and ovarian ageing in women. Apoptosis is established as essential for ovarian homeostasis at all stages of development and life. However, involvement of other cell death pathways in the ovary is less established. This review aims to summarize the most recent literature on cell death regulators in the ovary, with a particular focus on non-apoptotic pathways and their functions throughout the discrete stages of ovarian development and reproductive life.

SEARCH METHODS

Comprehensive literature searches were carried out using PubMed and Google Scholar for human, animal, and cellular studies published until August 2022 using the following search terms: oogenesis, follicle formation, follicle atresia, oocyte loss, oocyte apoptosis, regulated cell death in the ovary, non-apoptotic cell death in the ovary, premature ovarian insufficiency, primordial follicles, oocyte quality control, granulosa cell death, autophagy in the ovary, autophagy in oocytes, necroptosis in the ovary, necroptosis in oocytes, pyroptosis in the ovary, pyroptosis in oocytes, parthanatos in the ovary, and parthanatos in oocytes.

OUTCOMES

Numerous regulated cell death pathways operate in mammalian cells, including apoptosis, autophagic cell death, necroptosis, and pyroptosis. However, our understanding of the distinct cell death mediators in each ovarian cell type and follicle class across the different stages of life remains the source of ongoing investigation. Here, we highlight recent evidence for the contribution of non-apoptotic pathways to ovarian development and function. In particular, we discuss the involvement of autophagy during follicle formation and the role of autophagic cell death, necroptosis, pyroptosis, and parthanatos during follicle atresia, particularly in response to physiological stressors (e.g. oxidative stress).

WIDER IMPLICATIONS

Improved knowledge of the roles of each regulated cell death pathway in the ovary is vital for understanding ovarian development, as well as maintenance of ovarian function throughout the lifespan. This information is pertinent not only to our understanding of endocrine health, reproductive health, and fertility in women but also to enable identification of novel fertility preservation targets.

Radiotherapy exposure directly damages the uterus and causes pregnancy loss

Female cancer survivors are significantly more likely to experience infertility than the general population. It is well established that chemotherapy and radiotherapy can damage the ovary and compromise fertility, yet the ability of cancer treatments to induce uterine damage, and the underlying mechanisms, have been understudied. Here, we show that in mice total-body γ-irradiation (TBI) induced extensive DNA damage and apoptosis in uterine cells. We then transferred healthy donor embryos into ovariectomized adolescent female mice that were previously exposed to TBI to study the impacts of radiotherapy on the uterus independent from effects to ovarian endocrine function. Following TBI, embryo attachment and implantation were unaffected, but fetal resorption was evident at midgestation in 100% of dams, suggesting failed placental development. Consistent with this hypothesis, TBI impaired the decidual response in mice and primary human endometrial stromal cells. TBI also caused uterine artery endothelial dysfunction, likely preventing adequate blood vessel remodeling in early pregnancy. Notably, when pro-apoptotic protein Puma-deficient (Puma-/-) mice were exposed to TBI, apoptosis within the uterus was prevented, and decidualization, vascular function, and pregnancy were restored, identifying PUMA-mediated apoptosis as a key mechanism. Collectively, these data show that TBI damages the uterus and compromises pregnancy success, suggesting that optimal fertility preservation during radiotherapy may require protection of both the ovaries and uterus. In this regard, inhibition of PUMA may represent a potential fertility preservation strategy.

TAp63 determines the fate of oocytes against DNA damage

Cyclophosphamide and doxorubicin lead to premature ovarian insufficiency as an off-target effect. However, their oocyte death pathway has been debated. Here, we clarified the precise mechanism of ovarian depletion induced by cyclophosphamide and doxorubicin. Dormant oocytes instead of activated oocytes with high PI3K activity were more sensitive to cyclophosphamide. Checkpoint kinase 2 (CHK2) inhibitor rather than GNF2 protected oocytes from cyclophosphamide and doxorubicin, as cyclophosphamide up-regulated p-CHK2 and depleted primordial follicles in Abl1 knockout mice. Contrary to previous reports, TAp63 is pivotal in cyclophosphamide and doxorubicin-induced oocyte death. Oocyte-specific Trp63 knockout mice prevented primordial follicle loss and maintained reproductive function from cyclophosphamide and doxorubicin, indicated by undetectable levels of BAX and cPARP. Here, we demonstrated that TAp63 is fundamental in determining the signaling of oocyte death against DNA damage. This study establishes the role of TAp63 as a target molecule of adjuvant therapies to protect the ovarian reserve from different classes of chemotherapy.

Checkpoint inhibitor immunotherapy diminishes oocyte number and quality in mice

Loss of fertility is a major concern for female reproductive-age cancer survivors, since a common side-effect of conventional cytotoxic cancer therapies is permanent damage to the ovary. While immunotherapies are increasingly becoming a standard of care for many cancers-including in the curative setting-their impacts on ovarian function and fertility are unknown. We evaluated the effect of immune checkpoint inhibitors blocking programmed cell death protein ligand 1 and cytotoxic T lymphocyte-associated antigen 4 on the ovary using tumor-bearing and tumor-free mouse models. We find that immune checkpoint inhibition increases immune cell infiltration and tumor necrosis factor-α expression within the ovary, diminishes the ovarian follicular reserve and impairs the ability of oocytes to mature and ovulate. These data demonstrate that immune checkpoint inhibitors have the potential to impair both immediate and future fertility, and studies in women should be prioritized. Additionally, fertility preservation should be strongly considered for women receiving these immunotherapies, and preventative strategies should be investigated in future studies.

Ovarian Stem Cells (OSCs) from the Cryopreserved Ovarian Cortex: A Potential for Neo-Oogenesis in Women with Cancer-Treatment Related Infertility: A Case Report and a Review of Literature

Cancer treatment related infertility (CTRI) affects more than one third of young women undergoing anti-cancer protocols, inducing a premature exhaustion of the ovarian reserve. In addition to ovarian suppression by GnRHa, oocyte and cortex cryopreservation has gained interest in patients with estrogen-sensitive tumors for whom the hormonal burst to prompt the multiple follicular growth could provide a further pro-life tumor pulsing. On the other hand, cortex reimplantation implies a few drawbacks due to the unknown consistency of the follicles to be reimplanted or the risk of reintroducing malignant cells. The capability of ovarian stem cells (OCSs) from fresh ovarian cortex fragments to differentiate in vitro to mature oocytes provides a tool to overcome these drawbacks. In fact, since ovarian cortex sampling and cryopreservation is practicable before gonadotoxic treatments, the recruitment of OSCs from defrosted fragments could provide a novel opportunity to verify their suitability to be expanded in vitro as oocyte like cells (OLCs). Here, we describe in very preliminary experiments the consistency of an OSC population from a single cryopreserved ovarian cortex after thawing as well as both their viability and their suitability to be further explored in their property to differentiate in OLCs, thus reinforcing interest in stemness studies in the treatment of female CTRI.

Identification of Stem Cell-Like Cells in the Ovary

Understanding the function of stem cells and cellular microenvironments in in vitro oogenesis, including ovarian folliculogenesis, is crucial for reproductive biology. Because mammalian females cannot generate oocytes after birth, the number of oocyte decreases with the progression of reproductive age. Meanwhile, there is an emerging need for the neogenesis of female germ cells to treat the increasing infertility-related issues in cancer survivors. The concept of oocytes neogenesis came from the promising results of stem cells in reproductive medicine. The stem cells that generate oocytes are defined as stem cell-like cells in the ovary (OSCs). Several recent studies have focused on the origin, isolation, and characteristic of OSCs and the differentiation of OSCs into oocytes, ovarian follicles and granulosa cells. Hence, in this review, we focus on the experimental trends in OSC research and discuss the methods of OSC isolation. We further summarized the characteristics of OSCs and discuss the markers used to identify OSCs differentiated from various cell sources. We believe that this review will be beneficial for advancing the research and clinical applications of OSCs.

