Specificity of the requirement for Foxo3 in primordial follicle activation

Participation of preovulatory follicles in the activation of primordial follicles in mouse ovaries

The mechanisms behind the selection and initial recruitment of primordial follicles (PmFs) from the non-growing PmF pool during each estrous cycle in females remain largely unknown. This study demonstrates that PmFs closest to the ovulatory follicle are preferentially activated in mouse ovaries under physiological conditions. PmFs located within 40 μm of the ovulatory follicles were more likely to be activated compared to those situated further away during the peri-ovulation period. Repeated superovulation treatments accelerated the depletion of the PmF reserve, whereas continuous suppression of ovulation delayed PmF reserve consumption. Spatial transcriptome sequencing of peri-ovulatory follicles revealed that ovulation primarily induces the degradation and remodeling of the extracellular matrix (ECM). This ECM degradation reduces mechanical stress around PmFs, thereby triggering their activation. Specifically, Cathepsin L (CTSL), a cysteine proteinase and lysosomal enzyme involved in ECM degradation, initiates the activation of PmFs adjacent to ovulatory follicles in a distance-dependent manner. These findings highlight the link between ovulation and selective PmF activation, and underscore the role of CTSL in this process under physiological conditions.

Cystic endometrial hyperplasia-pyometra syndrome impairs the preantral follicle reserve in domestic bitches (Canis familiaris)

Cystic endometrial hyperplasia (CEH)-pyometra syndrome is the most common uterine pathological condition reported in breeding bitches, however, their described effects on fertility are limited to uterine disorders and conception rates. As the preantral follicle population represents the available reserve of gametes recruited during the lifespan, the aim of this study was to evaluate the effects of CEH-pyometra syndrome on the: (i) preantral follicle morphology, (ii) developing follicle rates, and (iii) preantral follicle and stromal cell densities. Ovarian fragments from bitches subjected to elective or therapeutic ovariohysterectomy were allocated according to uterine diagnosis as follows: control (n = 7, clinically healthy), CEH-mucometra (n = 8, uterine lumen filled with a sterile mucus), and pyometra (n = 17, presence of a purulent mucus) groups. Overall, the control group had 3.4 and 4.1-fold higher probability (P < 0.0001) of the presence of normal preantral follicles compared with CEH-mucometra and pyometra groups, respectively. Moreover, ovarian fragments from the pyometra group showed an increase in the percentage of developing follicles (P < 0.05) compared to the control. Both CEH-mucometra and pyometra groups showed lower (P < 0.05) preantral follicle and stromal cell densities (P < 0.05) compared to the control. In summary, the CEH-pyometra syndrome decreased the percentage of morphologically normal follicles and enhanced the developing follicle rates. Additionally, a reduction of preantral follicle and stromal cell densities suggests that the inappropriate uterine environment induced by CEH-pyometra syndrome can lead to premature depletion of ovarian reserve.

Genome-wide DNA methylation analysis of Astragalus and Danshen on the intervention of myofibroblast activation in idiopathic pulmonary fibrosis

BACKGROUND

Idiopathic pulmonary fibrosis (IPF), a chronic progressive interstitial lung disease of unknown etiology, is characterized by continuous damage to alveolar epithelial cells, abnormal repair of alveolar tissue, and alveolar wall scar formation. Currently, the recommended treatment for IPF in Western medicine is relatively limited. In contrast, traditional Chinese medicine and compound prescriptions show advantages in the diagnosis and treatment of IPF, which can be attributed to their multi-channel and multi-target characteristics and minimal side-effects. The purpose of this study was to further corroborate the effectiveness and significance of the traditional Chinese medications Astragalus and Danshen in IPF treatment.

METHODS

We performed whole-genome methylation analysis on nine rat lung tissue samples to determine the epigenetic variation between IPF and non-fibrotic lungs using Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses and quantitative reverse transcription polymerase chain reactions.

RESULTS

We identified differentially methylated regions and 105 associated key functional genes in samples related to IPF and Chinese medicine treatment. Based on the methylation levels and gene expression profiles between the Chinese medicine intervention and pulmonary fibrosis model groups, we speculated that Astragalus and Salvia miltiorrhiza (traditionally known as Danshen) act on the Isl1, forkhead box O3, and Sonic hedgehog genes via regulation at transcriptional and epigenetic levels during IPF.

CONCLUSIONS

These findings provide novel insights into the epigenetic regulation of IPF, indicate the effectiveness of Astragalus and Danshen in treating IPF, and suggest several promising therapeutic targets for preventing and treating IPF.

PARP1-catalyzed PARylation of YY1 mediates endoplasmic reticulum stress in granulosa cells to determine primordial follicle activation

Although only a small number of primordial follicles are known to be selectively activated during female reproductive cycles, the mechanisms that trigger this recruitment remain largely uncharacterized. Misregulated activation of primordial follicles may lead to the exhaustion of the non-renewable pool of primordial follicles, resulting in premature ovarian insufficiency. Here, we found that poly(ADP-ribose) polymerase 1 (PARP1) enzymatic activity in the surrounding granulosa cells (GCs) in follicles determines the subpopulation of the dormant primordial follicles to be awakened. Conversely, specifically inhibiting PARP1 in oocytes in an in vitro mouse follicle reconstitution model does not affect primordial follicle activation. Further analysis revealed that PARP1-catalyzed transcription factor YY1 PARylation at Y185 residue facilitates YY1 occupancy at Grp78 promoter, a key molecular chaperone of endoplasmic reticulum stress (ERS), and promotes Grp78 transcription in GCs, which is required for GCs maintaining proper ERS during primordial follicle activation. Inhibiting PARP1 prevents the loss of primordial follicle pool by attenuating the excessive ERS in GCs under fetal bisphenol A exposure. Together, we demonstrate that PARP1 in GCs acts as a pivotal modulator to determine the fate of the primordial follicles and may represent a novel therapeutic target for the retention of primordial follicle pool in females.

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

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

Squeezing the eggs to grow: The mechanobiology of mammalian folliculogenesis

The formation of functional eggs (oocyte) in ovarian follicles is arguably one of the most important events in early mammalian development since the oocytes provide the bulk genetic and cytoplasmic materials for successful reproduction. While past studies have identified many genes that are critical to normal ovarian development and function, recent studies have highlighted the role of mechanical force in shaping folliculogenesis. In this review, we discuss the underlying mechanobiological principles and the force-generating cellular structures and extracellular matrix that control the various stages of follicle development. We also highlight emerging techniques that allow for the quantification of mechanical interactions and follicular dynamics during development, and propose new directions for future studies in the field. We hope this review will provide a timely and useful framework for future understanding of mechano-signalling pathways in reproductive biology and diseases.

Tamoxifen Activates Dormant Primordial Follicles in Mouse Ovaries

Our previous study found that 17β-estradiol (E2) suppresses primordial follicle activation and growth in cultured mouse ovaries. In this study, we administered tamoxifen, an estrogen receptor antagonist, into the abdominal cavity of mice to clarify the relationship between primordial follicle activation and the physiological concentration of E2 in mouse ovaries. The results showed that tamoxifen promoted primordial follicle activation. Administration of tamoxifen promoted degradation of the extracellular matrix surrounding primordial follicles in the ovaries. Furthermore, tamoxifen decreased the expression of stefin A, an inhibitor of cathepsins that digest some proteins and extracellular matrix, in the ovaries. Mechanical stress produced by the extracellular matrix reportedly suppresses the activation of primordial follicles. The collective results show that tamoxifen can promote primordial follicle activation through the degradation of the extracellular matrix surrounding primordial follicles. Our results indicate that E2 suppresses primordial follicle activation in vivo and that tamoxifen may be useful as a therapeutic agent against infertility.

