Caspase 9 is constitutively activated in mouse oocytes and plays a key role in oocyte elimination during meiotic prophase progression

Sustained fertility from first-wave follicle oocytes that pause their growth

Ovulation results from the cyclical recruitment of non-renewing, quiescent oocytes for growth. Therefore, the primordial follicles that are established during development from an oocyte encapsulated by granulosa cells are thought to comprise the lifelong ovarian reserve 1-4. However, using oocyte lineage tracing in mice, we observed that a subset of oocytes recruited for growth in the first juvenile wave remain paused for many months before continuing growth, ovulation, fertilization and development into healthy offspring. This small subset of genetically-labeled fetal oocytes, labeled with Sycp3-CreERT2, is distinguished by earlier entry and slower dynamics of meiotic prophase I. While labeled oocytes were initially found in both primordial follicles and growing follicles of the first wave, they disappeared from primordial follicles by puberty. Unexpectedly, these first-wave labeled growing oocytes persisted throughout reproductive lifespan and contributed to offspring at a steady rate beyond 12 months of age, suggesting that follicles can pause mid-growth for extended periods then successfully resume. These results challenge the conclusion from lineage tracing of granulosa cells that first-wave follicles make a limited contribution to fertility5 and furthermore suggest that growth-paused oocytes comprise a second and previously unrecognized ovarian reserve.

Exploring caspase functions in mouse models

Caspases are enzymes with protease activity. Despite being known for more than three decades, caspase investigation still yields surprising and fascinating information. Initially associated with cell death and inflammation, their functions have gradually been revealed to extend beyond, targeting pathways such as cell proliferation, migration, and differentiation. These processes are also associated with disease mechanisms, positioning caspases as potential targets for numerous pathologies including inflammatory, neurological, metabolic, or oncological conditions. While in vitro studies play a crucial role in elucidating molecular pathways, they lack the context of the body's complexity. Therefore, laboratory animals are an indispensable part of successfully understanding and applying caspase networks. This paper aims to summarize and discuss recent knowledge, understanding, and challenges in caspase knock-out mice.

Heterozygous Eif4nif1 Stop Gain Mice Replicate the Primary Ovarian Insufficiency Phenotype in Women

We created the c.1286C>G stop-gain mutation found in a family with primary ovarian insufficiency (POI) at age 30 years. The Eif4enif1 C57/Bl6 transgenic mouse model contained a floxed exon 10-19 cassette with a conditional knock-in cassette containing the c.1286C>G stop-gain mutation in exon 10. The hybrid offspring of CMV- Cre mice with Eif4enif1 WT/flx mice were designated Eif4enif1 WT/ Δ for simplicity. A subset of female heterozygotes ( Eif4enif1 WT/ Δ ) had no litters. In those with litters, the final litter was earlier (5.4±2.6 vs. 10.5±0.7 months; p=0.02). Heterozygous breeding pair ( Eif4enif1 WT/ Δ x Eif4enif1 WT/ Δ ) litter size was 60% of WT litter size (3.9±2.0 vs. 6.5±3.0 pups/litter; p <0.001). The genotypes were 35% Eif4enif1 WT/flx and 65% Eif4enif1 WT/ Δ , with no homozygotes. Homozygote embryos did not develop beyond the 4-8 cell stage. The number of follicles in ovaries from Eif4enif1 WT/ Δ mice was lower starting at the primordial (499±290 vs. 1445±381) and primary follicle stage (1069±346 vs. 1450±193) on day 10 (p<0.05). The preantral follicle number was lower starting on day 21 (213±86 vs. 522±227; p<0.01). Examination of ribosome protected mRNAs (RPR) demonstrated altered mRNA expression. The Eif4enif1 stop-gain mice replicate the POI phenotype in women. The unique mouse model provides a platform to study regulation of protein translation across oocyte and embryo development in mammals.

A matter of new life and cell death: programmed cell death in the mammalian ovary

BACKGROUND

The mammalian ovary is a unique organ that displays a distinctive feature of cyclic changes throughout the entire reproductive period. The estrous/menstrual cycles are associated with drastic functional and morphological rearrangements of ovarian tissue, including follicular development and degeneration, and the formation and subsequent atrophy of the corpus luteum. The flawless execution of these reiterative processes is impossible without the involvement of programmed cell death (PCD).

MAIN TEXT

PCD is crucial for efficient and careful clearance of excessive, depleted, or obsolete ovarian structures for ovarian cycling. Moreover, PCD facilitates selection of high-quality oocytes and formation of the ovarian reserve during embryonic and juvenile development. Disruption of PCD regulation can heavily impact the ovarian functions and is associated with various pathologies, from a moderate decrease in fertility to severe hormonal disturbance, complete loss of reproductive function, and tumorigenesis. This comprehensive review aims to provide updated information on the role of PCD in various processes occurring in normal and pathologic ovaries. Three major events of PCD in the ovary-progenitor germ cell depletion, follicular atresia, and corpus luteum degradation-are described, alongside the detailed information on molecular regulation of these processes, highlighting the contribution of apoptosis, autophagy, necroptosis, and ferroptosis. Ultimately, the current knowledge of PCD aberrations associated with pathologies, such as polycystic ovarian syndrome, premature ovarian insufficiency, and tumors of ovarian origin, is outlined.

CONCLUSION

PCD is an essential element in ovarian development, functions and pathologies. A thorough understanding of molecular mechanisms regulating PCD events is required for future advances in the diagnosis and management of various disorders of the ovary and the female reproductive system in general.

Regulation of Oocyte Apoptosis: A View from Gene Knockout Mice

Apoptosis is a form of programmed cell death that plays a critical role in cellular homeostasis and development, including in the ovarian reserve. In humans, hundreds of thousands of oocytes are produced in the fetal ovary. However, the majority die by apoptosis before birth. After puberty, primordial follicles develop into mature follicles. While only a large dominant follicle is selected to ovulate, smaller ones undergo apoptosis. Despite numerous studies, the mechanism of oocyte death at the molecular level remains elusive. Over the last two and a half decades, many knockout mouse models disrupting key genes in the apoptosis pathway have been generated. In this review, we highlight some of the phenotypes and discuss distinct and overlapping roles of the apoptosis regulators in oocyte death and survival. We also review how the transcription factor p63 and its family members may trigger oocyte apoptosis in response to DNA damage.