Evaluation of Global DNA Methylation and Gene Expression of Izumo1 and Izumo1r in Gonads after High- and Low-Dose Radiation in Neonatal Mice

The intergenerational effects from chronic low-dose exposure are matters of concern. It is thus important to elucidate the radiation-induced effects of germ cell maturation, fertilization and embryonic development. It is well known that DNA methylation levels in CpG sites in gametes are reprogrammed in stages during their maturity. Furthermore, the binding of Izumo on the surface of sperm and Juno on the surface of oocytes is essential for fertilization. Thus, there is a possibility that these genes are useful indicators to evaluate fertility in mice after irradiation exposure. Therefore, in this study, we analyzed global DNA methylation patterns in the testes and gene expression of Izumo1 and Izumo1r (Juno) in the gonads of mice after neonatal acute high-dose ionizing radiation (HDR) and chronic low-dose ionizing radiation (LDR). One-week-old male and female mice were irradiated with a total dose of 4 Gy, with acute HDR at 7 days at a dose rate of 30 Gy/h and LDR continuously at a dose rate of 6 mGy/h from 7 to 35 days. Their gonads were subsequently analyzed. The results of global DNA methylation patterns in the testes showed that methylation level increased with age in the control group, the LDR group maintained its DNA methylation level, and the HDR group showed decreased DNA methylation levels with age. In the control group, the gene expression level of Izumo1 in the testis did not show age-related changes, although there was high expression at 100 days of age. However, in the LDR group, the expression level recovered after the end of irradiation, while it remained low regardless of age in the HDR group. Conversely, gene expression of Izumo1r (Izumo1 receptor) in the ovary decreased with age in the control group. Although the gene expression of Izumo1r decreased with age in the LDR group, it remained low in the HDR group. Our results indicate that LDR can induce different DNA methylation patterns, and both high- and low-dose radiation before sexual maturity might affect gametogenesis and fertility.

Effects of PD-1 blockade on ovarian follicles in a prepubertal female mouse

Immunotherapy has emerged at the forefront of cancer treatment. Checkpoint inhibitor pembrolizumab (KEYTRUDA), a chimeric antibody which targets programmed cell death protein 1 (PD-1), has been approved by the Food and Drug Administration (FDA) for use in pediatric patients with relapsed or refractory classical Hodgkin's lymphoma. However, there is currently no published data regarding the effects of pembrolizumab on the ovary of female pediatric patients. In this study, prepubertal immunocompetent and immunodeficient female mice were injected with pembrolizumab or anti-mouse PD-1 antibody. The number of primordial follicles significantly decreased post-injection of both pembrolizumab and anti-mouse PD-1 antibody in immunocompetent mice. However, no changes in follicle numbers were observed in immunodeficient nude mice. Superovulation test and vaginal opening experiments suggest that there is no difference in the number of cumulus-oocyte complexes (COCs) and the timing of puberty onset between the control and anti-mouse PD-1 antibody treatment groups, indicating that there is no effect on short-term fertility. Elevation of pro-inflammatory cytokine TNF-α following COX-2 upregulation was observed in the ovary. CD3+ T-cell infiltration was detected within some ovarian follicles and between stromal cells of the ovaries in mice following treatment with anti-mouse PD-1 antibody. Thus, PD-1 immune checkpoint blockade affects the ovarian reserve through a mechanism possibly involving inflammation following CD3+ T-cell infiltration.

Evaluation of mitochondria in mouse oocytes following cisplatin exposure

BACKGROUND

Cisplatin is a platinum-based chemotherapeutic that damages genomic DNA leading to cell death. It also damages mitochondrial DNA and induces high levels of mitochondrial reactive oxygen species (mtROS), further sensitising cells to apoptosis. Notably, immature oocytes are particularly vulnerable to cisplatin treatment, a common side effect of which is depletion of the primordial follicle reserve, leading to infertility and early menopause. Cisplatin is known to damage the DNA of oocytes, but the possibility that cisplatin also compromises oocyte survival and quality by damaging mitochondria, has not been investigated. To begin to address this question, neonatal mice were treated with saline or cisplatin (2 mg/kg or 4 mg/kg) and the short and long-term impacts on mitochondria in oocytes were characterised.

RESULTS

At 6 and 24 h after treatment, mitochondrial localisation, mass and ATP content in immature oocytes were similar between groups. However, TMRM staining intensity, a marker of mitochondrial membrane potential, was decreased in immature oocytes from cisplatin treated mice compared to saline treated controls, consistent with the induction of apoptosis. When mice were super ovulated 5 weeks after exposure, the number of mature oocytes harvested from cisplatin treated mice was significantly lower than controls. Mitochondrial localisation, mass, membrane potential and ATP levels showed no differences between groups.

CONCLUSIONS

These findings suggest that mitochondrial dysfunction may contribute to the depletion of the ovarian reserve caused by cisplatin, but long-term impacts on mitochondria may be minimal as those immature oocytes that survive cisplatin treatment develop into mature oocytes with normal mitochondrial parameters.

Female ageing affects the DNA repair capacity of oocytes in IVF using a controlled model of sperm DNA damage in mice

STUDY QUESTION

Does female ageing have a negative effect on the DNA repair capacity of oocytes fertilised by spermatozoa with controlled levels of DNA damage?

SUMMARY ANSWER

Compared to oocytes from younger females, oocytes from older females have a reduced capacity to repair damaged DNA introduced by spermatozoa.

WHAT IS KNOWN ALREADY

The reproductive lifespan in women declines with age predominantly due to poor oocyte quality. This leads to decreased reproductive outcomes for older women undergoing assisted reproductive technology (ART) treatments, compared to young women. Ageing and oocyte quality have been clearly associated with aneuploidy, but the range of factors that influence this change in oocyte quality with age remains unclear. The DNA repair activity prior to embryonic genomic activation is considered to be of maternal origin, with maternal transcripts and proteins controlling DNA integrity. With increasing maternal age, the number of mRNAs stored in oocytes decreases. This could result in diminished efficiency of DNA repair and/or negative effects on embryo development, especially in the presence of DNA damage.

STUDY DESIGN, SIZE, DURATION

Oocytes from two age groups of 30 super-ovulated female mice (young: 5-8 weeks old, n = 15; old: 42-45 weeks old, n = 15) were inseminated with sperm from five males with three different controlled DNA damage levels; control: ≤10%, 1 Gray (Gy): 11-30%, and 30 Gy: >30%. Inseminated oocytes (young: 125, old: 78) were assessed for the formation of zygotes (per oocyte) and blastocysts (per zygote). Five replicates of five germinal vesicles (GVs) and five MII oocytes from each age group were analysed for gene expression. The DNA damage response (DDR) was assessed in a minimum of three IVF replicates in control and 1 Gy zygotes and two-cell embryos using γH2AX labelling.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Swim-up sperm samples from the cauda epididymidis of C57BL6 mice were divided into control (no irradiation) and 1- and 30-Gy groups. Treated spermatozoa were irradiated at 1 and 30 Gy, respectively, using a linear accelerator Varian 21iX. Following irradiation, samples were used for DNA damage assessment (Halomax) and for insemination. Presumed zygotes were cultured in a time-lapse incubator (MIRI, ESCO). Gene expression of 91 DNA repair genes was assessed using the Fluidigm Biomark HD system. The DNA damage response in zygotes (6-8 h post-fertilisation) and two-cell embryos (22-24 h post-fertilisation) was assessed by immunocytochemical analysis of γH2AX using confocal microscopy (Olympus FV1200) and 3D volumetric analysis using IMARIS software.