DNA Damage Stress Response and Follicle Activation: Signaling Routes of Mammalian Ovarian Reserve

Chemotherapy regimens and radiotherapy are common strategies to fight cancer. In women, these therapies may cause side effects such as premature ovarian insufficiency (POI) and infertility. Clinical strategies to protect the ovarian reserve from the lethal effect of cancer therapies needs better understanding of the mechanisms underlying iatrogenic loss of follicle reserve. Recent reports demonstrate a critical role for p53 and CHK2 in the oocyte response to different DNA stressors, which are commonly used to treat cancer. Here we review the molecular mechanisms underlying the DNA damage stress response (DDR) and discuss crosstalk between DDR and signaling pathways implicated in primordial follicle activation.

High-Fat Diet and Female Fertility across Lifespan: A Comparative Lesson from Mammal Models

Female reproduction focuses mainly on achieving fully grown follicles and competent oocytes to be successfully fertilized, as well as on nourishing the developing offspring once pregnancy occurs. Current evidence demonstrates that obesity and/or high-fat diet regimes can perturbate these processes, leading to female infertility and transgenerational disorders. Since the mechanisms and reproductive processes involved are not yet fully clarified, the present review is designed as a systematic and comparative survey of the available literature. The available data demonstrate the adverse influences of obesity on diverse reproductive processes, such as folliculogenesis, oogenesis, and embryo development/implant. The negative reproductive impact may be attributed to a direct action on reproductive somatic and germinal compartments and/or to an indirect influence mediated by the endocrine, metabolic, and immune axis control systems. Overall, the present review highlights the fragmentation of the current information limiting the comprehension of the reproductive impact of a high-fat diet. Based on the incidence and prevalence of obesity in the Western countries, this topic becomes a research challenge to increase self-awareness of dietary reproductive risk to propose solid and rigorous preventive dietary regimes, as well as to develop targeted pharmacological interventions.

Maternal intake of fructose or artificial sweetener during pregnancy and lactation has persistent effects on metabolic and reproductive health of dams post-weaning

As rates of obesity, diabetes, and related comorbidities have increased, the consumption of artificial sweeteners (ASs) as sugar substitutes has also risen in popularity as they are perceived as a healthier alternative to sugar sweetened products. However, there is conflicting evidence regarding the impact of AS intake on metabolic and reproductive health. Glucose intolerance during pregnancy due to intake of sugar sweetened foods can result in an increased risk for the development of type 2 diabetes post-pregnancy. However, limited information exists on the impact of AS intake during pregnancy and lactation on the mother's health in later life. We hypothesised both AS and fructose would impair metabolic health post-partum (PP) following maternal consumption during pregnancy and lactation. Female C57Bl/6 mice received a standard control diet ad libitum with either water (CD), fructose (Fr; 34.7 mm intake), or AS (AS;12.5 mm Acesulfame-K) throughout pregnancy and lactation. Post-weaning, AS and Fr dams were fed the CD diet for the remainder of the experiment. Oral glucose tolerance tests were undertaken 8 weeks PP and dams were humanely killed at 9 weeks PP, with adipose tissue and ovaries collected for analysis. Experimental diets did not influence maternal bodyweight. At 8 weeks PP, increased glucose intolerance was evident in both AS and Fr dams. Adipocyte size was significantly increased in both the AS and Fr groups PP. Further, in the ovary, AS increased expression of genes associated with follicular development and ovulation. Therefore, ASs may not represent beneficial substitutes to fructose during pregnancy, with the potential to increase the risk of T2DM in later life in mothers.

The effect of mTOR activation and PTEN inhibition on human primordial follicle activation in ovarian tissue culture

PURPOSE

The effect of PTEN inhibitor (Bpv(HOpic); Bpv) and mTOR activators (phosphatidic acid (PA) and propranolol (PP)), were evaluated on the activation and subsequent development of human primordial follicles in ovarian tissue culture.

METHODS

Slow frozen-thawed human ovarian cortical strips were incubated for 24 h in different groups: (1) Control (base medium), (2) Bpv (100 µM), (3) PA (200 µM), (4) PA + PP (50 µm), and (5) Bpv + PA + PP. Afterward, the medium was exchanged, and all groups were cultured without stimulators for 6 additional days. The proportion of normal and degenerated follicles, estradiol secretion, and expression of RPS6, FOXO3a, and AKT proteins was evaluated and compared between groups.

RESULTS

After 24 h of culture, there was no significant difference between the proportion of primordial and growing follicles in either of the experimental groups. This non-significant change was also observed for the phosphorylated protein to total protein ratios of RPS6, FOXO3a, and AKT proteins. After 7 days of culture, the proportion of the transitional follicles was significantly higher in comparison to the primordial follicles for the PA, PA + PP, and Bpv + PA + PP groups. The estradiol level was significantly higher on the last day compared to the first day, in PA, PA + PP, and Bpv + PA + PP groups. Hormonal secretion was significantly higher in the PA and PA + PP groups and lower in the Bpv and Bpv + PA + PP groups compared to the control on day 7 of culture.

CONCLUSION

Temporary in vitro treatment of human ovarian tissue with mTOR activators enhances the initiation of primordial follicle development and positively influences steroidogenesis after short-term culture.

Oocyte Quiescence: From Formation to Awakening

Decades of work using various model organisms have resulted in an exciting and emerging field of oocyte maturation. High levels of insulin and active mammalian target of rapamycin signals, indicative of a good nutritional environment, and hormones such as gonadotrophin, indicative of the growth of the organism, work together to control oocyte maturation to ensure that reproduction happens at the right timing under the right conditions. In the wild, animals often face serious challenges to maintain oocyte quiescence under long-term unfavorable conditions in the absence of mates or food. Failure to maintain oocyte quiescence will result in activation of oocytes at the wrong time and thus lead to exhaustion of the oocyte pool and sterility of the organism. In this review, we discuss the shared mechanisms in oocyte quiescence and awakening and a conserved role of noradrenergic signals in maintenance of the quiescent oocyte pool under unfavorable conditions in simple model organisms.

Premature ovarian insufficiency: pathogenesis and therapeutic potential of mesenchymal stem cell

Primary ovarian insufficiency (POI) is defined as a reduction in ovarian function before the expected age of menopause. POI is known to increase the risk of cardiovascular disorders, osteoporosis, cognitive decline, and mood disorders, resulting in a reduced quality of life. Appropriate hormone replacement for premenopausal women decreases these adverse health risks and improves quality of life for women with POI, but does not prolong life expectancy. The potential etiologies of POI include chromosomal abnormalities and genetic mutations, autoimmune factors, and iatrogenic causes, including surgery, chemotherapy, and radiation therapy. A major association is suggested to exist between reproductive longevity and the DNA damage pathway response genes. DNA damage and repair in ovarian granulosa cells is strongly associated with POI. Depletion of oocytes with damaged DNA occurs through different cell death mechanisms, such as apoptosis, autophagy, and necroptosis, mediated by the phosphatase and tensin homolog (PTEN)/phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/forkhead transcription factors 3 (FOXO3) pathway. Mesenchymal stem cells (MSCs) are characterized by the ability of self-renewal and differentiation and play an important role in the regeneration of injured tissues. Transplantation of MSCs has been shown to functionally restore ovarian reserve in a POI mouse model. Recent advances in stem cell therapy are likely to be translated to new therapeutic options bringing new hope to patients with POI. The aim of this review is to summarize the pathogenic mechanisms that involve cell death and DNA damage and repair pathways and to discuss the stem cell-based therapies as potential therapeutic options for this gynecologic pathology.

In vitro activation of cryopreserved ovarian tissue: A single-arm meta-analysis and systematic review

OBJECTIVE

Primordial follicles in premature ovarian failure (POF) patients are very difficult to be activated spontaneously, so that mature oocytes are difficult to be obtained for in vitro fertilization. The aim of our review is to analyze and to systematize the published data regarding effectiveness of different strategies for in vitro activation of cryopreserved ovarian tissue.

STUDY DESIGN

According to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a review of the literature was performed for all relevant full-text articles published in PubMed in English. Meta-analysis conducted using STATA 14.0. The random-effects model was used to combine 8 study results because the examination of heterogeneity was minimal.