Reduced mitochondrial size in hippocampus and psychiatric behavioral changes in the mutant mice with homologous mutation of Timm8a1-I23fs49X

Background

Deafness-dystonia-optic neuronopathy (DDON) syndrome, a condition that predominantly affects males, is caused by mutations in translocase of mitochondrial inner membrane 8A (TIMM8A)/deafness dystonia protein 1 (DDP1) gene and characterized by progressive deafness coupled with other neurological abnormalities. In a previous study, we demonstrated the phenotype of male mice carrying the hemizygous mutation of Timm8a1-I23fs49X. In a follow-up to that study, this study aimed to observe the behavioral changes in the female mutant (MUT) mice with homologous mutation of Timm8a1 and to elucidate the underlying mechanism for the behavioral changes.

Materials and methods

Histological analysis, transmission electron microscopy (EM), Western blotting, hearing measurement by auditory brainstem response (ABR), and behavioral observation were compared between the MUT mice and wild-type (WT) littermates.

Results

The weight of the female MUT mice was less than that of the WT mice. Among MUT mice, both male and female mice showed hearing impairment, anxiety-like behavior by the elevated plus maze test, and cognitive deficit by the Morris water maze test. Furthermore, the female MUT mice exhibited coordination problems in the balance beam test. Although the general neuronal loss was not found in the hippocampus of the MUT genotype, EM assessment indicated that the mitochondrial size showing as aspect ratio and form factor in the hippocampus of the MUT strain was significantly reduced compared to that in the WT genotype. More importantly, this phenomenon was correlated with the upregulation of translation of mitochondrial fission process protein 1(Mtfp1)/mitochondrial 18 kDa protein (Mtp18), a key fission factor that is a positive regulator of mitochondrial fission and mitochondrial size. Interestingly, significant reductions in the size of the uterus and ovaries were noted in the female MUT mice, which contributed to significantly lower fertility in the MUT mice.

Conclusion

Together, a homologous mutation in the Timm8a1 gene caused the hearing impairment and psychiatric behavioral changes in the MUT mice; the latter phenotype might be related to a reduction in mitochondrial size regulated by MTP18.

The effects and mechanism of taxanes on chemotherapy-associated ovarian damage: A review of current evidence

Chemotherapy is often a cause of premature ovarian insufficiency and infertility since the ovarian follicles are extremely sensitive to the effects of chemotherapeutic agents. Different chemotherapeutic agents with varying mechanisms of action may damage ovarian function differently. Taxanes are widely used in clinical cancer treatment, but the specific reproductive toxicological information is still controversial. This review described the impact and duration of taxanes on ovarian function in women and analyzed the possible reasons for different conclusions. Furthermore, the toxicity of taxanes on ovarian function and its possible mechanisms were discussed. The potential protective strategies and agents against ovarian damage induced by taxanes are also reviewed.

The impact of oocyte death on mouse primordial follicle formation and ovarian reserve

Background

Ovaries, the source of oocytes, maintain the numbers of primordial follicles, develop oocytes for fertilization and embryonic development. Although it is well known that about two-thirds of oocytes are lost during the formation of primordial follicles through cyst fragmentation and the aggregation of oocytes within the cyst, the mechanism responsible for this remains unclear.

Methods

We provide an overview of cell death that is associated with the oocyte cyst breakdown and primordial follicle assembly along with our recent findings for mice that had been treated with a TNFα ligand inhibitor.

Main Findings

It is generally accepted that apoptosis is the major mechanism responsible for the depletion of germ cells. In fact, a gene deficiency or the overexpression of apoptosis regulators can have a great effect on follicle numbers and/or fertility. Apoptosis, however, may not be the only cause of the large-scale oocyte attrition during oocyte cyst breakdown, and other mechanisms, such as aggregation, may also be involved in this process.

Conclusion

The continued study of oocyte death during primordial follicle formation could lead to the development of novel strategies for manipulating the primordial follicle pool, leading to improved fertility by enhancing the ovarian reserve.

Bidirectional communication in oogenesis: a dynamic conversation in mice and Drosophila

In most animals, the oocyte is the largest cell by volume. The oocyte undergoes a period of large-scale growth during its development, prior to fertilization. At first glance, tissues that support the development of the oocyte in different organisms have diverse cellular characteristics that would seem to prohibit functional comparisons. However, these tissues often act with a common goal of establishing dynamic forms of two-way communication with the oocyte. We propose that this bidirectional communication between oocytes and support cells is a universal phenomenon that can be directly compared across species. Specifically, we highlight fruit fly and mouse oogenesis to demonstrate that similarities and differences in these systems should be used to inform and design future experiments in both models.

PI3K/PTEN/AKT Signaling Pathways in Germ Cell Development and Their Involvement in Germ Cell Tumors and Ovarian Dysfunctions

Several studies indicate that the PI3K/PTEN/AKT signaling pathways are critical regulators of ovarian function including the formation of the germ cell precursors, termed primordial germ cells, and the follicular pool maintenance. This article reviews the current state of knowledge of the functional role of the PI3K/PTEN/AKT pathways during primordial germ cell development and the dynamics of the ovarian primordial follicle reserve and how dysregulation of these signaling pathways may contribute to the development of some types of germ cell tumors and ovarian dysfunctions.