MAIN RESULTS AND THE ROLE OF CHANCE

The average sperm DNA damage for the three groups was statistically different (control: 6.1%, 1 Gy: 16.1%, 30 Gy: 53.1%, P < 0.0001), but there were no significant differences in fertilisation rates after IVF within or between the two age groups [(young; control: 86.79%, 1 Gy: 82.75%, 30 Gy: 76.74%) (old; control: 93.1%, 1 Gy: 70.37%, 30 Gy: 68.18%) Fisher's exact]. However, blastocyst rates were significantly different (P < 0.0001) among the groups [(young; control: 86.95%, 1 Gy: 33.33%, 30 Gy: 0.0%) (old; control: 70.37%, 1 Gy: 0.0%, 30 Gy: 0.0%)]. Between the age groups, 1-Gy samples showed a significant decrease in the blastocyst rate in old females compared to young females (P = 0.0166). Gene expression analysis revealed a decrease in relative expression of 21 DNA repair genes in old GV oocytes compared to young GV oocytes (P < 0.05), and similarly, old MII oocytes showed 23 genes with reduced expression compared to young MII oocytes (P < 0.05). The number of genes with decreased expression in older GV and MII oocytes significantly affected pathways such as double strand break (GV: 5; MII: 6), nucleotide excision repair (GV: 8; MII: 5) and DNA damage response (GV: 4; MII: 8). There was a decreased DDR in zygotes and in two-cell embryos from old females compared to young regardless of sperm treatment (P < 0.05). The decrease in DNA repair gene expression of oocytes and decreased DDR in embryos derived from older females suggests that ageing results in a diminished DNA repair capacity.

LARGE-SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

Ionising radiation was used only for experimental purposes, aiming at controlled levels of sperm DNA damage; however, it can also damage spermatozoa proteins. The female age groups selected in mice were intended to model effects in young and old women, but clinical studies are required to demonstrate a similar effect.

WIDER IMPLICATIONS OF THE FINDINGS

Fertilisation can occur with sperm populations with medium and high DNA damage, but subsequent embryo growth is affected to a greater extent with aging females, supporting the theory that oocyte DNA repair capacity decreases with age. Assessment of the oocyte DNA repair capacity may be a useful diagnostic tool for infertile couples.

STUDY FUNDING/COMPETING INTEREST(S)

Funded by the Education Program in Reproduction and Development, Department of Obstetrics and Gynaecology, Monash University. None of the authors has any conflict of interest to report.

The PARP inhibitor, olaparib, depletes the ovarian reserve in mice: implications for fertility preservation

STUDY QUESTION

What is the impact of the poly(ADP-ribose) polymerase (PARP) inhibitor, olaparib, alone or in combination with chemotherapy on the ovary in mice?

SUMMARY ANSWER

Olaparib treatment, when administered alone, depletes primordial follicle oocytes, but olaparib does not exacerbate chemotherapy-mediated ovarian follicle loss in mice.

WHAT IS KNOWN ALREADY

The ovary contains a finite number of oocytes stored within primordial follicles, which give rise to all mature ovulatory oocytes. Unfortunately, they are highly sensitive to exogenous DNA damaging insults, such as cytotoxic cancer treatments. Members of the PARP family of enzymes are central to the repair of single-strand DNA breaks. PARP inhibitors have shown promising clinical efficacy in reducing tumour burden, by blocking DNA repair capacity. Olaparib is a PARP1/2 inhibitor recently FDA-approved for treatment of BRCA1 and BRCA2 mutation carriers with metastatic breast cancer. It is currently being investigated as an adjunct to standard treatment at an earlier stage, potentially curable, BRCA1- and BRCA2-associated breast cancer which affects reproductive age women. Despite this, there is no preclinical or clinical information regarding the potential impacts of olaparib on the ovary or on female fertility. Unfortunately, it may be many years before clinical data on fertility outcomes for women treated with PARP inhibitors becomes available, highlighting the importance of rigorous preclinical research using animal models to establish the potential for new cancer therapies to affect the ovary in humans. We aimed to comprehensively determine the impact of olaparib alone, or following chemotherapy, on the ovary in mice.

STUDY DESIGN, SIZE, DURATION

On Day 0, mice (n = 5/treatment group) were administered a single intraperitoneal dose of cyclophosphamide (75 mg/kg/body weight), doxorubicin (10 mg/kg), carboplatin (80 mg/kg), paclitaxel (7.5 mg/kg) or vehicle control. From Days 1 to 28, mice were administered subcutaneous olaparib (50 mg/kg) or vehicle control. This regimen is proven to reduce tumour burden in preclinical mouse studies and is also physiologically relevant for women.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Adult female wild-type C57BL6/J mice at peak fertility (8 weeks) were administered a single intraperitoneal dose of chemotherapy, or vehicle, then either subcutaneous olaparib or vehicle for 28 days. Vaginal smears were performed on each animal for 14 consecutive days from Days 15 to 28 to monitor oestrous cycling. At 24 h after final treatment, ovaries were harvested for follicle enumeration and immunohistochemical analysis of primordial follicle remnants (FOXL2 expressing granulosa cells), DNA damage (γH2AX) and analysis of apoptosis by TUNEL assay. Serum was collected to measure circulating anti-Müllerian hormone (AMH) concentrations by ELISA.

MAIN RESULTS AND THE ROLE OF CHANCE

Olaparib significantly depleted primordial follicles by 36% compared to the control (P < 0.05) but had no impact on other follicle classes, serum AMH, corpora lutea number (indicative of ovulation) or oestrous cycling. Primordial follicle remnants were rarely detected in control ovaries but were significantly elevated in ovaries from mice treated with olaparib alone (P < 0.05). Similarly, DNA damage denoted by γH2AX foci was completely undetectable in primordial follicles of control animals but was observed in ∼10% of surviving primordial follicle oocytes in mice treated with olaparib alone. These observations suggest that functional PARPs are essential for primordial follicle oocyte maintenance and survival. Olaparib did not exacerbate chemotherapy-mediated follicle depletion in the wild-type mouse ovary.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

This study was performed in mice, so the findings may not translate to women and further studies utilizing human ovarian tissue and sera samples should be performed in the future. Only one long-term time point was analysed, therefore olaparib-mediated follicle damage should be assessed at more immediate time points in the future to support our mechanistic findings.

WIDER IMPLICATIONS OF THE FINDINGS

Olaparib dramatically depleted primordial follicles and this could be attributed to loss of intrinsic PARP-mediated DNA repair mechanisms. Importantly, diminished ovarian reserve can result in premature ovarian insufficiency and infertility. Notably, the extent of follicle depletion might be enhanced in BRCA1 and BRCA2 mutation carriers, and this is the subject of current investigations. Together, our data suggest that fertility preservation options should be considered for young women prior to olaparib treatment, and that human studies of this issue should be prioritized.

STUDY FUNDING/COMPETING INTEREST(S)

This work was made possible through Victorian State Government Operational Infrastructure Support and Australian Government NHMRC IRIISS. This work was supported by funding from the National Health and Medical Research Council (NHMRC); (K.J.H. #1050130) (A.L.W. #1120300). K.A.P. is a National Breast Cancer Foundation Fellow (Australia-PRAC-17-004). K.A.P. is the Breast Cancer Trials (Australia) Study Chair for the OlympiA clinical trial sponsored by AstraZeneca, the manufacturer of olaparib. All other authors declare no competing financial or other interests.