RESULTS

One hundred and seventy seven patients after in vitro activation treatment (IVA) of ovarian tissue had accumulatively 26 pregnancies through IVF or natural pregnancy and then produced 18 live births. The random-effects model showed that the total clinical pregnancy and baby born rates reported in 8 studies evidence about effectiveness of IVA.

CONCLUSION

In vitro activation of primordial follicles as a new potential treatment for ovarian disorder patients, can be a promising option for fertility preservation. Drug-free activation of ovarian tissue in comparison with drug-included activation seemed to be more efficient.

Adoptive transfers of CD4+ CD25+ Tregs partially alleviate mouse premature ovarian insufficiency

This study was designed to investigate the protective effect of CD4⁺ CD25⁺ regulatory T cells (Tregs) against zona pellucida glycoprotein 3 peptide (pZP3) immunization-induced premature ovarian insufficiency (POI) in mice. A mouse POI model was induced by two subcutaneous injections of pZP3 (50 nmol/L). Mice in the pZP3-Treg group were intraperitoneally injected with 5 × 10⁵ CD4⁺ CD25⁺ Tregs after the POI model was established. Sex hormone levels, follicle numbers, apoptotic events, and the Akt/FOXO3a signaling pathway molecules in the ovaries were assessed. Compared with control group, the weight of ovaries in both pZP3 group and pZP3-Treg group was decreased and no difference was found between them. The number of follicles in the Treg transferred mice, like in pZP3 group, was significantly reduced compared to the control group, but showed a modest improvement when compared the pZP3 group alone. Significantly lower serum concentrations of follicle-stimulating hormone, luteinizing hormone, and anti-zona pellucida antibodies (AZPAbs) were found, while the concentrations of estradiol and anti-Mullerian hormone increased. In mechanism, Treg cell transfer to ZP3 treated mice restored the levels of Caspase3 to control levels, and partially restored Bax, however, had no effect on Bcl-2. Moreover, Treg cell transfer to ZP3 treated mice partially restored the levels of Akt and FOXO3a, and partially restored the ratios of p-Akt/Akt and p-FOXO3a/FOXO3a. In conclusion, Treg cells improved some aspects of ZP3-induced POI which may be mediate by suppressing ovarian cells apoptosis and involving the Akt/FOXO3a signaling pathway. Therefore, Treg cells may be protective against autoimmune POI.

PTEN and FOXO3 expression in the prenatal and postnatal human ovary

PURPOSE

The objective of this study was to analyse the expression and cellular localization of FOXO3, pFOXO3 and PTEN throughout human ovary development both before and after birth.

METHODS

Foetal, pubertal and adult paraffin-embedded ovarian samples were analysed by immunohistochemistry for cellular localization of FOXO3, pFOXO3 and PTEN proteins. Protein and mRNA expression were analysed by western blot and real time PCR, respectively, from fresh biopsies.

RESULTS

PTEN was not detected by immunohistochemistry in germ cells and follicles of foetal, pubertal and adult ovaries. Occasional PTEN immunoreactive granulosa cells were found in atretic antral follicles in the adult ovary. Western blot analysis showed low levels of PTEN protein. Nuclear FOXO3-expressing primordial follicles represented a variable proportion of the ovarian reserve. The presence of FOXO3-expressing primordial follicles was very low in foetal ovary; although always represented in a low proportion, prevalence increased during pubertal and adult life.

CONCLUSION

Our results seem to indicate that two subpopulations of primordial follicles, i.e. nuclear FOXO3-expressing and no FOXO3-expressing primordial follicles are found in the postnatal human ovary. This scenario suggests that FOXO3 could be acting as in the mouse model, preventing primordial follicle activation. However, the strategy would not be an "all or nothing" system as in mouse ovary but rather a selected subpopulation of primordial follicles preserved to ensure long-term fertility.

Transcriptome of tambaqui Colossoma macropomum during gonad differentiation: Different molecular signals leading to sex identity

Tambaqui (Colossoma macropomum) is the major native species in Brazilian aquaculture, and we have shown that females exhibit a higher growth compared to males, opening up the possibility for the production of all-female population. To date, there is no information on the sex determination and differentiation molecular mechanisms of tambaqui. In the present study, transcriptome sequencing of juvenile trunks was performed to understand the molecular network involved in the gonadal sex differentiation. The results showed that before differentiation, components of the Wnt/β-catenin pathway, fox and fst genes imprint female sex development, whereas antagonistic pathways (gsk3b, wt1 and fgfr2), sox9 and genes for androgen synthesis indicate male differentiation. Hence, in undifferentiated tambaqui, the Wnt/β-catenin exerts a role on sex differentiation, either upregulated in female-like individuals, or antagonized in male-like individuals.

Periovulatory insulin signaling is essential for ovulation, granulosa cell differentiation, and female fertility

Recent studies have demonstrated an essential role for insulin signaling in folliculogenesis as conditional ablation of Igf1r in primary follicles elicits defective follicle-stimulating hormone responsiveness blocking development at the preantral stage. Thus the potential role of insulin action in the periovulatory window and in the corpus luteum is unknown. To examine this, we generated conditional Insr,Igf1r, and double receptor knockout mice driven by Pgr-Cre. These models escape the preantral follicle block and in response to superovulatory gonadotropins exhibit normal distribution of ovarian follicles and corpora lutea. However, single ablation of Igf1r leads to subfertility and mice lacking both receptors are infertile. Double knockout mice have impaired oocyte development and ovulation. While some oocytes are released and fertilized, subsequent embryo development is retarded, and the embryos potentially fail to thrive due to lack of luteal support. In support of this, we found reduced expression of key enzymes in the steroid synthesis pathway and reduced serum progesterone. In addition to metabolic and steroidogenic pathways, RNA-sequencing analysis revealed transcription factor-3 as an important transcription factor downstream of insulin signaling. Collectively, these results highlight the importance of growth factors of the insulin family during two distinct windows of follicular development, ovulation, and luteinization.

Primordial follicle reserve, DNA damage and macrophage infiltration in the ovaries of the long-living Ames dwarf mice

The aim of this study was to evaluate the effect of growth hormone (GH) deficiency in primordial follicle reserve, DNA damage and macrophage infiltration in the ovaries of young mice. Ovaries from six-month-old GH-deficient Ames Dwarf (df/df) and Normal (N/df) mice were used. The number of primordial follicles was higher in df/df mice (p = 0.0026). Also, df/df mice had a lower number of primary (p = 0.023), secondary (p = 0.0052) and tertiary (p = 0.019) follicles. These findings indicate a slower rate of primordial follicle activation in df/df mice. Female df/df mice had decreased γH2AX foci intensity in oocytes of primordial (p = 0.015) and primary (p = 0.0004) follicles compared to N/df mice. Also, df/df mice had reduced γH2AX intensity in granulosa cells of primordial (p = 0.0002) and primary (p < 0.0001) follicles. Overall, this indicate to us that df/df mice accumulate less DNA damage in the ovarian reserve compared to N/df mice. Additionally, macrophage infiltration was also reduced in ovaries of df/df mice compared to N/df mice (p = 0.033). Interestingly, df/df mice had a reduced number of granulosa cells around primordial (p = 0.0024) and primary (p = 0.007) follicles compared to N/df mice. Also, df/df mice had a small diameter of primordial follicle nuclei (p = 0.0093), secondary follicle oocyte (p = 0.046) and tertiary follicle (p = 0.012). This points to the role of granulosa cell proliferation and oocyte growth for primordial follicle activation. The current study points to the role of the GH/IGF-I axis in extending lifespan of reproductive health, along with maintenance of oocyte DNA integrity and reduced ovarian inflammation.