The brominated flame retardant, TBCO, impairs oocyte maturation in zebrafish (Danio rerio)

The brominated flame retardant, 1,2,5,6-tetrabromocyclooctane (TBCO), has been shown to decrease fecundity in Japanese medaka (Oryzias latipes) and there is indirect evidence from analysis of the transcriptome and proteome that this effect might be due to impaired oogenesis. An assay for disruption of oocyte maturation by chemical stressors has not been developed in Japanese medaka. Thus, using zebrafish (Danio rerio) as a model, objectives of the present study were to determine whether exposure to TBCO has effects on maturation of oocytes and to investigate potential mechanisms. Sexually mature female zebrafish were given a diet of 35.3 or 628.8 μg TBCO / g food for 14 days after which, stage IV oocytes were isolated to assess maturation in response to maturation inducing hormone. To explore potential molecular mechanisms, abundances of mRNAs of a suite of genes that regulate oocyte maturation were quantified by use of quantitative real-time PCR, and abundances of microRNAs were determined by use of miRNAseq. Ex vivo maturation of oocytes from fish exposed to TBCO was significantly less than maturation of oocytes from control fish. The percentage of oocytes which matured from control fish and those exposed to low and high TBCO were 89, 71, and 67%, respectively. Among the suite of genes known to regulate oocyte maturation, mRNA abundance of insulin like growth factor-3 was decreased by 1.64- and 3.44-fold in stage IV oocytes from females given the low and high concentrations of TBCO, respectively, compared to the control group. Abundances of microRNAs regulating the expression of proteins that regulate oocyte maturation, including processes related to insulin-like growth factor, were significantly different in stage IV oocytes from fish exposed to TBCO. Overall, results of this study indicated that impaired oocyte maturation might be a mechanism of reduced reproductive performance in TBCO-exposed fish. Results also suggested that effects of TBCO on oocyte maturation might be due to molecular perturbations on insulin-like growth factor signaling and expression of microRNAs.

Dynamics of apoptosis-related gene expression during follicular development in yak

The potential reproduction power of domestic animals is limited by a complicated follicular atresia process. P53, caspase-9 (Casp9), Bax, Bcl-2 and Fas play a crucial role in the ovarian mitochondrion-dependent apoptosis and death receptor pathway. In accordance with this study, the expression levels of Casp9, Bax, Bcl-2 and Fas were analysed in ovaries and oviducts of yak by immunohistochemistry (IHC). P53 and the above in ovarian granulosa cells (GCs) from atretic (3-6 mm) to healthy follicles (6-8 mm) and in oviducts were examined from the luteal phase to the follicular phase during the oestrous circle by Western blot (WB) and real-time PCR (RT-PCR). Results demonstrated that typical classic apoptotic factors Casp9, Bax, Bcl-2 and Fas were expressed in the cytoplasm and zonal pellucida of oocytes, primordial follicles, primary follicles, ovarian surface epithelium, ovarian GCs, granular lutein cells, surface epithelia in oviduct uterotubal junction and oviduct ampulla during the luteal phase. RT-PCR and WB revealed that P53 and Fas significantly increased in GCs of atretic follicles. P53 and Casp9 increased in oviduct epithelium during the luteal phase, but Fas was unchanged. A contrary tendency was noted in Bcl-2 and Bax expression. Overall, P53 and Fas play an essential role in inducing GC apoptosis, and Bax, Bcl-2, Casp9 and P53 are involved in oviduct epithelial regeneration in yak.

Ovarian Aging: Role of Pituitary-Ovarian Axis Hormones and ncRNAs in Regulating Ovarian Mitochondrial Activity

The number of mitochondria in the oocyte along with their functions (e.g., energy production, scavenger activity) decline with age progression. Such multifaceted functions support several processes during oocyte maturation, ranging from energy supply to synthesis of the steroid hormones. Hence, it is hardly surprising that their impairment has been reported in both physiological and premature ovarian aging, wherein they are crucial players in the apoptotic processes that arise in aged ovaries. In any form, ovarian aging implies the progressive damage of the mitochondrial structure and activities as regards to ovarian germ and somatic cells. The imbalance in the circulating hormones and peptides (e.g., gonadotropins, estrogens, AMH, activins, and inhibins), active along the pituitary-ovarian axis, represents the biochemical sign of ovarian aging. Despite the progress accomplished in determining the key role of the mitochondria in preserving ovarian follicular number and health, their modulation by the hormonal signalling pathways involved in ovarian aging has been poorly and randomly explored. Yet characterizing this mechanism is pivotal to molecularly define the implication of mitochondrial dysfunction in physiological and premature ovarian aging, respectively. However, it is fairly difficult considering that the pathways associated with ovarian aging might affect mitochondria directly or by altering the activity, stability and localization of proteins controlling mitochondrial dynamics and functions, either unbalancing other cellular mediators, released by the mitochondria, such as non-coding RNAs (ncRNAs). We will focus on the mitochondrial ncRNAs (i.e., mitomiRs and mtlncRNAs), that retranslocate from the mitochondria to the nucleus, as active players in aging and describe their role in the nuclear-mitochondrial crosstalk and its modulation by the pituitary-ovarian hormone dependent pathways. In this review, we will illustrate mitochondria as targets of the signaling pathways dependent on hormones and peptides active along the pituitary/ovarian axis and as transducers, with a particular focus on the molecules retrieved in the mitochondria, mainly ncRNAs. Given their regulatory function in cellular activities we propose them as potential diagnostic markers and/or therapeutic targets.

The antiandrogenic vinclozolin induces differentiation delay of germ cells and changes in energy metabolism in 3D cultures of fetal ovaries

Vinclozolin is a pesticide with antiandrogenic activity as an endocrine disruptor compound. Its effects upon the progression of primordial follicles were assessed in cultures of mouse fetal ovaries from the onset of meiotic differentiation of germ cells (13.5 days post coitum) and from both in vivo exposed mice and in vitro exposed ovaries. Exposure of ovaries to vinclozolin—at in vitro dosages ranging from 10 to 200 μM and in 3D ex vivo culture following in vivo exposure to 50 mg/kg bw/day—showed delays in meiocyte differentiation and in follicle growth, even at the lowest in vitro dose exposure. Immunofluorescent analysis showed the presence of the proteins MSY2 and NOBOX in the primary follicles but no difference in the level of protein signals or in the number of follicles in relation to treatment. However, assessing the cytological differentiation of germ cells by detecting the synaptonemal complex protein SYCP3, the exposure to vinclozolin delayed meiotic differentiation from both in vitro- and in vivo-exposed ovaries. These effects were concomitant with changes in the energy metabolism, detected as a relative increase of glycolytic metabolism in live-cell metabolic assays in exposed ovaries.

