Loss of the proapoptotic BH3-only protein BCL-2 modifying factor prolongs the fertile life span in female mice

Multiple treatments with human embryonic stem cell-derived mesenchymal progenitor cells preserved the fertility and ovarian function of perimenopausal mice undergoing natural aging

OBJECTIVES

Currently, no approved stem cell-based therapies for preserving ovarian function during aging. To solve this problem, we developed a long-term treatment for human embryonic stem cell-derived mesenchymal progenitor cells (hESC-MPCs). We investigated whether the cells retained their ability to resist ovarian aging, which leads to delayed reproductive senescence.

MATERIALS AND METHODS

In a middle-aged female model undergoing natural aging, we analyzed whether hESC-MPCs benefit the long-term maintenance of reproductive fecundity and ovarian reservoirs and how their transplantation regulates ovarian function.

RESULTS

The number of primordial follicles and mice with regular estrous cycles were increased in perimenopausal mice who underwent multiple introductions of hESC-MPCs compared to age-matched controls. The estradiol levels in the hESC-MPCs group were restored to those in the young and adult groups. Embryonic development and live birth rates were higher in the hESC-MPC group than in the control group, suggesting that hESC-MPCs delayed ovarian senescence. In addition to their direct effects on the ovary, multiple-treatments with hESC-MPCs reduced ovarian fibrosis by downregulating inflammation and fibrosis-related genes via the suppression of myeloid-derived suppressor cells (MDSCs) produced in the bone marrow.

CONCLUSIONS

Multiple introductions of hESC-MPCs could be a useful approach to prevent female reproductive senescence and that these cells are promising sources for cell therapy to postpone the ovarian aging and retain fecundity in perimenopausal women.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

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.

Smad4 mediates Bmf involvement in sheep granulosa cell apoptosis

Bcl-2-modifying factor (Bmf) functions to mediate follicular atresia and oocyte growth in mice. It has been proven that TGF-β can induce Bmf expression via the Smad4 pathway in a variety of cells, and then induce cell apoptosis. Based on this, we hypothesized that Smad4 and Bmf may play important roles in the apoptosis of granulosa cells (GCs) in domestic animals. This study used small-tailed Han sheep follicular GCs cultured in vitro as a model system, and overexpression or interference experiments, to explore the biological roles of Bmf and reveal the preliminary regulatory mechanisms between Smad4 and Bmf in the process of GCs' apoptosis. We found that the proliferation rate of sheep GCs was significantly increased after the knockdown of Bmf, whereas overexpressing Bmf increased the apoptosis rate of GCs, results also verified by the expression patterns of PCNA, Bcl-2, and Bax genes. After the Smad4 knockdown, the apoptosis rate of GCs was increased, while the mRNA and protein expression of Bmf was significantly up-regulated. A rescue experiment verified that the Bmf knockdown could alleviate GCs' apoptosis induced by Smad4 knockdown. In conclusion, our study not only elucidated an important role for Bmf in the apoptosis of sheep GCs but also revealed a new regulatory pathway between Smad4 and Bmf in this process.

Lifelong exposure to pyrethroid insecticide cypermethrin at environmentally relevant doses causes primary ovarian insufficiency in female mice

Pyrethroids are a class of widely used insecticides. Our recent epidemiological study of Chinese women reported that pyrethroid exposure was positively associated with the risk of primary ovarian insufficiency (POI). In this study, we utilized cypermethrin (CP), the most frequently detected pyrethroid in the environment, to recognize how lifelong and low-dose exposure to pyrethroids affects ovarian functions and the underlying mechanism(s). Female mice were exposed to CP at doses of human dietary intake of 6.7 μg/kg/day, an acceptable daily intake (ADI) of 20 μg/kg/day, or the chronic reference dose (RfD) of 60 μg/kg/day, starting from gestational day 0.5 until 44-week-old. We assessed effects on fertility, serum hormone levels, ovarian follicular development and ovarian transcriptomic profiles. Chronic exposure to CP at doses of ADI and RfD caused a significant reduction in the size of the primordial follicle pool on postnatal day (PND) 5 and the number of all types of follicles in 44-week-old mice, lower estrogen and higher gonadotropin levels, as well as decreased fertility. Significant increase in apoptosis and decrease in cell proliferation were observed in CP-exposed ovarian follicles from PND 5 and 44-week-old mice. Ovarian transcriptomic data showed that the pro-apoptotic protein BMF and the cell cycle inhibitor p27 were significantly up-regulated in CP-exposed ovaries. Cyp17a1, Cyp19a1 and Hsd17b1 genes involved in the key steps of steroidogenesis were down-regulated in the ovaries of female mice exposed to CP. This study first reported that lifelong exposure to CP at doses of ADI or RfD caused an ovarian phenotype similar to human POI in female mice and provided a mechanistic explanation. Our findings suggest that lifelong exposure to pyrethroids of low doses, which are recommended as 'safe' dosages, may have a significant impact on the ovarian health of female mammals and humans.