Crosstalk between PTEN/PI3K/Akt Signalling and DNA Damage in the Oocyte: Implications for Primordial Follicle Activation, Oocyte Quality and Ageing

The preservation of genome integrity in the mammalian female germline from primordial follicle arrest to activation of growth to oocyte maturation is fundamental to ensure reproductive success. As oocytes are formed before birth and may remain dormant for many years, it is essential that defence mechanisms are monitored and well maintained. The phosphatase and tensin homolog of chromosome 10 (PTEN)/phosphatidylinositol 3-kinase (PI3K)/protein kinase B (PKB, Akt) is a major signalling pathway governing primordial follicle recruitment and growth. This pathway also contributes to cell growth, survival and metabolism, and to the maintenance of genomic integrity. Accelerated primordial follicle activation through this pathway may result in a compromised DNA damage response (DDR). Additionally, the distinct DDR mechanisms in oocytes may become less efficient with ageing. This review considers DNA damage surveillance mechanisms and their links to the PTEN/PI3K/Akt signalling pathway, impacting on the DDR during growth activation of primordial follicles, and in ovarian ageing. Targeting DDR mechanisms within oocytes may be of value in developing techniques to protect ovaries against chemotherapy and in advancing clinical approaches to regulate primordial follicle activation.

Insulin-like growth factor-1 (IGF-1) promotes primordial follicle growth and reduces DNA fragmentation through the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) signalling pathway

We investigated the effects of insulin-like growth factor 1 (IGF-1) on the morphology and follicular activation of ovine preantral follicles cultured in situ and whether the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) pathway is involved in IGF-1 action in the sheep ovary. Ovine ovarian fragments were fixed for histological and terminal deoxynucleotidyl transferase dUTP nick-end labelling (TUNEL) analyses (fresh control) or cultured in supplemented alpha-minimum essential medium (α-MEM+; control) or α-MEM+ with IGF-1 (1, 10, 50, 100 or 200ngmL-1) for 7 days. Follicles were classified as normal or atretic, primordial or growing and the oocyte and follicle diameters were measured. DNA fragmentation was evaluated by TUNEL assay. Proliferating cell nuclear antigen (PCNA) immunohistochemistry was performed on the fresh control, α-MEM+ and 100ngmL-1 IGF-1 samples. Inhibition of PI3K activity was performed through pretreatment with the PI3K inhibitor LY294002 and phosphorylated AKT (pAKT) expression was analysed after culture in the absence or presence of LY294002. IGF-1 at 100ngmL-1 increased (P<0.05) follicular activation compared with α-MEM+ and decreased TUNEL-positive cells (P<0.05) compared with other treatments. PCNA-positive cells also increased (P<0.05) in 100ngmL-1 IGF-1. LY294002 significantly inhibited follicular activation stimulated by α-MEM+ and 100ngmL-1 IGF-1 and reduced pAKT expression in follicles. Overall, IGF-1 at 100ngmL-1 promoted primordial follicle activation, cell proliferation and reduced DNA fragmentation after in situ culture through the PI3K/AKT pathway.

Involvement of Bone Morphogenetic Proteins (BMP) in the Regulation of Ovarian Function

Primordial germ cells migrate to the fetal gonads and proliferate during gestation to generate a fixed complement of primordial follicles, the so-called ovarian reserve. Primordial follicles comprise an oocyte arrested at the diplotene stage of meiosis, surrounded by a layer of pregranulosa cells. Activation of primordial follicles to grow beyond this arrested stage is of particular interest because, once activated, they are subjected to regulatory mechanisms involved in growth, selection, maturation, and ultimately, ovulation or atresia. The vast majority of follicles succumb to atresia and are permanently lost from the quiescent or growing pool of follicles. The bone morphogenetic proteins (BMPs), together with other intraovarian growth factors, are intimately involved in regulation of follicle recruitment, dominant follicle selection, ovulation, and atresia. Activation of primordial follicles appears to be a continuous process, and the number of small antral follicles at the beginning of the menstrual cycle provides an indirect indication of ovarian reserve. Continued antral follicle development during the follicular phase of the menstrual cycle is driven by follicle stimulating hormone (FSH) and luteinizing hormone (LH) in conjunction with many intraovarian growth factors and inhibitors interrelated in a complex web of regulatory balance. The BMP signaling system has a major intraovarian role in many species, including the human, in the generation of transcription factors that influence proliferation, steroidogenesis, cell differentiation, and maturation prior to ovulation, as well as formation of corpora lutea after ovulation. At the anterior pituitary level, BMPs also contribute to the regulation of gonadotrophin production.

Ovarian aging and the activation of the primordial follicle reserve in the long-lived Ames dwarf and the short-lived bGH transgenic mice

The aim of this study was to evaluate the effect of growth hormone (GH) in the maintenance of the ovarian primordial follicle reserve. Ovaries from 16 mo old GH-deficient Ames Dwarf (df/df) and Normal (N/df) mice were used. A subgroup of df/df and N mice received GH or saline injections for six weeks starting at 14 mo of age. In addition, ovaries from 12 mo old mice overexpressing bovine GH (bGH) and controls were used. df/df mice had higher number of primordial and total follicles than N/df mice (p < 0.05), while GH treatment decreased follicle counts in both genotypes (p < 0.05). In addition, bGH mice had lower number of primordial and total follicles than the controls (p < 0.05). pFoxO3a levels were higher in mice treated with GH and in bGH mice (p < 0.05) when comparing with age match controls. These results indicate that increased circulating GH is associated with a reduced ovarian primordial follicle reserve and increased pFoxO3a content in oocytes.

Low expression of SEMA6C accelerates the primordial follicle activation in the neonatal mouse ovary

The primordial follicle assembly, activation and the subsequent development are critical processes for female reproduction. A limited number of primordial follicles are activated to enter the growing follicle pool each wave, and the primordial follicle pool progressively diminishes over a woman's life‐time. The number of remaining primordial follicles represents the ovarian reserve. Identification and functional investigation of the factors involved in follicular initial recruitment will be of great significance to the understanding of the female reproduction process and ovarian ageing. In this study, we aimed to study whether and how semaphorin 6C (Sema6c) regulated the primordial follicle activation in the neonatal mouse ovary. The attenuation of SEMA6C expression by SiRNA accelerated the primordial follicle activation in the in vitro ovary culture system. PI3K‐AKT‐rpS6 pathway was activated when SEMA6C expression was down‐regulated. And the LY294002 could reverse the effect of low SEMA6C expression on primordial follicle activation. Our findings revealed that Sema6c was involved in the activation of primordial follicles, and the down‐regulation of SEMA6C led to massive primordial follicle activation by interacting with the PI3K‐AKT‐rpS6 pathway, which might also provide valuable information for understanding premature ovarian failure and ovarian ageing.

Single nucleotide polymorphisms at miR-146a/196a2 and their primary ovarian insufficiency-related target gene regulation in granulosa cells

MicroRNAs post-transcriptionally regulate gene expression in animals and plants. The aim of this study was to identify new target genes for microRNA polymorphisms (miR-146aC>G and miR-196a2T>C) in primary ovarian insufficiency (POI). We cloned and transfected miR-146aC>G and miR-196a2T>C into human granulosa cells and used microarrays and qPCR-arrays to examine the changes in the messenger RNA expression profile. We show miR-146aC>G and miR-196a2T>C change the mRNA expression patterns in granulosa cell. In each case, mRNAs were up or down-regulated after treatments with miR-146a C or G and miR-196a2 T or C. We found that miR-146a led to a significantly altered regulation of the mRNA levels of FOXO3, FOXL2 and CCND2 compared to controls. We also found that the polymorphisms of miR-146a led to a significantly altered regulation of CCND2 and FOXO3. Our results suggest that miR-146aC>G and miR-196a2T>C can regulate the levels of many of their target transcripts. In addition, specific target genes of miR-146aC>G polymorphisms may be involved in granulosa cell regulation.