X-linked α-thalassemia with mental retardation is downstream of protein kinase A in the meiotic cell cycle signaling cascade in Xenopus oocytes and is dynamically regulated in response to DNA damage†

X-linked α-thalassemia with mental retardation (ATRX) is a chromatin remodeling protein that belongs to the SWItch/sucrose non-fermentable (SWI2/SNF2) family of helicase/ATPases. During meiosis, ATRX is necessary for heterochromatin formation and maintenance of chromosome stability in order to ensure proper assembly of the metaphase II spindle. Previously, we established ATRX as a novel progesterone regulated protein during bovine meiotic maturation, in addition to being dynamically regulated in response to DNA damage in oocytes. In the present study, we utilize the Xenopus laevis model system to further elucidate the signaling pathways regulating ATRX expression within the oocyte. Here, we present an analysis of endogenous ATRX protein expression during oogenesis, oocyte meiotic maturation, and early embryonic development. ATRX expression is dynamically regulated as evidenced by loss of the protein in metaphase II of meiosis. The downstream activation of meiosis via protein kinase A inhibition resulted in a similar decrease in ATRX protein expression. We demonstrate that the ATRX protein is detected in ubiquitin immuno-precipitates from germinal vesicle oocyte extracts and experimentally demonstrate that proteosomal degradation is responsible for the decreased expression of ATRX during meiosis. ATRX expression is significantly increased in response to gamma-irradiation induced DNA damage in oocytes and embryos. This increased expression is independent of p53 protein expression in apoptotic embryos, as determined by the expression of active caspase-3. Thus, regulation of ATRX protein expression impacts on G2-M progression and ultimately has consequences for cell survival.

Interplay between Caspase 9 and X-linked Inhibitor of Apoptosis Protein (XIAP) in the oocyte elimination during fetal mouse development

Mammalian female fertility is limited by the number and quality of oocytes in the ovarian reserve. The number of oocytes is finite since all germ cells cease proliferation to become oocytes in fetal life. Moreover, 70-80% of the initial oocyte population is eliminated during fetal and neonatal development, restricting the ovarian reserve. Why so many oocytes are lost during normal development remains an enigma. In Meiotic Prophase I (MPI), oocytes go through homologous chromosome synapsis and recombination, dependent on formation and subsequent repair of DNA double strand breaks (DSBs). The oocytes that have failed in DSB repair or synapsis get eliminated mainly in neonatal ovaries. However, a large oocyte population is eliminated before birth, and the cause or mechanism of this early oocyte loss is not well understood. In the current paper, we show that the oocyte loss in fetal ovaries was prevented by a deficiency of Caspase 9 (CASP9), which is the hub of the mitochondrial apoptotic pathway. Furthermore, CASP9 and its downstream effector Caspase 3 were counteracted by endogenous X-linked Inhibitor of Apoptosis (XIAP) to regulate the oocyte population; while XIAP overexpression mimicked CASP9 deficiency, XIAP deficiency accelerated oocyte loss. In the CASP9 deficiency, more oocytes were accumulated at the pachytene stage with multiple γH2AFX foci and high LINE1 expression levels, but with normal levels of synapsis and overall DSB repair. We conclude that the oocytes with LINE1 overexpression were preferentially eliminated by CASP9-dependent apoptosis in balance with XIAP during fetal ovarian development. When such oocytes were retained, however, they get eliminated by a CASP9-independent mechanism during neonatal development. Thus, the oocyte is equipped with multiple surveillance mechanisms during MPI progression to safe-guard the quality of oocytes in the ovarian reserve.

Exposure to Zinc oxide nanoparticles during pregnancy induces oocyte DNA damage and affects ovarian reserve of mouse offspring

Zinc oxide nanoparticles (nZnO) have been shown to have higher toxic effects likely due to their ion-shedding ability and low solubility under neutral conditions. In order to investigate whether exposure to nZnO during embryonic development affects ovary development, 12.5 day post coitum (dpc) fetal mouse ovaries were cultured in the presence of nZnO for 6 days. We found that the nanoparticles (NPs) accumulated within the oocyte cytoplasm in a dose dependent manner, caused DNA damage and apoptosis, and result in a significant decrease in oocyte numbers. No such effects were observed when the ovaries were incubated in the presence of ZnSO₄ or bulk ZnO as controls. In addition, we injected intravenously 16 mg/kg body weight nZnO in 12.5 dpc pregnant mice on two consecutive days and analyzed the ovaries of fetuses or offspring at three critical periods of oogenesis: 17.5 dpc, 3 days post-partum (dpp) and 21 dpp. Evidence of increased DNA damage in pachytene oocytes in fetal ovaries and impaired primordial follicle assembly and folliculogenesis dynamics in the ovaries of the offspring were found. Our results indicate that certain types of NPs affect pre- and post-natal oogenesis in vitro and in vivo.

Effects of paclitaxel and cisplatin on in vitro ovarian follicle development

INTRODUCTION

Despite its importance in pre-chemotherapy counselling, specific reproductive toxicological information about cisplatin and paclitaxel is very rare. This study aimed to investigate the concentrations at which cisplatin and paclitaxel, alone or combined, affect the in vitro development of ovarian follicles. Their differential effects on the oocytes and surrounding granulosa cells was also evaluated.

MATERIAL AND METHODS

Ovarian follicles were cultured in vitro using gonadotropins and treated with 10^-8-10^-10 M of cisplatin, paclitaxel, or both. At day 13, granulosa cells and oocytes were retrieved and used for imaging and functional analyses.

RESULTS

Follicular survival and growth was significantly suppressed in all treatment groups at 10^-9 M or higher concentrations, and additive effects were observed in the combination group (p < 0.01). Oocyte-specific genes such as growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) were more suppressed in the paclitaxel group than in the cisplatin group. Granulosa cell-specific gene suppression and its electron microscopic alteration were more prominent in the cisplatin group than in the paclitaxel group. X-linked inhibitor of apoptosis protein (XIAP) expression of granulosa cells was also further down-regulated in the cisplatin group.