Effects of Continuous In Utero Low- and Medium-Dose-Rate Gamma-Ray Exposure on Fetal Germ Cells

The effects of radiation exposure on germ cells and the gonads have been well studied at acute high-dose exposures, but the effects of chronic low-dose-rate (LDR) irradiation, particularly relevant for radiation protection, on germ cells and the gonads are largely unknown. Our previous study revealed that chronic exposure of mice to medium-dose-rate (MDR, 200 or 400 mGy/day) gamma-rays in utero for the entire gestation period (18 days) induced only a mild degree of general growth retardation, but with very drastic effects on the gonads and germ cells. In the current study, we further investigated the histomorphological changes in the gonads and the number of germ cells from gestation day (GD) 18 fetuses irradiated with MDR throughout the entire gestation period. The germ cells in the testes and ovaries of the MDR-irradiated fetuses were almost obliterated. Gestation day 18 fetuses exposed to LDR (20 mGy/day) radiation for the entire gestation period showed decreases in the number of the germ cells, which were not statistically significant or only marginally significant at most. Further investigations on the effects of LDR irradiation in utero using more sensitive methods are necessary.

DNA repair in primordial follicle oocytes following cisplatin treatment

PURPOSE

Genotoxic chemotherapy and radiotherapy can cause DNA double stranded breaks (DSBs) in primordial follicle (PMF) oocytes, which then undergo apoptosis. The development of effective new fertility preservation agents has been hampered, in part, by a limited understanding of DNA repair in PMF oocytes. This study investigated the induction of classical DSB repair pathways in the follicles of wild type (WT) and apoptosis-deficient Puma^-/- mice in response to DSBs caused by the chemotherapy agent cisplatin.

METHODS

Adult C57BL/6 WT and Puma^-/- mice were injected i.p. with saline or cisplatin (5 mg/kg); ovaries were harvested at 8 or 24 h. Follicles were counted, and H2A histone family member (γH2AX) immunofluorescence used to demonstrate DSBs. DNA repair protein RAD51 homolog 1 (RAD51) and DNA-dependent protein kinase, catalytic subunit (DNA-PKcs) immunofluorescence were used to identify DNA repair pathways utilised.

RESULTS

Puma^-/- mice retained 100% of follicles 24 h after cisplatin treatment. Eight hours post-treatment, γH2AX immunofluorescence showed DSBs across follicular stages in Puma^-/- mice; staining returned to control levels in PMFs within 5 days, suggesting repair of PMF oocytes in this window. RAD51 immunofluorescence eight hours post-cisplatin was positive in damaged cell types in both WT and Puma^-/- mice, demonstrating induction of the homologous recombination pathway. In contrast, DNA-PKcs staining were rarely observed in PMFs, indicating non-homologous end joining plays an insignificant role.

CONCLUSION

PMF oocytes are able to conduct high-fidelity repair of DNA damage accumulated during chemotherapy. Therefore, apoptosis inhibition presents a viable strategy for fertility preservation in women undergoing treatment.

Mtor inhibition by INK128 extends functions of the ovary reconstituted from germline stem cells in aging and premature aging mice

Stem cell transplantation has been generally considered as promising therapeutics in preserving or recovering functions of lost, damaged, or aging tissues. Transplantation of primordial germ cells (PGCs) or oogonia stem cells (OSCs) can reconstitute ovarian functions that yet sustain for only short period of time, limiting potential application of stem cells in preservation of fertility and endocrine function. Here, we show that mTOR inhibition by INK128 extends the follicular and endocrine functions of the reconstituted ovaries in aging and premature aging mice following transplantation of PGCs/OSCs. Follicular development and endocrine functions of the reconstituted ovaries by transplanting PGCs into kidney capsule of the recipient mice were maintained by INK128 treatment for more than 12 weeks, in contrast to the controls for only about 4 weeks without receiving the mTOR inhibitors. Comparatively, rapamycin also can prolong the ovarian functions but for limited time. Furthermore, our data reveal that INK128 promotes mitochondrial function in addition to its known function in suppression of immune response and inflammation. Taken together, germline stem cell transplantation in combination with mTOR inhibition by INK128 improves and extends the reconstituted ovarian and endocrine functions in reproductive aging and premature aging mice.

The different doses of radiation therapy-induced damage to the ovarian environment in rats

PURPOSE

The sequelae of premature loss of ovarian function can undoubtedly have undesirable effects for a woman although radiotherapy is one of the most relevant treatment modalities for various types of malignancies. The aim of this study was to determine the effect of different doses of radiation on ovarian folliculogenesis, inflammation, and apoptotic markers.

MATERIALS AND METHODS

For this purpose, 40 healthy Wistar albino female rats divided into four groups: 1) Control group; 2) those that were exposed to total body 1 Gy of gamma rays; 3) those that were exposed to the total body 5 Gy of gamma rays, and 4) those that were exposed to total body 10 Gy of gamma rays. External irradiation to the total body was given with gamma irradiation delivered by the Co60 teletherapy machine. The day after radiation application the rats were sacrificed and the ovaries were removed in all groups. Histopathologic examination, follicle counting, and classification were performed in the ovarian tissues. The expression of AMH, TNF-α, IL1-β, Bax, and Bcl-2 was detected. The stained sections were examined for caspase 3 positive apoptotic cell numbers.

RESULTS

The recorded results revealed that increased radiation dose induced obvious ovarian injuries that were indicated by histopathological, and immunohistochemical alterations, including elevation of ovarian injury markers. A significantly lower number of total and primordial follicles was detected with increasing radiation dose compared with the control group. According to our immunohistochemical results, 10 Gy of gamma rays group had the lowest AMH expression levels, while had the highest TNF-α, IL1-β expression level compared to the control group. When the groups were evaluated in terms of apoptosis, it was seen that the number of caspase 3 positive cells and Bax immunoreactivity intensity increased with radiation dose. In contrast, Bcl-2 immunoreactivity intensity decreased with increasing radiation dose compared with the control group.

CONCLUSIONS

We demonstrate here that dose rate plays an important role when estimating the relation between exposure to an increased dose of ionizing radiation and the risk of ovarian disease. According to these results, certain factors have to be optimized before introducing them into clinics.

Ageing and ovarian stimulation modulate the relative levels of transcript abundance of oocyte DNA repair genes during the germinal vesicle-metaphase II transition in mice

PURPOSE

Oocyte quality and reproductive outcome are negatively affected by advanced maternal age, ovarian stimulation and method of oocyte maturation during assisted reproduction; however, the mechanisms responsible for these associations are not fully understood. The aim of this study was to compare the effects of ageing, ovarian stimulation and in-vitro maturation on the relative levels of transcript abundance of genes associated with DNA repair during the transition of germinal vesicle (GV) to metaphase II (MII) stages of oocyte development.

METHODS

The relative levels of transcript abundance of 90 DNA repair-associated genes was compared in GV-stage and MII-stage oocytes from unstimulated and hormone-stimulated ovaries from young (5-8-week-old) and old (42-45-week-old) C57BL6 mice. Ovarian stimulation was conducted using pregnant mare serum gonadotropin (PMSG) or anti-inhibin serum (AIS). DNA damage response was quantified by immunolabeling of the phosphorylated histone variant H2AX (γH2AX).

RESULTS

The relative transcript abundance in DNA repair genes was significantly lower in MII oocytes compared to GV oocytes in young unstimulated and PMSG stimulated but was higher in AIS-stimulated mice. Interestingly, an increase in the relative level of transcript abundance of DNA repair genes was observed in MII oocytes from older mice in unstimulated, PMSG-stimulated and AIS-stimulated mice. Decreased γH2AX levels were found in both GV oocytes (82.9%) and MII oocytes (37.5%) during ageing in both ovarian stimulation types used (PMSG/AIS; p < 0.05).

CONCLUSIONS

In conclusion, DNA repair relative levels of transcript abundance are altered by maternal age and the method of ovarian stimulation during the GV-MII transition in oocytes.