Conserved insulin signaling in the regulation of oocyte growth, development, and maturation

Insulin signaling regulates various aspects of physiology, such as glucose homeostasis and aging, and is a key determinant of female reproduction in metazoans. That insulin signaling is crucial for female reproductive health is clear from clinical data linking hyperinsulinemic and hypoinsulinemic condition with certain types of ovarian dysfunction, such as altered steroidogenesis, polycystic ovary syndrome, and infertility. Thus, understanding the signaling mechanisms that underlie the control of insulin-mediated ovarian development is important for the accurate diagnosis of and intervention for female infertility. Studies of invertebrate and vertebrate model systems have revealed the molecular determinants that transduce insulin signaling as well as which biological processes are regulated by the insulin-signaling pathway. The molecular determinants of the insulin-signaling pathway, from the insulin receptor to its downstream signaling components, are structurally and functionally conserved across evolution, from worms to mammals-yet, physiological differences in signaling still exist. Insulin signaling acts cooperatively with gonadotropins in mammals and lower vertebrates to mediate various aspects of ovarian development, mainly owing to evolution of the endocrine system in vertebrates. In contrast, insulin signaling in Drosophila and Caenorhabditis elegans directly regulates oocyte growth and maturation. In this review, we compare and contrast insulin-mediated regulation of ovarian functions in mammals, lower vertebrates, C. elegans, and Drosophila, and highlight conserved signaling pathways and regulatory mechanisms in general while illustrating insulin's unique role in specific reproductive processes.

MAPK3/1 participates in the activation of primordial follicles through mTORC1-KITL signaling

The majority of ovarian primordial follicles are preserved in a dormant state to maintain the female reproductive lifespan, and only a few primordial follicles are activated to enter the growing follicle pool in each wave. Recent studies have shown that primordial follicular activation depends on mammalian target of rapamycin complex 1 (mTORC1)-KIT ligand (KITL) signaling in pre-granulosa cells and its receptor (KIT)-phosphoinositol 3 kinase (PI3K) signaling in oocytes. However, the upstream regulator of mTORC1 signaling is unclear. The results of the present study showed that the phosphorylated mitogen-activated protein kinase3/1 (MAPK3/1) protein is expressed in some primordial follicles and all growing follicles. Culture of 3 days post-parturition (dpp) ovaries with the MAPK3/1 signaling inhibitor U0126 significantly reduced the number of activated follicles and was accompanied by dramatically reduced granulosa cell proliferation and increased oocyte apoptosis. Western blot and immunofluorescence analyses showed that U0126 significantly decreased the phosphorylation levels of Tsc2, S6K1, and rpS6 and the expression of KITL, indicating that U0126 inhibits mTORC1-KITL signaling. Furthermore, U0126 decreased the phosphorylation levels of Akt, resulting in a decreased number of oocytes with Foxo3 nuclear export. To further investigate MAPK3/1 signaling in primordial follicle activation, we used phosphatase and tensin homolog deleted on chromosome 10 (PTEN) inhibitor bpV(HOpic) to promote primordial follicle activation. In this model, U0126 also inhibited the activation of primordial follicles and mTORC1 signaling. Thus, these results suggest that MAPK3/1 participates in primordial follicle activation through mTORC1-KITL signaling.

Ovarian transcriptome associated with reproductive senescence in the long-living Ames dwarf mice

The aim of the current work was to evaluate the ovarian follicle reserve and the ovarian transcriptome in Ames dwarf (df/df) mice. The results suggest a delayed ovarian aging in df/df mice compared to normal (N) mice. Although a high number of genes were differentially expressed during aging of N mice, only a small fraction of these changed with aging in df/df mice. These alterations involved more than 500 categorized biological processes. The majority of these biological processes, including inflammatory/immune responses, were up-regulated with aging in N mice, while old df/df mice were characterized by down-regulation of these same processes in comparison to age matched N mice. However, biological processes related to DNA damage and repairing were commonly down-regulated with aging in both genotypes. In conclusion, delayed ovarian aging in long-living df/df mice was associated with reduced expression of genes related to the inflammatory and immune responses.

Effects of the IGF-1/PTEN/Akt/FoxO signaling pathway on male reproduction in rats subjected to water immersion and restraint stress

The aim of the present study was to determine the effects of the insulin-like growth factor 1 (IGF-1)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/forkhead box (FoxO) signaling pathway on male reproduction in rats subjected to water immersion and restraint stress (WRS). Sperm morphology, sperm malformation rate, and serum testosterone concentration were analyzed following WRS. In addition, the expression levels and immunolocalization of IGF-1, PTEN, Akt and FoxO proteins, as well as the rate of cell apoptosis in rat testes, were investigated. The results indicated that sperm malformation rate, serum testosterone concentration, and the number of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive cells were increased in the testes after WRS. Furthermore, IGF-1 and FoxO1 proteins were predominantly localized in the sperm cytoplasm during the late stages of spermatogenesis. FoxO1 protein was also localized in Leydig cell cytoplasm. PTEN and total Akt proteins were predominantly expressed in the cytoplasm of Leydig cells and spermatogonia. PTEN protein was also detected in vascular endothelial cells. In addition, IGF-1, PTEN, Akt1, Akt2, FoxO3 and FoxO4 gene expression levels were upregulated following WRS, and peaked after 7 h of WRS. During the recovery period, the expression levels of these genes gradually returned to normal levels. The present study demonstrated that WRS induced sperm damage in the testes. In addition, the results indicated that the IGF-1/PTEN/Akt/FoxO signaling pathway may serve an anti-stress role in the testes of rats subjected to WRS.

Effect of high-fat diet-induced obesity on the Akt/FoxO/Smad signaling pathway and the follicular development of the mouse ovary

Obesity has a negative effect on ovarian functions, which is reported to increase the risk of infertility. The mechanism underlying obesity‑induced infertility is not yet clear. The present study established a high‑fat diet (HFD)‑induced obesity mouse model to elucidate the mechanisms underlying the effect of HFD‑induced obesity on follicular development in the mouse ovary. The 4‑week‑old female mice were fed with HFD or normal control (NC) diet for 15 or 20 weeks. Body mass index was used to demonstrate that the mice were obese following HFD treatment. The follicular development of the ovaries from the HFD group mice was retarded in a time‑dependent manner, as demonstrated by morphological and histological examination of the ovaries. Further investigation via western blot analysis demonstrated that the activity of the transcription factor, forkhead box O3a (FoxO3a), was increased by HFD through downregulated FoxO3a phosphorylation, which may contribute to the inhibited development of ovarian follicles. To determine the regulatory mechanism of FoxO3 on the follicular development, the expression levels of FoxO3a target protein, Smad1/5/8, were also determined and there was significant decrease in phosphorylated Smad1/5/8 in the ovaries from the HFD group compared with the NC group, indicating that FoxO3a/Smad1/5/8 may be important in the regulation of follicular development. The expression levels of the upstream regulator of FoxO3a, Akt, were also examined and it was demonstrated that Akt phosphorylation was significantly reduced in the HFD group compared with the NC group, indicating that Akt/FoxO3a may be also involved in follicular development. Together, the experiments demonstrated that HFD‑induced obesity affected the activity of the Akt/FoxO3a/Smad1/5/8 signaling pathway in a time‑dependent manner during the follicular development of the mouse ovary, leading to abnormal follicular development. These findings may provide part of a theoretical basis for the clinical prevention and treatment of obesity-associated female infertility.

Control of Oocyte Reawakening by Kit

In mammals, females are born with finite numbers of oocytes stockpiled as primordial follicles. Oocytes are “reawakened” via an ovarian-intrinsic process that initiates their growth. The forkhead transcription factor Foxo3 controls reawakening downstream of PI3K-AKT signaling. However, the identity of the presumptive upstream cell surface receptor controlling the PI3K-AKT-Foxo3 axis has been questioned. Here we show that the receptor tyrosine kinase Kit controls reawakening. Oocyte-specific expression of a novel constitutively-active Kit^D818V allele resulted in female sterility and ovarian failure due to global oocyte reawakening. To confirm this result, we engineered a novel loss-of-function allele, Kit^L. Kit inactivation within oocytes also led to premature ovarian failure, albeit via a contrasting phenotype. Despite normal initial complements of primordial follicles, oocytes remained dormant with arrested oocyte maturation. Foxo3 protein localization in the nucleus versus cytoplasm explained both mutant phenotypes. These genetic studies provide formal genetic proof that Kit controls oocyte reawakening, focusing future investigations into the causes of primary ovarian insufficiency and ovarian aging.