CONCLUSIONS

These data provide an insight into the critical concentrations regarding in vitro follicular development and the differential effects of chemotherapeutic effects on oocytes and granulosa cells. Further studies are necessary to develop more efficient pre-chemotherapeutic fertility-sparing medical treatment that can evade oocyte-specific damage.

Initiation of sex change and gonadal gene expression in black sea bass (Centropristis striata) exposed to exemestane, an aromatase inhibitor

Many teleost fishes exhibit sequential hermaphroditism, where male or female gonads develop first and later undergo sex change. Model sex change species are characterized by social hierarchies and coloration changes, which enable experimental manipulations to better understand these processes. However, other species such as the protogynous black sea bass (Centropristis striata) do not exhibit these characteristics and instead receive research attention due to their importance in fisheries or aquaculture. Black sea bass social structure is unknown, which makes sex change sampling difficult, and few molecular resources are available. The purpose of the present study was to induce sex change using exemestane, an aromatase inhibitor, and assess gonadal gene expression using sex markers (amh, zpc2) and genes involved in steroidogenesis (cyp19a1a, cyp11b), estrogen signaling (esr1, esr2b), and apoptosis or atresia (aen, casp9, fabp11, parg, pdcd4, rif1). Overall, dietary exemestane treatment was effective, and most exposed females exhibited early histological signs of sex change and significantly higher rates of ovarian atresia relative to control females. Genes associated with atresia did not reflect this, however, as expression patterns in sex changing gonads were overall similar to those of ovaries, likely due to a whole ovary dilution effect of the RNA. Still, small but insignificant expression decreases during early sex change were detected for ovary-related genes (aen, casp9, fabp11, zpc2) and anti-apoptotic factors (parg, rif1). Exemestane treatment did not impact spermatogenesis or testicular gene expression, but testes were generally characterized by elevated steroidogenic enzyme and estrogen receptor mRNAs. Further research will be needed to understand these processes in black sea bass, using isolated ovarian follicles and multiple stages of sex change.

Mitochondrial DNA copy number in peripheral blood: a potential non-invasive biomarker for female subfertility

PURPOSE

Low mitochondrial DNA (mtDNA) content in oocytes and in cumulus cells is an indicator of poor oocyte quality. Moreover, initial evidence showed a correlation between mtDNA content in cumulus cells and mtDNA copy number in peripheral blood cells. On these bases, we deemed of interest investigating the correlation between mtDNA copy number in peripheral blood and natural fecundity.

METHODS

This is a nested case-control study drawn from a prospective cohort of pregnant women referred for routine first trimester screening for aneuploidies (from 11 + 0 to 12 + 6 weeks of gestation) between January 2012 and March 2013 at the "Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico" of Milan, Italy. Cases were subfertile women who attempted to become pregnant for 12-24 months. Controls were the two subsequently age-matched women who became pregnant in less than 1 year. MtDNA was quantified using real-time PCR and normalized to nuclear DNA.

RESULTS

One hundred and four subfertile women and 208 controls were selected. The median (IQR) mtDNA copy number was 95 (73-124) and 145 (106-198), respectively (p < 0.001). The area under the ROC curve was 0.73 (95% CI 0.67-0.79) (p < 0.001). The Youden index was 105 mtDNA copy number. The crude OR for subfertility in women with mtDNA copy number below this threshold was 5.72 (95% CI 3.43-9.55). The accuracy of mtDNA copy number assessment in peripheral blood progressively decreased with increasing female age.

CONCLUSIONS

Low mtDNA copy number in peripheral blood is associated with an increased risk of subfertility and may represent a biomarker of natural fecundity.

Programmed cell death in the human ovary

During a woman's reproductive life, only about 400 primordial follicles will develop into a preovulatory follicle and undergo ovulation, releasing an oocyte available for fertilization. The process of formation and selection of these follicles is complex and involves a multistep process characterized by a balance between survival and death of the oocytes and the surrounding follicular cells. Although the mechanisms underlying such process are not completely clarified yet, it is common idea that they can occur through various types of programmed cellular death (PCD). Since atresia is the principal destiny of the ovarian follicles, it is relevant to understand how this process takes place and how it is regulated. In this review, after a summary description of folliculogenesis in humans, the main mechanisms of atresia reported to occur during folliculogenesis from birth to adult age, in the human ovary and in other mammals when appropriate, are described.

FSH and eCG impact follicles development and expression of ovarian FSHR and caspase-9 in mice

The study aimed to investigate the effects of FSH and eCG on the ovarian and follicular development, expression levels of FSHR and caspase-9 of ovaries in vivo. One hundred and five prepuberty mice were allocated into FSH-1, FSH-2, FSH-3, eCG-1, eCG-2, eCG-3 groups and control group (CG). Mice in FSH-1, FSH-2 and FSH-3 were intramuscularly injected with 5, 10 and 20 IU FSH twice (on day 0 and 4), respectively. Mice in eCG-1, eCG-2 and eCG-3 were intraperitoneally injected with 10, 20 and 40 IU eCG on day 0 and 4. Mice in the CG were injected with 0.5 ml normal saline on day 0 and 4. Left and right ovaries of each mouse were dissected aseptically on days 7, 14 and 21, respectively. The results showed that on days 14 and 21 the ovarian sizes and follicle numbers of FSH-3 and eCG-3 groups were greater than CG (P<0.05). FSHR mRNA of FSH-2 and eCG-1 were higher than CG on days 14 and 21 (P<0.05). FSHR proteins of FSH-3 were higher than CG on days 14 and 21 (P<0.05). Caspase-9 mRNA in FSH and eCG groups was less than CG. There were positive correlations between follicle numbers and FSH and eCG doses. FSHR protein expressions had positive correlations between ovarian weights and sizes of ovary and follicle numbers (r=0.971, P<0.05) in FSH-treated mice. Serum FSH concentrations of FSH-2, FSH-3, eCG-2 and eCG-3 groups were greater than that of CG. In conclusion, eCG and FSH promoted the ovarian development, follicle genesis, FSH secretion, FSHR mRNA and protein expressions in ovaries of mice. FSH and eCG inhibited the expression of ovarian caspase-9 mRNA.