Features of DNA Repair in the Early Stages of Mammalian Embryonic Development

Cell repair machinery is responsible for protecting the genome from endogenous and exogenous effects that induce DNA damage. Mutations that occur in somatic cells lead to dysfunction in certain tissues or organs, while a violation of genomic integrity during the embryonic period often leads to death. A mammalian embryo’s ability to respond to damaged DNA and repair it, as well as its sensitivity to specific lesions, is still not well understood. In this review, we combine disparate data on repair processes in the early stages of preimplantation development in mammalian embryos.

DNA damage and repair in the female germline: contributions to ART

BACKGROUND

DNA integrity and stability are critical determinants of cell viability. This is especially true in the female germline, wherein DNA integrity underpins successful conception, embryonic development, pregnancy and the production of healthy offspring. However, DNA is not inert; rather, it is subject to assault from various environment factors resulting in chemical modification and/or strand breakage. If structural alterations result and are left unrepaired, they have the potential to cause mutations and propagate disease. In this regard, reduced genetic integrity of the female germline ranks among the leading causes of subfertility in humans. With an estimated 10% of couples in developed countries taking recourse to ART to achieve pregnancy, the need for ongoing research into the capacity of the oocyte to detect DNA damage and thereafter initiate cell cycle arrest, apoptosis or DNA repair is increasingly more pressing.

OBJECTIVE AND RATIONALE

This review documents our current knowledge of the quality control mechanisms utilised by the female germline to prevent and remediate DNA damage during their development from primordial follicles through to the formation of preimplantation embryos.

SEARCH METHODS

The PubMed database was searched using the keywords: primordial follicle, primary follicle, secondary follicle, tertiary follicle, germinal vesical, MI, MII oocyte, zygote, preimplantation embryo, DNA repair, double-strand break and DNA damage. These keywords were combined with other phrases relevant to the topic. Literature was restricted to peer-reviewed original articles in the English language (published 1979-2018) and references within these articles were also searched.

OUTCOMES

In this review, we explore the quality control mechanisms utilised by the female germline to prevent, detect and remediate DNA damage. We follow the trajectory of development from the primordial follicle stage through to the preimplantation embryo, highlighting findings likely to have important implications for fertility management, age-related subfertility and premature ovarian failure. In addition, we survey the latest discoveries regarding DNA repair within the metaphase II (MII) oocyte and implicate maternal stores of endogenous DNA repair proteins and mRNA transcripts as a primary means by which they defend their genomic integrity. The collective evidence reviewed herein demonstrates that the MII oocyte can engage in the activation of major DNA damage repair pathway(s), therefore encouraging a reappraisal of the long-held paradigm that oocytes are largely refractory to DNA repair upon reaching this late stage of their development. It is also demonstrated that the zygote can exploit a number of protective strategies to mitigate the risk and/or effect the repair, of DNA damage sustained to either parental germline; affirming that DNA protection is largely a maternally driven trait but that some aspects of repair may rely on a collaborative effort between the male and female germlines.

WIDER IMPLICATIONS

The present review highlights the vulnerability of the oocyte to DNA damage and presents a number of opportunities for research to bolster the stringency of the oocyte's endogenous defences, with implications extending to improved diagnostics and novel therapeutic applications to alleviate the burden of infertility.

Oocytes can efficiently repair DNA double-strand breaks to restore genetic integrity and protect offspring health

Female fertility and offspring health are critically dependent on an adequate supply of high-quality oocytes, the majority of which are maintained in the ovaries in a unique state of meiotic prophase arrest. While mechanisms of DNA repair during meiotic recombination are well characterized, the same is not true for prophase-arrested oocytes. Here we show that prophase-arrested oocytes rapidly respond to γ-irradiation-induced DNA double-strand breaks by activating Ataxia Telangiectasia Mutated, phosphorylating histone H2AX, and localizing RAD51 to the sites of DNA damage. Despite mobilizing the DNA repair response, even very low levels of DNA damage result in the apoptosis of prophase-arrested oocytes. However, we show that, when apoptosis is inhibited, severe DNA damage is corrected via homologous recombination repair. The repair is sufficient to support fertility and maintain health and genetic fidelity in offspring. Thus, despite the preferential induction of apoptosis following exogenously induced genotoxic stress, prophase-arrested oocytes are highly capable of functionally efficient DNA repair. These data implicate DNA repair as a key quality control mechanism in the female germ line and a critical determinant of fertility and genetic integrity.

Efficiently accumulating germ-like stem cells from mouse postnatal ovary by in situ tissue culture

Although recent studies have revealed that germline stem cells (GSCs) exist in the mouse postnatal ovary, how to efficiently obtain GSCs for regenerating neo-oogenesis is still a technical challenge. Here, we report that using in situ tissue culture we can efficiently accumulate large amounts of proliferating germ-like cells from mouse postnatal ovaries. Usually, more than 10,000 germ-like cells can be derived from one ovary by this method, and over 20% of these cells can grow into germ-like cells with self-renewal, which thus can serve as a good cell pool to isolate GSCs by other cell assorting methods such as FACS. This method is simple and time-saving, which should be useful for in future studies on mouse GSCs.

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.

Mechanistic approach of the inhibitory effect of chrysin on inflammatory and apoptotic events implicated in radiation-induced premature ovarian failure: Emphasis on TGF-β/MAPKs signaling pathway

Radiotherapy is one of the most relevant treatment modalities for various types of malignancies. However, it causes premature ovarian failure (POF) and subsequent infertility in women of reproductive age; hence urging the development of effective radioprotective agents. Chrysin, a natural flavone, possesses several pharmacological activities owing to its antioxidant, anti-inflammatory and anti-apoptotic properties. Therefore, the aim of this study was to investigate the efficacy of chrysin in limiting γ-radiation-mediated POF and to elucidate the underlying molecular mechanisms. Immature female Sprague-Dawley rats were subjected to a single dose of γ-radiation (3.2 Gy) and/or treated with chrysin (50 mg/kg) once daily for two weeks before and three days post-irradiation. Chrysin prevented the radiation-induced ovarian dysfunction by restoring estradiol levels, preserving the normal ovarian histoarchitecture and combating the follicular loss. Eelectron microscopic analysis showed that the disruption of ultrastructure components due to radiation exposure was hampered by chrysin administration. Mechanistically, chrsyin was able to reduce the levels of the inflammatory markers NF-κB, TNF-α, iNOS and COX-2 in radiation-induced ovarian damage. Chrysin also exhibited potent anti-apoptotic effects against radiation-induced cell death by downregulating the expression of cytochrome c and caspase 3. Radiation obviously induced upregulation of TGF-β protein with subsequent phospholyration and hence activation of downstream mitogen-activated protein kinases (MAPKs); p38 and JNK. Notably, administration of chrysin successfully counteracted these effects. These findings revealed that chrysin may be beneficial in ameliorating radiation-induced POF, predominantly via downregulating TGF-β/MAPK signaling pathways leading subsequently to hindering inflammatory and apoptotic signal transduction pathways implicated in POF.

The capacity of oocytes for DNA repair

Female fertility and offspring health are critically dependent on the maintenance of an adequate supply of high-quality oocytes. Like somatic cells, oocytes are subject to a variety of different types of DNA damage arising from endogenous cellular processes and exposure to exogenous genotoxic stressors. While the repair of intentionally induced DNA double strand breaks in gametes during meiotic recombination is well characterised, less is known about the ability of oocytes to repair pathological DNA damage and the relative contribution of DNA repair to oocyte quality is not well defined. This review will discuss emerging data suggesting that oocytes are in fact capable of efficient DNA repair and that DNA repair may be an important mechanism for ensuring female fertility, as well as the transmission of high-quality genetic material to subsequent generations.