In mammals, oocyte reawakening controls female fertility, the onset of the menopause, and thus, overall aging. We demonstrate here through complementary genetic experiments that Kit is the upstream receptor regulating oocyte reawakening. Although other cell surface receptors have been proposed as candidates, the data have remained contradictory, and definitive genetic evidence in support of any one receptor has been lacking. We engineered two novel Kit alleles in mice, one an activating (gain-of-function) mutation, the other an inactivating (loss-of-function) mutation. These alleles permitted us to conduct elegant genetic experiments whereby Kit was activated or inactivated in the oocytes of newborn mice. The results were complementary and striking. Oocyte-specific Kit activation resulted in female sterility due to reawakening of all oocytes, leading to premature ovarian failure. In contrast, Kit inactivation also led to female sterility and ovarian failure, but through a contrasting and opposite phenotype: a complete failure of primordial follicle reawakening. Additional studies demonstrated that Foxo3, a known regulator of reawakening, was the mediator of both phenotypes, linking our findings to prior discoveries. These complementary genetic experiments thus definitively incriminate Kit as the upstream receptor regulating reawakening.

Inhibitors of c-Jun phosphorylation impede ovine primordial follicle activation

STUDY HYPOTHESIS

Is the c-Jun-N-terminal kinase (JNK) pathway implicated in primordial follicle activation?

STUDY FINDING

Culture of ovine ovarian cortex in the presence of two different c-Jun phosphorylation inhibitors impeded pre-antral follicle activation.

WHAT IS KNOWN ALREADY

Despite its importance for fertility preservation therapies, the mechanisms of primordial follicle activation are poorly understood. Amongst different signalling pathways potentially involved, the JNK pathway has been previously shown to be essential for cell cycle progression and pre-antral follicle development in mice.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Ovine ovarian cortex pieces were cultured with varying concentrations of SP600125, JNK inhibitor VIII or anti-Mullerian hormone (AMH) in the presence of FSH for 9 days. Follicular morphometry and immunohistochemistry for proliferating cell nuclear antigen (PCNA), apoptosis and follicle activation (Foxo3a) were assessed.

MAIN RESULTS AND THE ROLE OF CHANCE

Inhibition of primordial follicle activation occurred in the presence of SP600125, JNK inhibitor VIII and AMH when compared with controls (all P < 0.05) after 2 days of culture. However, only in the highest concentrations used was the inhibition of activation associated with induction of follicular apoptosis (P < 0.05). In growing follicles, PCNA antigen expression was reduced when the JNK inhibitors or AMH were used (P < 0.05 versus control), indicating reduced proliferation of the somatic compartment.

LIMITATIONS, REASONS FOR CAUTION

Although we evaluated the effects of inhibition of c-Jun phosphorylation on primordial follicle development, we did not determine the cellular targets and mechanism of action of the inhibitors.

WIDER IMPLICATIONS OF THE FINDINGS

These results are the first to implicate the JNK pathway in primordial follicle activation and could have significant consequences for the successful development of fertility preservation strategies and our understanding of primordial follicle activation.

LARGE SCALE DATA

n/a.

STUDY FUNDING AND COMPETING INTERESTS

Dr Michael J. Bertoldo and the laboratories involved in the present study were supported by a grant from 'Région Centre' (CRYOVAIRE, Grant number #320000268). There are no conflicts of interest to declare.

Regulation of FOXO3 subcellular localization by Kit ligand in the neonatal mouse ovary

PURPOSE

Foxo3 protein is required in the oocyte nucleus for the maintenance of primordial follicles in a dormant state. PI3K/AKT-dependent phosphorylation of Foxo3 leads to its relocalization to the cytoplasm and subsequent follicular activation. However, the nature of the upstream signals controlling Foxo3 activity and subcellular localization remains unknown. We aimed to study the in vitro effects of Kit ligand (stem cell factor) on the subcellular localization of Foxo3 in primordial follicles within the postnatal mouse ovary.

METHODS

This was an in vitro study using explants of intact neonatal mouse ovaries. The study was performed in laboratory animal facility and basic science research laboratory at a University Hospital. The animals used for this study were FVB mice. Neonatal FVB mice ovaries at postnatal day 7 (PD7) were harvested and incubated in culture medium (DMEM) at 37 °C and 5 % CO(2) for 60-90 min with (n = 3) or without (n = 3) Kit ligand at 150 ng/mL (8 nM). Similar experimental conditions were used to establish a dose-response curve for the effects of Kit ligand and assess the effects of imatinib (small molecule inhibitor of the Kit receptor). Immunofluorescence was used to identify the subcellular location of Foxo3 in oocytes. Proportions of cytoplasmic versus nuclear Foxo3 in primordial follicles were determined.

RESULTS

Kit ligand treatment increased the cytoplasmic localization of Foxo3 from 40 % in the untreated ovaries to 74 % in the treated group (p = 0.007 in paired samples and p = 0.03 in unpaired samples). Furthermore, this effect was reversible with imatinib (p = 0.005). A dose-response curve for Kit ligand treatment showed that maximum effect was seen at 150 ng/mL.

CONCLUSION

Kit ligand treatment in vitro increases the proportion of cytoplasmic Foxo3 in primordial follicles at PD7, lending support to the idea that Kit receptor/ligand controls Foxo3 activity in the context of primordial follicle activation.

FOXO1, FOXO3, AND FOXO4 are differently expressed during mouse oocyte maturation and preimplantation embryo development

Preimplantation embryo development is affected by its environment. FoxO transcription factors are regulated by PI3K/Akt signaling pathway that essentially supports growth and development. FoxO transcription factors are at the interface of crucial cellular processes, orchestrating programs of gene expression that regulate apoptosis, cell-cycle arrest, oxidative stress resistance, DNA repair, glucose metabolism, and differentiation. In the presence of growth factors, FoxO transcription factors are localized in the cytoplasm, whereas under stress conditions they move to the nucleus and trigger transcriptional activities of their target genes. The aim of the present study is to investigate whether FoxO transcription factors are present during in vivo oocyte maturation and preimplantation embryo development. Presence and localizations of FoxO1, FoxO3 and FoxO4 proteins have been determined with immunofluorescence staining. Our results have confirmed that FoxO1, FoxO3 and FoxO4 proteins are differentially expressed in prophase I, metaphase I, metaphase II oocytes, as well as in fertilized oocyte, 2-cell embryo, 4-cell embryo, 8-cell embryo, morula, and blastocyst. FoxOs translocate to nucleus in embryos with developmental delay. Our findings indicate that FoxO transcription factors are present during both oocyte and embryo in vivo maturation and provide fundamental knowledge that FoxOs may regulate in vitro embryo development under stress conditions.

Cell autonomous phosphoinositide 3-kinase activation in oocytes disrupts normal ovarian function through promoting survival and overgrowth of ovarian follicles

In this study, we explored the effects of oocytic phosphoinositide 3-kinase (PI3K) activation on folliculogensis by generating transgenic mice, in which the oocyte-specific Cre-recombinase induces the expression of constitutively active mutant PI3K during the formation of primordial follicles. The ovaries of neonatal transgenic (Cre+) mice showed significantly reduced apoptosis in follicles, which resulted in an excess number of follicles per ovary. Thus, the elevation of phosphatidylinositol (3,4,5)-trisphosphate levels within oocytes promotes the survival of follicles during neonatal development. Despite the increase in AKT phosphorylation, primordial follicles in neonatal Cre+ mice remained dormant demonstrating a nuclear accumulation of phosphatase and tensin homolog deleted on chromosome 10 (PTEN). These primordial follicles containing a high level of nuclear PTEN persisted in postpubertal females, suggesting that PTEN is the dominant factor in the maintenance of female reproductive lifespan through the regulation of primordial follicle recruitment. Although the oocytic PI3K activity and PTEN levels were elevated, the activation of primordial follicles and the subsequent accumulation of antral follicles with developmentally competent oocytes progressed normally in prepubertal Cre+ mice. However, mature Cre+ female mice were anovulatory. Because postnatal day 50 Cre+ mice released cumulus-oocyte complexes with developmentally competent oocytes in response to super-ovulation treatment, the anovulatory phenotype was not due to follicular defects but rather endocrine abnormalities, which were likely caused by the excess number of overgrown follicles. Our current study has elucidated the critical role of oocytic PI3K activity in follicular function, as well as the presence of a PTEN-mediated mechanism in the prevention of immature follicle activation.