Ovarian ageing: the role of mitochondria in oocytes and follicles

BACKGROUND

There is a great inter-individual variability of ovarian ageing, and almost 20% of patients consulting for infertility show signs of premature ovarian ageing. This feature, taken together with delayed childbearing in modern society, leads to the emergence of age-related ovarian dysfunction concomitantly with the desire for pregnancy. Assisted reproductive technology is frequently inefficacious in cases of ovarian ageing, thus raising the economic, medical and societal costs of the procedures.

OBJECTIVE AND RATIONAL

Ovarian ageing is characterized by quantitative and qualitative alteration of the ovarian oocyte reserve. Mitochondria play a central role in follicular atresia and could be the main target of the ooplasmic factors determining oocyte quality adversely affected by ageing. Indeed, the oocyte is the richest cell of the body in mitochondria and depends largely on these organelles to acquire competence for fertilization and early embryonic development. Moreover, the oocyte ensures the uniparental transmission and stability of the mitochondrial genome across the generations. This review focuses on the role played by mitochondria in ovarian ageing and on the possible consequences over the generations.

SEARCH METHODS

PubMed was used to search the MEDLINE database for peer-reviewed original articles and reviews concerning mitochondria and ovarian ageing, in animal and human species. Searches were performed using keywords belonging to three groups: 'mitochondria' or 'mitochondrial DNA'; 'ovarian reserve', 'oocyte', 'ovary' or 'cumulus cells'; and 'ageing' or 'ovarian ageing'. These keywords were combined with other search phrases relevant to the topic. References from these articles were used to obtain additional articles.

OUTCOMES

There is a close relationship, in mammalian models and humans, between mitochondria and the decline of oocyte quality with ageing. Qualitatively, ageing-related mitochondrial (mt) DNA instability, which leads to the accumulation of mtDNA mutations in the oocyte, plays a key role in the deterioration of oocyte quality in terms of competence and of the risk of transmitting mitochondrial abnormalities to the offspring. In contrast, some mtDNA haplogroups are protective against the decline of ovarian reserve. Quantitatively, mitochondrial biogenesis is crucial during oogenesis for constituting a mitochondrial pool sufficiently large to allow normal early embryonic development and to avoid the untimely activation of mitochondrial biogenesis. Ovarian ageing also seriously affects the dynamic nature of mitochondrial biogenesis in the surrounding granulosa cells that may provide interesting alternative biomarkers of oocyte quality.

WIDER IMPLICATIONS

A fuller understanding of the involvement of mitochondria in cases of infertility linked to ovarian ageing would contribute to a better management of the disorder in the future.

BCL2-modifying factor promotes germ cell loss during murine oogenesis

Apoptosis plays a prominent role during ovarian development by eliminating large numbers of germ cells from the female germ line. However, the precise mechanisms and regulatory proteins involved in germ cell death are yet to be determined. In this study, we characterised the role of the pro-apoptotic BH3-only protein, BCL2-modifying factor (BMF), in germ cell apoptosis in embryonic and neonatal mouse ovaries. BMF protein was immunohistochemically localised to germ cells at embryonic days 15.5 (E15.5) and E17.5 and postnatal day 1 (PN1), coincident with entry into the meiotic prophase, but was undetectable at E13.5, and only present at low levels at PN3 and PN5. Consistent with this expression pattern, loss of BMF in female mice was associated with a decrease in apoptosis at E15.5 and E17.5. Furthermore, increased numbers of germ cells were found in ovaries from Bmf−/− mice compared with WT animals at E15.5 and PN1. However, germ cell numbers were comparable between Bmf−/− and WT ovaries at PN3, PN5 and PN10. Collectively, these data indicate that BMF mediates foetal oocyte loss and its action limits the maximal number of germ cells attained in the developing ovary, but does not influence the number of primordial follicles initially established in ovarian reserve.

The role of sex chromosomes in mammalian germ cell differentiation: can the germ cells carrying X and Y chromosomes differentiate into fertile oocytes?

The sexual differentiation of germ cells into spermatozoa or oocytes is strictly regulated by their gonadal environment, testis or ovary, which is determined by the presence or absence of the Y chromosome, respectively. Hence, in normal mammalian development, male germ cells differentiate in the presence of X and Y chromosomes, and female germ cells do so in the presence of two X chromosomes. However, gonadal sex reversal occurs in humans as well as in other mammalian species, and the resultant XX males and XY females can lead healthy lives, except for a complete or partial loss of fertility. Germ cells carrying an abnormal set of sex chromosomes are efficiently eliminated by multilayered surveillance mechanisms in the testis, and also, though more variably, in the ovary. Studying the molecular basis for sex-specific responses to a set of sex chromosomes during gametogenesis will promote our understanding of meiotic processes contributing to the evolution of sex determining mechanisms. This review discusses the fate of germ cells carrying various sex chromosomal compositions in mouse models, the limitation of which may be overcome by recent successes in the differentiation of functional germ cells from embryonic stem cells under experimental conditions.

Control of Oocyte Growth and Development by Intercellular Communication Within the Follicular Niche

In the mammalian ovary, each oocyte grows and develops within its own structural and developmental niche-the follicle. Together with the female germ cell in the follicle are somatic granulosa cells, specialized companion cells that surround the oocyte and provide support to it, and an outer layer of thecal cells that serve crucial roles including steroid synthesis. These follicular compartments function as a single physiological unit whose purpose is to produce a healthy egg, which upon ovulation can be fertilized and give rise to a healthy embryo, thus enabling the female germ cell to fulfill its reproductive potential. Beginning from the initial stage of follicle formation and until terminal differentiation at ovulation, oocyte and follicle growth depend absolutely on cooperation between the different cellular compartments. This cooperation synchronizes the initiation of oocyte growth with follicle activation. During growth, it enables metabolic support for the follicle-enclosed oocyte and allows the follicle to fulfill its steroidogenic potential. Near the end of the growth period, intra-follicular interactions prevent the precocious meiotic resumption of the oocyte and ensure its nuclear differentiation. Finally, cooperation enables the events of ovulation, including meiotic maturation of the oocyte and expansion of the cumulus granulosa cells. In this chapter, we discuss the cellular interactions that enable the growing follicle to produce a healthy oocyte, focusing on the communication between the germ cell and the surrounding granulosa cells.