Loss of PUMA protects the ovarian reserve during DNA-damaging chemotherapy and preserves fertility

Female gametes are stored in the ovary in structures called primordial follicles, the supply of which is non-renewable. It is well established that DNA-damaging cancer treatments can deplete the ovarian reserve of primordial follicles, causing premature ovarian failure and infertility. The precise mechanisms underlying this chemotherapy-driven follicle loss are unclear, and this has limited the development of targeted ovarian-protective agents. To address this fundamental knowledge gap, we used gene deletion mouse models to examine the role of the DNA damage-induced pro-apoptotic protein, PUMA, and its transcriptional activator TAp63, in primordial follicle depletion caused by treatment with cyclophosphamide or cisplatin. Cyclophosphamide caused almost complete destruction of the primordial follicle pool in adult wild-type (WT) mice, and a significant destructive effect was also observed for cisplatin. In striking contrast, Puma-/- mice retained 100% of their primordial follicles following either genotoxic treatment. Furthermore, elimination of PUMA alone completely preserved fertility in cyclophosphamide-treated mice, indicating that oocytes rescued from DNA damage-induced death can repair themselves sufficiently to support reproductive function and offspring health. Primordial follicles were also protected in TAp63-/- mice following cisplatin treatment, but not cyclophosphamide, suggesting mechanistic differences in the induction of apoptosis and depletion of the ovarian reserve in response to these different chemotherapies. These studies identify PUMA as a crucial effector of apoptosis responsible for depletion of primordial follicles following exposure to cyclophosphamide or cisplatin, and this indicates that inhibition of PUMA may be an effective ovarian-protective strategy during cancer treatment in women.

Radiation-induced ovarian follicle loss occurs without overt stromal changes

Radiation damage due to total body irradiation (TBI) or targeted abdominal radiation can deplete ovarian follicles and accelerate reproductive aging. We characterized a mouse model of low-dose TBI to investigate how radiation affects the follicular and stromal compartments of the ovary. A single TBI dose of either 0.1 Gy or 1 Gy (Cesium-137 γ) was delivered to reproductively adult CD1 female mice, and sham-treated mice served as controls. Mice were euthanized either 2 weeks or 5 weeks post exposure, and ovarian tissue was harvested. To assess the ovarian reserve, we classified and counted the number of morphologically normal follicles in ovarian histologic sections for all experimental cohorts using an objective method based on immunohistochemistry for an oocyte-specific protein (MSY2). 0.1 Gy did not affect that total number of ovarian follicles, whereas 1 Gy resulted in a dramatic loss. At two weeks, there was a significant reduction in all preantral follicles, but early antral and antral follicles were still present. By five weeks, there was complete depletion of all follicle classes. We examined stromal quality using histologic stains to visualize ovarian architecture and fibrosis and by immunohistochemistry and quantitative microscopy to assess cell proliferation, cell death and vasculature. There were no differences in the ovarian stroma across cohorts with respect to these markers, indicating that this compartment is more radio-resistant relative to the germ cells. These findings have implications for reproductive health and the field of fertility preservation because the radiation doses we examined mimic scatter doses experienced in typical therapeutic regimens.

The importance of DNA repair for maintaining oocyte quality in response to anti-cancer treatments, environmental toxins and maternal ageing

BACKGROUND

Within the ovary, oocytes are stored in long-lived structures called primordial follicles, each comprising a meiotically arrested oocyte, surrounded by somatic granulosa cells. It is essential that their genetic integrity is maintained throughout life to ensure that high quality oocytes are available for ovulation. Of all the possible types of DNA damage, DNA double-strand breaks (DSBs) are considered to be the most severe. Recent studies have shown that DNA DSBs can accumulate in oocytes in primordial follicles during reproductive ageing, and are readily induced by exogenous factors such as γ-irradiation, chemotherapy and environmental toxicants. DSBs can induce oocyte death or, alternatively, activate a program of DNA repair in order to restore genetic integrity and promote survival. The repair of DSBs has been intensively studied in the context of meiotic recombination, and in recent years more detail is becoming available regarding the repair capabilities of primordial follicle oocytes.

OBJECTIVE AND RATIONALE

This review discusses the induction and repair of DNA DSBs in primordial follicle oocytes.

SEARCH METHODS

PubMed (Medline) and Google Scholar searches were performed using the key words: primordial follicle oocyte, DNA repair, double-strand break, DNA damage, chemotherapy, radiotherapy, ageing, environmental toxicant. The literature was restricted to papers in the English language and limited to reports in animals and humans dated from 1964 until 2017. The references within these articles were also manually searched.

OUTCOMES

Recent experiments in animal models and humans have provided compelling evidence that primordial follicle oocytes can efficiently repair DNA DSBs arising from diverse origins, but this capacity may decline with increasing age.

WIDER IMPLICATIONS

Primordial follicle oocytes are vulnerable to DNA DSBs emanating from endogenous and exogenous sources. The ability to repair this damage is essential for female fertility. In the long term, augmenting DNA repair in primordial follicle oocytes has implications for the development of novel fertility preservation agents for female cancer patients and for the management of maternal ageing. However, further work is required to fully characterize the specific proteins involved and to develop strategies to bolster their activity.

Bone Marrow Stem Cells Do Not Contribute to Endometrial Cell Lineages in Chimeric Mouse Models

Studies from five independent laboratories conclude that bone marrow stem cells transdifferentiate into endometrial stroma, epithelium, and endothelium. We investigated the nature of bone marrow-derived cells in the mouse endometrium by reconstituting irradiated wild type recipients with bone marrow containing transgenic mTert-green fluorescent protein (GFP) or chicken β-actin (Ch β-actin)-GFP reporters. mTert-GFP is a telomerase marker identifying hematopoietic stem cells and subpopulations of epithelial, endothelial, and immune cells in the endometrium. Ch β-actin-GFP is a ubiquitous reporter previously used to identify bone marrow-derived cells in the endometrium. Confocal fluorescence microscopy for GFP and markers of endometrial and immune cells were used to characterize bone marrow-derived cells in the endometrium of transplant recipients. No evidence of GFP⁺ bone marrow-derived stroma, epithelium, or endothelium was observed in the endometrium of mTert-GFP or Ch β-actin-GFP recipients. All GFP⁺ cells detected in the endometrium were immune cells expressing the pan leukocyte marker CD45, including CD3⁺ T cells and F4/80⁺ macrophages. Further examination of the Ch β-actin-GFP transplant model revealed that bone marrow-derived F4/80⁺ macrophages immunostained weakly for CD45. These macrophages were abundant in the stroma, infiltrated the epithelial and vascular compartments, and could easily be mistaken for bone marrow-derived endometrial cells. We conclude that it is unlikely that bone marrow cells are able to transdifferentiate into endometrial stroma, epithelium, and endothelium. This result has important therapeutic implications, as the expectation that bone marrow stem cells contribute directly to endometrial regeneration is shaping strategies designed to regenerate endometrium in Asherman's syndrome and to control aberrant endometrial growth in endometriosis. Stem Cells 2018;36:91-102.

Neo-oogenesis in mammals

Recently, the existence of a mechanism for neo-oogenesis in the ovaries of adult mammals has generated much controversy within reproductive biology. This mechanism, which proposes that the ovary has cells capable of renewing the follicular reserve, has been described for various species of mammals. The first evidence was found in prosimians and humans. However, these findings were not considered relevant because the predominant dogma for reproductive biology at the time was that of Zuckerman. This dogma states that female mammals are born with finite numbers of oocytes that decline throughout postnatal life. Currently, the concept of neo-oogenesis has gained momentum due to the discovery of cells with mitotic activity in adult ovaries of various mammalian species (mice, humans, rhesus monkeys, domestic animals such as pigs, and wild animals such as bats). Despite these reports, the concept of neo-oogenesis has not been widely accepted by the scientific community, generating much criticism and speculation about its accuracy because it has been impossible to reproduce some evidence. This controversy has led to the creation of two positions: one in favour of neo-oogenesis and the other against it. Various animal models have been used in support of both camps, including both classic laboratory animals and domestic and wild animals. The aim of this review is to critically present the current literature on the subject and to evaluate the arguments pro and contra neo-oogenesis in mammals.