PI3K/PTEN/Akt and TSC/mTOR signaling pathways, ovarian dysfunction, and infertility: an update

Abnormalities in ovarian function, including defective oogenesis and folliculogenesis, represent a key female reproductive deficiency. Accumulating evidence in the literature has shown that the PI3K/PTEN/Akt and TSC/mTOR signaling pathways are critical regulators of ovarian function including quiescence, activation, and survival of primordial follicles, granulosa cell proliferation and differentiation, and meiotic maturation of oocytes. Dysregulation of these signaling pathways may contribute to infertility caused by impaired follicular development, intrafollicular oocyte development, and ovulation. This article reviews the current state of knowledge of the functional role of the PI3K/PTEN/Akt and TSC/mTOR pathways during mammalian oogenesis and folliculogenesis and their association with female infertility.

Genome-wide identification, phylogeny, and gonadal expression of fox genes in Nile tilapia, Oreochromis niloticus

The fox genes play important roles in various biological processes, including sexual development. In the present study, we isolated 65 fox genes, belonging to 18 subfamilies named A-R, from Nile tilapia through genome-wide screening. Twenty-four of them have two or three (foxm1) copies. Furthermore, 16, 25, 68, and 45 fox members were isolated from nematodes, protochordates, teleosts, and tetrapods, respectively. Phylogenetic analyses indicated fox gene family had undergone three expansions parallel to the three rounds of genome duplication during evolution. We also analyzed the clustered fox genes and found that apparent linkage duplication existed in teleosts, which further supported fish-specific genome duplication hypothesis. In addition, species- and lineage-specific duplication is another reason for fox gene family expansion. Based on the four pairs of XX and XY gonadal transcriptome data from four critical developmental stages, we analyzed the expression profile of all fox genes and identified sexually dimorphic fox genes at each stage. All fox genes were detected in gonads, with 15 of them at the background expression level (total read per kb per million reads, RPKM < 10), 29 at moderate expression level (10 < total RPKM < 100), and 21 at high expression level (total RPKM > 100). There are 27, 24, 28, and 9 sexually dimorphic fox genes at 5, 30, 90, and 180 days after hatching (dah), respectively. foxq1a, foxf1, foxr1, and foxr1 were identified as the most differentially expressed genes at each stage. foxl2 was characterized as XX-dominant gene, while foxd5, foxi3, foxn3, foxj1a, foxj3b, and foxo6b were characterized as XY-dominant genes. qPCR and in situ hybridization of foxh1 and foxj1a were performed to confirm the expression profiles and to validate the transcriptome data. Our results suggest that fox genes might play important roles in sex determination and gonadal development in teleosts.

The small GTPase Rheb is required for spermatogenesis but not oogenesis

The process of germ cell development is under the tight control of various signaling pathways, among which the PI3K-Akt-mTOR pathway is of critical importance. Previous studies have demonstrated sex-specific roles for several components of this pathway. In the current study, we aimed to evaluate the role of Rheb, a member of the small GTPase superfamily and a critical component for mTORC1 activation, in male and female gametogenesis. The function of Rheb in development and the nervous system has been extensively studied, but little is known about its role in the germ line. We have exploited genetic approaches in the mouse to study the role of Rheb in the germ line and have identified an essential role in spermatogenesis. Conditional knockout (cKO) of Rheb in the male germ line resulted in severe oligoasthenoteratozoospermia and male sterility. More detailed phenotypic analyses uncovered an age-dependent meiotic progression defect combined with subsequent abnormalities in spermiogenesis as evidenced by abnormal sperm morphology. In the female, however, germ-cell specific inactivation of Rheb was not associated with any discernible abnormality; these cKO mice were fertile with morphologically unremarkable ovaries, normal primordial follicle formation, and subsequent follicle maturation. The absence of an abnormal ovarian phenotype is striking given previous studies demonstrating a critical role for the mTORC1 pathway in the maintenance of primordial follicle pool. In conclusion, our findings demonstrate an essential role of Rheb in diverse aspects of spermatogenesis but suggest the existence of functionally redundant factors that can compensate for Rheb deficiency within oocytes.

Ovarian germline stem cells: an unlimited source of oocytes?

While there has been progress in directing the development of embryonic stem cells and induced pluripotent stem cells toward a germ cell state, their ability to serve as a source of functional oocytes in a clinically relevant model or situation has yet to be established. Recent studies suggest that the adult mammalian ovary is not endowed with a finite number of oocytes, but instead possesses stem cells that contribute to their renewal. The ability to isolate and promote the growth and development of such ovarian germline stem cells (GSCs) would provide a novel means to treat infertility in women. Although such ovarian GSCs are well characterized in nonmammalian model organisms, the findings that support the existence of adult ovarian GSCs in mammals have been met with considerable evidence that disputes their existence. This review details the lessons provided by model organisms that successfully utilize ovarian GSCs to allow for a continual and high level of female germ cell production throughout their life, with a specific focus on the cellular mechanisms involved in GSC self-renewal and oocyte development. Such an overview of the role that oogonial stem cells play in maintaining fertility in nonmammalian species serves as a backdrop for the data generated to date that supports or disputes the existence of GSCs in mammals as well as the future of this area of research in terms of its potential for any application in reproductive medicine.

Transforming growth factor-β signaling participates in the maintenance of the primordial follicle pool in the mouse ovary

Physiologically, only a few primordial follicles are activated to enter the growing follicle pool each wave. Recent studies in knock-out mice show that early follicular activation depends on signaling from the tuberous sclerosis complex, the mammalian target of rapamycin complex 1 (mTORC1), phosphatase and tensin homolog deleted on chromosome 10, and phosphatidylinositol 3-kinase (PI3K) pathways. However, the manner in which these pathways are normally regulated, and whether or not TGF-β acts on them are poorly understood. So, this study aims to identify whether or not TGF-β acts on the process. Ovary organ culture experiments showed that the culture of 18.5 days post-coitus (dpc) ovaries with TGF-β1 reduced the total population of oocytes and activated follicles, accelerated oocyte growth was observed in ovaries treated with TGF-βR1 inhibitor 2-(5-chloro-2-fluorophenyl)pteridin-4-yl]pyridin-4-yl-amine (SD208) compared with control ovaries, the down-regulation of TGF-βR1 gene expression also activated early primordial follicle oocyte growth. We further showed that there was dramatically more proliferation of granulosa cells in SD208-treated ovaries and less proliferation in TGF-β1-treated ovaries. Western blot and morphological analyses indicated that TGF-β signaling manipulated primordial follicle growth through tuberous sclerosis complex/mTORC1 signaling in oocytes, and the mTORC1-specific inhibitor rapamycin could partially reverse the stimulated effect of SD208 on the oocyte growth and decreased the numbers of growing follicles. In conclusion, our results suggest that TGF-β signaling plays an important physiological role in the maintenance of the dormant pool of primordial follicles, which functions through activation of p70 S6 kinase 1 (S6K1)/ribosomal protein S6 (rpS6) signaling in mouse ovaries.

Genomic markers of ovarian reserve

Ovarian reserve and its utilization, over a reproductive life span, are determined by genetic, epigenetic, and environmental factors. The establishment of the primordial follicle pool and the rate of primordial follicle activation have been under intense study to determine genetic factors that affect reproductive lifespan. Much has been learned from transgenic animal models about the developmental origins of the primordial follicle pool and mechanisms that lead to primordial follicle activation, folliculogenesis, and the maturation of a single oocyte with each menstrual cycle. Recent genome-wide association studies on the age of human menopause have identified approximately 20 loci, and shown the importance of factors involved in double-strand break repair and immunology. Studies to date from animal models and humans show that many genes determine ovarian aging, and that there is no single dominant allele yet responsible for depletion of the ovarian reserve. Personalized genomic approaches will need to take into account the high degree of genetic heterogeneity, family pedigree, and functional data of the genes critical at various stages of ovarian development to predict women's reproductive life span.