Relationship between diminished ovarian reserve and mitochondrial biogenesis in cumulus cells

STUDY QUESTION

What part do mitochondria play in cases of diminished ovarian reserve (DOR)?

SUMMARY ANSWER

Mitochondrial biogenesis in cumulus cells may be linked with impaired oocyte competence in patients with DOR.

WHAT IS KNOWN ALREADY

DOR, one of the causes of infertility even in young women, is characterized by the depletion of the ovarian pool associated with a decline in oocyte competence. Mitochondria, which play a role in oocyte quality, could be involved in the pathogenesis of DOR. The study of cumulus cells offers an interesting non-invasive approach for evaluating oocyte quality and the metabolic processes on which it depends. If mitochondrial dysfunction is involved in DOR, it is likely to have an impact on the functioning of cumulus cells.

STUDY DESIGN, SIZE, DURATION

This is an observational study of 74 immature oocyte-cumulus complexes retrieved from 47 women undergoing in vitro fertilization with intracytoplasmic sperm injection at the University Hospital of Angers, France, from March 2013 to March 2014. The women were divided into two groups: one group included 26 women with DOR, and the other, which included 21 women with a normal ovarian reserve (NOR), served as a control group.

PARTICIPANTS/MATERIALS, SETTINGS, METHODS

The oocyte mitochondrial content and the average mitochondrial content of the cumulus cells were assessed by mitochondrial (mt)DNA quantification using a quantitative real-time PCR technique. Microfluidic-based quantitative RT-PCR assays were used to quantify the expression of 13 genes involved in mitochondrial functions such as apoptosis and antioxidant activity or in mitochondrial biogenesis. We used orthogonal partial least-squares discriminant analysis (OPLS-DA) to distinguish between the DOR group and the NOR group of patients, and an OPLS model to predict the value of the oocyte mtDNA content that could be used as a critical marker of oocyte quality.

MAIN RESULTS AND THE ROLE OF CHANCE

The OPLS-DA model showed a good predictive capability (Q2 = 0.543). Using the variable importance in projection (VIP) metric we found three mitochondrial variables distinguishing the DOR group from the NOR group of patients, i.e. the oocyte mtDNA content (VIP = 0.92), the cumulus cell mtDNA content (VIP = 0.95) and the expression in cumulus cells of peroxisome proliferator-activated receptor γ coactivator 1 alpha (PPARGC-1A) (VIP = 1.10), all of which were lower in the DOR group than in the NOR group of patients. The OPLS model was able to satisfactorily predict the oocyte mtDNA content in only the NOR group of patients (Q2 = 0.506). We found four new variables positively linked to the oocyte mitochondrial mass, i.e. the cumulus cell mtDNA content (VIP = 1.19), and the expression in cumulus cells of three factors of mitochondrial biogenesis: polymerase gamma (POLG) (VIP = 2.13), optic atrophy 1 (OPA1) (VIP = 1.89) and the transcription factor associated with mitochondria (TFAM) (VIP = 1.32).

LIMITATIONS, REASONS OF CAUTION

This is a descriptive study. Because of ethical concerns in human clinical practice, this study has been performed only on immature oocytes and corresponding cumulus cells, which are usually discarded during in vitro fertilization procedures.

WIDER IMPLICATIONS OF THE FINDINGS

Cumulus cells may govern mitochondrial biogenesis, creating an adequate oocyte mitochondrial pool to promote embryonic development. The alteration of this process in patients with DOR may account for the impairment of oocyte quality. This suggests that some mitochondrial characteristics of cumulus cells may serve as indicators of oocyte competence and that oocyte quality may be improved by products enhancing mitochondrial biogenesis.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by a grant from the University Hospital of Angers, France: 'Appel d'offre interne à la recherche 2014'.

TRIAL REGISTRATION NUMBER

N/A.

Taking control of the female fertile lifespan: a key role for Bcl-2 family proteins

Precisely how the length of the female fertile lifespan is regulated is poorly understood and it is likely to involve complex factors, one of which is follicle number. Indeed, the duration of female fertility appears to be intimately linked to the number of available oocytes, which are stored in the ovary as primordial follicles. There is mounting evidence implicating the intrinsic apoptosis pathway, which is controlled by members of the B-cell lymphoma-2 (BCL-2) family, as a key regulator of the number of primordial follicles established in the ovary at birth and maintained throughout reproductive life. Consequently, the pro- and anti-apoptotic BCL-2 family proteins are emerging as key determinants of the length of the female fertile lifespan. This review discusses the relationship between the intrinsic apoptosis pathway, follicle number and length of the female fertile lifespan.

Enhanced susceptibility of ovaries from obese mice to 7,12-dimethylbenz[a]anthracene-induced DNA damage