Oocyte stem cells: fact or fantasy?

For many decades, the dogma prevailed that female mammals had a finite pool of oocytes at birth and this was gradually exhausted during a lifetime of reproductive function. However, in 2004, a new era began in the field of female oogenesis. A study was published that appeared to detect oocyte-stem cells capable of generating new eggs within mouse ovaries. This study was highly controversial and the years since this initial finding have produced extensive research and even more extensive debate into their possibility. Unequivocal evidence testifying to the existence of oocyte-stem cells (OSCs) has yet to be produced, meanwhile the spectrum of views from both sides of the debate are wide-ranging and surprisingly passionate. Although recent studies have presented some convincing results that germ cells exist and are capable of creating new oocytes, many questions remain. Are these cells present in humans? Do they exist in physiological conditions in a dormant state? This comprehensive review first examines where and how the dogma of a finite pool was established, how this has been challenged over the years and addresses the most pertinent questions as to the current status of their existence, their role in female fertility, and perhaps most importantly, if they do exist, how can we harness these cells to improve a woman's oocyte reserve and treat conditions such as premature ovarian insufficiency (POI: also known as premature ovarian failure, POF).

DNA damage responses in mammalian oocytes

DNA damage acquired during meiosis can lead to infertility and miscarriage. Hence, it should be important for an oocyte to be able to detect and respond to such events in order to make a healthy egg. Here, the strategies taken by oocytes during their stages of growth to respond to DNA damaging events are reviewed. In particular, recent evidence of a novel pathway in fully grown oocytes helps prevent the formation of mature eggs with DNA damage. It has been found that fully grown germinal vesicle stage oocytes that have been DNA damaged do not arrest at this point in meiosis, but instead undergo meiotic resumption and stall during the first meiotic division. The Spindle Assembly Checkpoint, which is a well-known mitotic pathway employed by somatic cells to monitor chromosome attachment to spindle microtubules, appears to be utilised by oocytes also to respond to DNA damage. As such maturing oocytes are arrested at metaphase I due to an active Spindle Assembly Checkpoint. This is surprising given this checkpoint has been previously studied in oocytes and considered to be weak and ineffectual because of its poor ability to be activated in response to microtubule attachment errors. Therefore, the involvement of the Spindle Assembly Checkpoint in DNA damage responses of mature oocytes during meiosis I uncovers a novel second function for this ubiquitous cellular checkpoint.

Primordial Follicle Transplantation within Designer Biomaterial Grafts Produce Live Births in a Mouse Infertility Model

The gonadotoxic effects of chemotherapy and radiation may result in premature ovarian failure in premenopausal oncology patients. Although autotransplantation of ovarian tissue has led to successful live births, reintroduction of latent malignant cells inducing relapse is a significant concern. In this report, we investigated the design of biomaterial grafts for transplantation of isolated ovarian follicles as a means to preserve fertility. Primordial and primary ovarian follicles from young female mice were extracted and encapsulated into biomaterials for subsequent transplantation into adult mice. Among the formulations tested, aggregated follicles encapsulated within fibrin had enhanced survival and integration with the host tissue following transplantation relative to the fibrin-alginate and fibrin-collagen composites. All mice transplanted with fibrin-encapsulated follicles resumed cycling, and live births were achieved only for follicles transplanted within VEGF-loaded fibrin beads. The extent to which these procedures reduce the presence of metastatic breast cancer cells among the isolated follicles was evaluated, with significantly reduced numbers of cancer cells present relative to intact ovaries. This ability to obtain live births by transplanting isolated primordial and primary follicles, while also reducing the risk of re-seeding disease relative to ovarian tissue transplantation, may ultimately provide a means to preserve fertility in premenopausal oncology patients.

The developmental origins of the mammalian ovarian reserve

The adult mammalian ovary is devoid of definitive germline stem cells. As such, female reproductive senescence largely results from the depletion of a finite ovarian follicle pool that is produced during embryonic development. Remarkably, the crucial nature and regulation of follicle assembly and survival during embryogenesis is just coming into focus. This developmental pathway involves the coordination of meiotic progression and the breakdown of germ cell cysts into individual oocytes housed within primordial follicles. Recent evidence also indicates that genetic and environmental factors can specifically perturb primordial follicle assembly. Here, we review the cellular and molecular mechanisms by which the mammalian ovarian reserve is established, highlighting the presence of a crucial checkpoint that allows survival of only the highest-quality oocytes.

Ovary and fimbrial stem cells: biology, niche and cancer origins

The mammalian ovary is covered by a single-layered epithelium that undergoes rupture and remodelling following each ovulation. Although resident stem cells are presumed to be crucial for this cyclic regeneration, their identity and mode of action have been elusive. Surrogate stemness assays and in vivo fate-mapping studies using recently discovered stem cell markers have identified stem cell pools in the ovary and fimbria that ensure epithelial homeostasis. Recent findings provide insights into intrinsic mechanisms and local extrinsic cues that govern the function of ovarian and fimbrial stem cells. These discoveries have advanced our understanding of stem cell biology in the ovary and fimbria, and lay the foundations for evaluating the contribution of resident stem cells to the initiation and progression of human epithelial ovarian cancer.

Use of an organotypic mammalian in vitro follicle growth assay to facilitate female reproductive toxicity screening

Screening of pharmaceutical, chemical and environmental compounds for their effects on reproductive health relies on in vivo studies. More robust and efficient methods to assess these effects are needed. Herein we adapted and validated an organotypic in vitro follicle growth (IVFG) assay to determine the impact of compounds on markers of ovarian function. We isolated mammalian follicles and cultured them in the presence of compounds with: (1) known fertotoxicity (i.e. toxicity to the reproductive system; cyclophosphamide and cisplatin); (2) no known fertotoxicity (nalbuphine); and (3) unknown fertotoxicity (Corexit EC 9500 A; CE, Nalco, Chicago, IL, USA). For each compound, we assayed follicle growth, hormone production and the ability of follicle-enclosed oocytes to resume meiosis and produce a mature egg. Cyclophosphamide and cisplatin caused dose-dependent disruption of follicle dynamics, whereas nalbuphine did not. The reproductive toxicity of CE, an oil dispersant used heavily during the 2010 Deepwater Horizon oil spill, has never been examined in a mammalian system. In the present study, CE compromised follicle morphology and functional parameters. Our findings demonstrate that this IVFG assay system can be used to distinguish fertotoxic from non-toxic compounds, providing an in vitro tool to assess the effects of chemical compounds on reproductive function and health.

Early-life nutritional effects on the female reproductive system

There is now considerable epidemiological and experimental evidence indicating that early-life environmental conditions, including nutrition, affect subsequent development in later life. These conditions induce highly integrated responses in endocrine-related homeostasis, resulting in persistent changes in the developmental trajectory producing an altered adult phenotype. Early-life events trigger processes that prepare the individual for particular circumstances that are anticipated in the postnatal environment. However, where the intrauterine and postnatal environments differ markedly, such modifications to the developmental trajectory may prove maladaptive in later life. Reproductive maturation and function are similarly influenced by early-life events. This should not be surprising, because the primordial follicle pool is established early in life and is thus vulnerable to early-life events. Results of clinical and experimental studies have indicated that early-life adversity is associated with a decline in ovarian follicular reserve, changes in ovulation rates, and altered age at onset of puberty. However, the underlying mechanisms regulating the relationship between the early-life developmental environment and postnatal reproductive development and function are unclear. This review examines the evidence linking early-life nutrition and effects on the female reproductive system, bringing together clinical observations in humans and experimental data from targeted animal models.