Gonadal expression of Foxo1, but not Foxo3, is conserved in diverse Mammalian species

The Foxos are key effectors of the PI3K/Akt signaling pathway and regulate diverse physiologic processes. Two of these factors, Foxo1 and Foxo3, serve specific roles in reproduction in the mouse. Foxo3 is required for suppression of primordial follicle activation in females, while Foxo1 regulates spermatogonial stem cell maintenance in males. In the mouse ovary, Foxo1 is highly expressed in somatic cells (but not in oocytes), suggesting an important functional role for Foxo1 in these cells. Given that invertebrate model species such as Caenorhabditis elegans and Drosophila melanogaster harbor a single ancestral Foxo homolog, these observations suggest that gene duplication conferred a selective advantage by permitting the Foxos to adopt distinct roles in oogenesis and spermatogenesis. Our objective was to determine if the remarkably specific expression patterns of Foxo1 and Foxo3 in mouse gonads (and, by inference, Foxo function) are conserved in diverse mammalian species. Western blotting was used to validate isoform-specific antibodies in rodents, companion animals, farm animals, nonhuman primates, and humans. Following validation of each antibody, immunohistochemistry was performed to ascertain Foxo1 and Foxo3 gonadal expression patterns. While Foxo1 expression in spermatogonia and granulosa cells was conserved in each species evaluated, Foxo3 expression in oocytes was not. Our findings suggest that Foxo3 is not uniquely required for primordial follicle maintenance in nonrodent species and that other Foxos, particularly Foxo1, may contribute to oocyte maintenance in a functionally redundant manner.

Ovarian Stem Cells-the Pros and Cons

The potential for postnatal de novo oogenesis in mammals and in humans has become very controversial in the fields of reproductive science and biology. Historically, it has been thought that females of most mammalian species lose the ability to produce oocytes at birth. A contemporary understanding of stem cell biology together with novel experimental methods has challenged the model of a prenatal fixed ovarian primordial follicle pool that declines with age. Researchers have suggested replenishment of post-natal oocytes by germ-line stem cells (GSCs). According to this theory, GSCs produce oocytes and primordial follicles throughout the lifetime of the adult female. This review describes recent approaches supporting the revolutionary idea of de novo oogenesis in mammals and humans of reproductive-age and provides counter arguments from opponents of this novel and innovative concept.

Specific disruption of Tsc1 in ovarian granulosa cells promotes ovulation and causes progressive accumulation of corpora lutea

Tuberous sclerosis complex 1 (Tsc1) is a tumor suppressor negatively regulating mammalian target of rapamycin complex 1 (mTORC1). It is reported that mice lacking Tsc1 gene in oocytes show depletion of primordial follicles, resulting in premature ovarian failure and subsequent infertility. A recent study indicated that deletion of Tsc1 in somatic cells of the reproductive tract caused infertility of female mice. However, it is not known whether specific disruption of Tsc1 in granulosa cells influences the reproductive activity of female mice. To clarify this problem, we mated Tsc1^flox/flox mice with transgenic mice strain expressing cyp19-cre which exclusively expresses in granulosa cells of the ovary. Our results demonstrated that Tsc1^flox/flox; cyp19-cre mutant mice were fertile, ovulating more oocytes and giving birth to more pups than control Tsc1^flox/flox mice. Progressive accumulation of corpora lutea occurred in the Tsc1^flox/flox; cyp19-cre mutant mice with advanced age. These phenotypes could be explained by the elevated activity of mTORC1, as indicated by increased phosphorylation of rpS6, a substrate of S6 in the Tsc1^flox/flox; cyp19-cre mutant granulosa cells. In addition, rapamycin, a specific mTORC1 inhibitor, effectively rescued the phenotype caused by increased mTORC1 activity in the Tsc1^cko ovaries. Our data suggest that conditional knockout of Tsc1 in granulosa cells promotes reproductive activity in mice.

Oogenesis: transcriptional regulators and mouse models

Oocyte differentiation into a totipotent cell requires initial germ cell cyst breakdown to form primordial follicles, recruitment of primordial follicles for development into primary follicles and remarkable growth of the ovarian follicle which culminates in ovulation. During oogenesis, the oocyte undergoes dynamic alterations in gene expression which are regulated by a set of well-coordinated transcription factors active in the germ line and soma. A number of germ cell specific as well as somatic expressed transcriptional regulators are critical in ovarian formation and folliculogenesis. These transcriptional regulators include: Foxo3, Foxl2, Figla, Lhx8, Nobox, Sohlh1 and Sohlh2. A subset of these transcriptional regulators is mutated in women with ovarian insufficiency and infertility. Studies on transcriptional regulators preferentially expressed in the ovary are important to develop a better understanding of the mechanisms of activation and survival of ovarian follicles, as well as an understanding of ovary specific pathways that can be modulated in the future to regulate fertility and protect against external insults such as chemotherapy.

Suppressor of cytokine signaling 4 (SOCS4): moderator of ovarian primordial follicle activation

Mammalian ovarian primordial follicle activation and regulation is considered as one of the most important stages of folliculogenesis and as such requires exquisite control. Selection of quiescent follicles to enter the growing pool determines the rate of supply of maturing follicles over the female reproductive lifespan. To coordinate this process a range of positive and negative input signals contribute to determine follicle fate. This study demonstrates that the cytokine Leukemia Inhibitory Factor (LIF) activates the Janus Kinase 1/Signal Transducers and Activators of Transcription 3 (JAK1/STAT3) signaling pathway in pre-granulosa cells and positively regulates primordial follicle activation. Negative regulation of the JAK/STAT pathway is controlled by the suppressor of cytokine signaling 4 (SOCS4) protein, which target members of negative feedback loops, Cardiotrophin like Cytokine (CLC), Poly (rC) Binding Protein 1 (PCBP1), and Cytosolic Malate Dehydrogenase (MDH1) to suppress follicle growth and development.

Deletion of tuberous sclerosis 1 in somatic cells of the murine reproductive tract causes female infertility

Tumors develop with dysregulated activation of mammalian target of rapamycin (mTOR), the kinase activity of which is kept in an inactive state by a tumor suppressor dimer containing tuberous sclerosis 1 (TSC1) and TSC2. We examined whether conditional deletion of TSC1 by a knock-in allele of the anti-Müllerian hormone type 2 receptor (Amhr2) driving Cre expression and subsequent activation of mTOR in granulosa cells and in oviductal and uterine stromal cells affects fertility in female mice. Increased phosphorylation of ribosomal protein S6, a downstream target of activated mTOR, was observed in all AMHR2-expressing tissues examined, indicating loss of TSC1 activity. TSC1 deletion in granulosa cells led to the detection of significantly fewer primordial follicles in mutant mice at 12 wk, suggesting premature ovarian insufficiency, which might be related to the significantly increased time mutant mice spent in estrus. Although the number of good-quality ovulated oocytes was not significantly different compared with controls, there was a significantly higher number of degenerated oocytes after normal and superovulation, suggesting compromised oocyte quality, as well. Natural mating also showed severalfold higher numbers of degenerate bodies in the mutants that collected in bilateral swellings resembling hydrosalpinges that formed in all mice examined because of occlusion of the proximal oviduct. Attempts to transfer control embryos into mutant uteri also failed, indicating that implantation was compromised. Endometrial epithelial cells continued to proliferate, and quantitative RT-PCR showed that mucin 1 expression persisted during the window of implantation in mutant uteri, without any changes in progesterone receptor mRNA expression, suggesting a mechanism that does not involve disrupted estradiol-regulated progesterone receptor expression. Homozygous deletion of TSC1 in reproductive tract somatic tissues of mice rendered females completely infertile, which is likely due to these pleiotropic effects on follicle recruitment, oviductal development, and blastocyst implantation.