7,12-Dimethylbenz[a]anthracene (DMBA) depletes ovarian follicles and induces DNA damage in extra-ovarian tissues, thus, we investigated ovarian DMBA-induced DNA damage. Additionally, since obesity is associated with increased offspring birth defect incidence, we hypothesized that a DMBA-induced DNA damage response (DDR) is compromised in ovaries from obese females. Wild type (lean) non agouti (a/a) and KK.Cg-Ay/J heterozygote (obese) mice were dosed with sesame oil or DMBA (1mg/kg; intraperitoneal injection) at 18weeks of age, for 14days. Total ovarian RNA and protein were isolated and abundance of Ataxia telangiectasia mutated (Atm), X-ray repair complementing defective repair in Chinese hamster cells 6 (Xrcc6), breast cancer type 1 (Brca1), Rad 51 homolog (Rad51), poly [ADP-ribose] polymerase 1 (Parp1) and protein kinase, DNA-activated, catalytic polypeptide (Prkdc) were quantified by RT-PCR or Western blot. Phosphorylated histone H2AX (γH2AX) level was determined by Western blotting. Obesity decreased (P<0.05) basal protein abundance of PRKDC and BRCA1 proteins but increased (P<0.05) γH2AX and PARP1 proteins. Ovarian ATM, XRCC6, PRKDC, RAD51 and PARP1 proteins were increased (P<0.05) by DMBA exposure in lean mice. A blunted DMBA-induced increase (P<0.05) in XRCC6, PRKDC, RAD51 and BRCA1 was observed in ovaries from obese mice, relative to lean counterparts. Taken together, DMBA exposure induced γH2AX as well as the ovarian DDR, supporting that DMBA causes ovarian DNA damage. Additionally, ovarian DDR was partially attenuated in obese females raising concern that obesity may be an additive factor during chemical-induced ovotoxicity.

A role for retrotransposon LINE-1 in fetal oocyte attrition in mice

Fetal oocyte attrition (FOA) is a conserved but poorly understood process of elimination of more than two-thirds of meiotic prophase I (MPI) oocytes before birth. We now implicate retrotransposons LINE-1 (L1), activated during epigenetic reprogramming of the embryonic germline, in FOA in mice. We show that wild-type fetal oocytes possess differential nuclear levels of L1ORF1p, an L1-encoded protein essential for L1 ribonucleoprotein particle (L1RNP) formation and L1 retrotransposition. We demonstrate that experimental elevation of L1 expression correlates with increased MPI defects, FOA, oocyte aneuploidy, and embryonic lethality. Conversely, reverse transcriptase (RT) inhibitor AZT has a profound effect on the FOA dynamics and meiotic recombination, and it implicates an RT-dependent trigger in oocyte elimination in early MPI. We propose that FOA serves to select oocytes with limited L1 activity that are therefore best suited for the next generation.

Figla-Cre transgenic mice expressing myristoylated EGFP in germ cells provide a model for investigating perinatal oocyte dynamics

FIGLA (Factor in the germline, alpha) is a bHLH transcription factor expressed abundantly in female and less so in male germ cells. Mice lacking FIGLA do not form primordial follicles in the ovary and females are sterile, but there is no obvious phenotype in males. Using the Figla promoter to express Cre recombinase, we have established mEGFP/mTomato reporter mice with green germ cells and red somatic tissue. These mice were crossed into the Figla null background to accelerate perinatal oocyte loss. Live imaging of cultured newborn ovaries provides evidence that few oocytes egress and the vast majority disappear within the confines of the ovary. Although a cohort of mobile, phagocytic cells was observed, macrophage depletion in Csf1^op/op mice did not affect oocyte loss. Investigations with TUNEL assays and caspase inhibitors suggest that apoptosis plays a role in the perinatal loss of oocyte in female mice. These results establish the utility of Figla-EGFP/Cre; mTomato/mEGFP in investigating germ cell dynamics in prepubertal mice.

Mitochondria and mammalian reproduction

Mitochondria are cellular organelles with crucial roles in ATP synthesis, metabolic integration, reactive oxygen species (ROS) synthesis and management, the regulation of apoptosis (namely via the intrinsic pathway), among many others. Additionally, mitochondria in different organs or cell types may have distinct properties that can decisively influence functional analysis. In terms of the importance of mitochondria in mammalian reproduction, and although there are species-specific differences, these aspects involve both energetic considerations for gametogenesis and fertilization, control of apoptosis to ensure the proper production of viable gametes, and ROS signaling, as well as other emerging aspects. Crucially, mitochondria are the starting point for steroid hormone biosynthesis, given that the conversion of cholesterol to pregnenolone (a common precursor for all steroid hormones) takes place via the activity of the cytochrome P450 side-chain cleavage enzyme (P450scc) on the inner mitochondrial membrane. Furthermore, mitochondrial activity in reproduction has to be considered in accordance with the very distinct strategies for gamete production in the male and female. These include distinct gonad morpho-physiologies, different types of steroids that are more prevalent (testosterone, estrogens, progesterone), and, importantly, the very particular timings of gametogenesis. While spermatogenesis is complete and continuous since puberty, producing a seemingly inexhaustible pool of gametes in a fixed environment; oogenesis involves the episodic production of very few gametes in an environment that changes cyclically. These aspects have always to be taken into account when considering the roles of any common element in mammalian reproduction.

TUNEL analysis of DNA fragmentation in mouse unfertilized oocytes: the effect of microorganisms within human follicular fluid collected during IVF cycles

Recently we reported the presence of bacteria within follicular fluid. Previous studies have reported that DNA fragmentation in human spermatozoa after in vivo or in vitro incubation with bacteria results in early embryo demise and a reduced rate of ongoing pregnancy, but the effect of bacteria on oocytes is unknown. This study examined the DNA within mouse oocytes after 12 hours' incubation within human follicular fluids (n=5), which were collected from women undergoing in vitro fertilization (IVF) treatment. Each follicular fluid sample was cultured to detect the presence of bacteria. Terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling (TUNEL) was used to label DNA fragmentation in ovulated, non-fertilized mouse oocytes following in vitro incubation in human follicular fluid. The bacteria Streptococcus anginosus and Peptoniphilus spp., Lactobacillus gasseri (low-dose), L. gasseri (high-dose), Enterococcus faecalis, or Propionibacterium acnes were detected within the follicular fluids. The most severe DNA fragmentation was observed in oocytes incubated in the follicular fluids containing P. acnes or L. gasseri (high-dose). No DNA fragmentation was observed in the mouse oocytes incubated in the follicular fluid containing low-dose L. gasseri or E. faecalis. Low human oocyte fertilization rates (<29%) were associated with extensive fragmentation in mouse oocytes (80-100%). Bacteria colonizing human follicular fluid in vivo may cause DNA fragmentation in mouse oocytes following 12h of in vitro incubation. Follicular fluid bacteria may result in poor quality oocytes and/or embryos, leading to poor IVF outcomes.