Cancer etiology. Variation in cancer risk among tissues can be explained by the number of stem cell divisions

Some tissue types give rise to human cancers millions of times more often than other tissue types. Although this has been recognized for more than a century, it has never been explained. Here, we show that the lifetime risk of cancers of many different types is strongly correlated (0.81) with the total number of divisions of the normal self-renewing cells maintaining that tissue's homeostasis. These results suggest that only a third of the variation in cancer risk among tissues is attributable to environmental factors or inherited predispositions. The majority is due to "bad luck," that is, random mutations arising during DNA replication in normal, noncancerous stem cells. This is important not only for understanding the disease but also for designing strategies to limit the mortality it causes.

Control of hair cell development by molecular pathways involving Atoh1, Hes1 and Hes5

Atoh1, Hes1 and Hes5 are crucial for normal inner ear hair cell development. They regulate the expression of each other in a complex network, while they also interact with many other genes and pathways, such as Notch, FGF, SHH, WNT, BMP and RA. This paper summarized molecular pathways that involve Atoh1, Hes1, and Hes5. Some of the pathways and gene regulation mechanisms discussed here were studied in other tissues, yet they might inspire studies in inner ear hair cell development. Thereby, we presented a complex regulatory network involving these three genes, which might be crucial for proliferation and differentiation of inner ear hair cells.

Life-long in vivo cell-lineage tracing shows that no oogenesis originates from putative germline stem cells in adult mice

Whether or not oocyte regeneration occurs in adult life has been the subject of much debate. In this study, we have traced germ-cell lineages over the life spans of three genetically modified mouse models and provide direct evidence that oogenesis does not originate from any germline stem cells (GSCs) in adult mice. By selective ablation of all existing oocytes in a Gdf9-Cre;iDTR mouse model, we have demonstrated that no new germ cells were ever regenerated under pathological conditions. By in vivo tracing of oocytes and follicles in the Sohlh1-CreER(T2);R26R and Foxl2-CreER(T2);mT/mG mouse models, respectively, we have shown that the initial pool of oocytes is the only source of germ cells throughout the life span of the mice and that no adult oogenesis ever occurs under physiological conditions. Our findings clearly show that there are no GSCs that contribute to adult oogenesis in mice and that the initial pool of oocytes formed in early life is the only source of germ cells throughout the entire reproductive life span.

The role of BH3-only proteins in apoptosis within the ovary

BH3-only proteins are pro-apoptotic members of the BCL2 family that play pivotal roles in embryonic development, tissue homeostasis and immunity by triggering cell death through the intrinsic apoptosis pathway. Recent in vitro and in vivo studies have demonstrated that BH3-only proteins are also essential mediators of apoptosis within the ovary and are responsible for the initiation of the cell death signalling cascade in a cell type and stimulus-specific fashion. This review gives a brief overview of the intrinsic apoptosis pathway and summarise the roles of individual BH3-only proteins in the promotion of apoptosis in embryonic germ cells, oocytes, follicular granulosa cells and luteal cells. The role of these proteins in activating apoptosis in response to developmental cues and cell stressors, such as exposure to chemotherapy, radiation and environmental toxicants, is described. Studies on the function of BH3-only proteins in the ovary are providing valuable insights into the regulation of oocyte number and quality, as well as ovarian endocrine function, which collectively influence the female reproductive lifespan and health.

Bortezomib prevents acute doxorubicin ovarian insult and follicle demise, improving the fertility window and pup birth weight in mice

Increasing numbers of female patients survive cancer, but succumb to primary ovarian insufficiency after chemotherapy. We tested the hypothesis that Bortezomib (Bort) protects ovaries from doxorubicin (DXR) chemotherapy by treating female mice with Bort 1 hour prior to DXR. By preventing DXR accumulation in the ovary, Bort attenuated DXR-induced DNA damage in all ovarian cell types, subsequent γH2AFX phosphorylation, and resulting apoptosis in preantral follicles. Bort pretreatment extended the number of litters per mouse, improved litter size and increased pup weight following DXR treatment, thus increasing the duration of post-chemotherapy fertility and improving pup health. As a promising prophylactic ovoprotective agent, Bort does not interfere with cancer treatment, and is currently used as a chemotherapy adjuvant. Bort-based chemoprotection may preserve ovarian function in a non-invasive manner that avoids surgical ovarian preservation, thus diminishing the health complications of premature menopause following cancer treatment.

No evidence for neo-oogenesis may link to ovarian senescence in adult monkey

Female germline or oogonial stem cells transiently residing in fetal ovaries are analogous to the spermatogonial stem cells or germline stem cells (GSCs) in adult testes where GSCs and meiosis continuously renew. Oocytes can be generated in vitro from embryonic stem cells and induced pluripotent stem cells, but the existence of GSCs and neo-oogenesis in adult mammalian ovaries is less clear. Preliminary findings of GSCs and neo-oogenesis in mice and humans have not been consistently reproducible. Monkeys provide the most relevant model of human ovarian biology. We searched for GSCs and neo-meiosis in ovaries of adult monkeys at various ages, and compared them with GSCs from adult monkey testis, which are characterized by cytoplasmic staining for the germ cell marker DAZL and nuclear expression of the proliferative markers PCNA and KI67, and pluripotency-associated genes LIN28 and SOX2, and lack of nuclear LAMIN A, a marker for cell differentiation. Early meiocytes undergo homologous pairing at prophase I distinguished by synaptonemal complex lateral filaments with telomere perinuclear distribution. By exhaustive searching using comprehensive experimental approaches, we show that proliferative GSCs and neo-meiocytes by these specific criteria were undetectable in adult mouse and monkey ovaries. However, we found proliferative nongermline somatic stem cells that do not express LAMIN A and germ cell markers in the adult ovaries, notably in the cortex and granulosa cells of growing follicles. These data support the paradigm that adult ovaries do not undergo germ cell renewal, which may contribute significantly to ovarian senescence that occurs with age.

Rapamycin-sensitive mTORC1 signaling is involved in physiological primordial follicle activation in mouse ovary

In mammals, resting female oocytes reside in primordial ovarian follicles. An individual primordial follicle may stay quiescent for a protracted period of time before initiating follicular growth, which is also termed “activation.” Female reproductive capacity is sustained by the gradual, streamlined activation of the entire population of primordial follicles, but this process also results in reproductive senescence in older animals. Based on the recent findings that genetically triggered, excessive mammalian target of rapamycin complex 1 (mTORC1) activation in mouse oocytes leads to accelerated primordial follicle activation, we examined the necessity of mTORC1 signaling in physiological primordial follicle activation. We found that induction of oocyte mTORC1 activity is associated with early follicular growth in neonatal mouse ovaries. Pharmacological inhibition of mTORC1 activity in vivo by rapamycin treatment leads to a marked, but partial, suppression of primordial follicle activation. The suppressive effect of rapamycin on primordial follicle activation was reproduced in cultured ovaries. While rapamycin did not apparently affect several plausible cellular targets in neonatal mouse ovaries, such as mTORC2, AKT, or cyclin-dependent kinase (CDK) inhibitor p27-KIP1, its inhibitory effect on Cyclin A2 gene expression implies that mTORC1 signaling in oocytes may engage a Cyclin A/CDK regulatory network that promotes primordial follicle activation. The current work strengthens the concept that mTORC1-dependent events in the oocytes of primordial follicles may represent potential targets for intervention in humans to slow the depletion of the ovarian reserve.