MiR-125b-5p ameliorates hypoxia/reoxygenation-induced endothelial cell dysfunction and attenuates reduced uterine perfusion pressure-induced hypertension in pregnant rats via targeting BMF

BACKGROUND AND PURPOSE

MicroRNA-125b-5p (miR-125b-5p) is downregulated in patients with gestational hypertension signs. However, the role of miR-125b-5p in pregnancy-induced hypertension (PIH) remains unknown.

METHODS

The human placental microvascular endothelial cells (HPMECs) have undergone hypoxia and reoxygenation (H/R) treatment to establish PIH cellular model. Rats were performed with reduced uterine perfusion pressure (RUPP) operation to establish PIH animal model.

RESULTS

MiR-125b-5p promoted viability while inhibited the apoptosis of H/R-treated HPMECs by downregulating BMF. MiR-125b-5p alleviated hypertensive symptoms and improved pregnancy outcomes in RUPP rats.

CONCLUSION

MiR-125b-5p ameliorates H/R-induced HPMEC dysfunction and attenuates RUPP-induced hypertension in pregnant rats by downregulating BMF.

Effects and mechanisms of mUCMSCs on ovarian structure and function in naturally ageing C57 mice

BACKGROUND

The ovaries are the core reproductive organs in women and are critical for maintaining normal reproductive function and endocrine system stability. An ageing C57 mouse model was used to evaluate the efficacy and mechanism of mouse umbilical cord mesenchymal stem cells (mUCMSCs) and to explore the mechanism by which mUCMSCs promote the antioxidant repair of mouse granulosa cells (mGCs).

RESULTS

Eighteen-month-old C57 mice were randomly divided into a model group and a treatment group. At the same time, 2-month-old C57 mice were established as a young group (15 mice per group). The mice in the treatment group were injected via the tail vein with GFP-labelled mUCMSCs. The ovarian volume in ageing C57 mice was decreased, and there were no follicles at any stage. After mUCMSC transplantation, the mouse ovaries increased in size, follicles at various stages were observed in the cortex, and the antral follicle counts increased. The serum E2, AMH, and INH-B levels of mice in the treatment group were significantly higher than those of mice in the model control group (P < 0.05). mUCMSCs downregulated the expression of the autophagy-related gene LC3b and the apoptosis-related genes Bax and Caspase-3, upregulated the expression of SOD2 and the peroxidase gene PRDX IV, and reduced apoptosis rates and reactive oxygen species (ROS) levels in granulosa cells.

CONCLUSIONS

mUCMSCs play roles in promoting the repair of ageing ovaries by regulating immunity, anti-inflammatory responses and the PI3K-Akt signalling pathway.

APOE4 genotype exacerbates the impact of menopause on cognition and synaptic plasticity in APOE-TR mice

The impact of sex and menopausal status in Alzheimer's disease remains understudied despite increasing evidence of greater female risk, particularly in APOE4 carriers. Utilizing female APOE-TR mice maintained on a high-fat diet background we induced ovarian failure through repeated VCD injections, to mimic human menopause. At 12 months of age, recognition memory and spatial memory were assessed using object recognition, Y-maze spontaneous alternation, and Barnes maze. A VCD*genotype interaction reduced the recognition memory (P < .05), with APOE4 VCD-treated animals unable to distinguish between novel and familiar objects. APOE4 mice displayed an additional 37% and 12% reduction in Barnes (P < .01) and Y-maze (P < .01) performance, indicative of genotype-specific spatial memory impairment. Molecular analysis indicated both VCD and genotype-related deficits in synaptic plasticity with BDNF, Akt, mTOR, and ERK signaling compromised. Subsequent reductions in the transcription factors Creb1 and Atf4 were also evident. Furthermore, the VCD*genotype interaction specifically diminished Ephb2 expression, while Fos, and Cnr1 expression reduced as a consequence of APOE4 genotype. Brain DHA levels were 13% lower in VCD-treated animals independent of genotype. Consistent with this, we detected alterations in the expression of the DHA transporters Acsl6 and Fatp4. Our results indicate that the combination of ovarian failure and APOE4 leads to an exacerbation of cognitive and neurological deficits.

FBW7 suppresses ovarian cancer development by targeting the N6-methyladenosine binding protein YTHDF2

Background

The tumor suppressor FBW7 is the substrate recognition component of the SCF E3-ubiquitin ligase complex that mediates proteolytic degradation of various oncogenic proteins. However, the role of FBW7 in ovarian cancer progression remains inadequately understood.

Methods

IP-MASS, co-IP, immunohistochemistry, and western blotting were used to identify the potential substrate of FBW7 in ovarian cancer. The biological effects of FBW7 were investigated using in vitro and in vivo models. LC/MS was used to detect the m⁶A levels in ovarian cancer tissues. MeRIP-Seq and RNA-Seq were used to assess the downstream targets of YTHDF2.

Results

We unveil that FBW7 is markedly down-regulated in ovarian cancer tissues and its high expression is associated with favorable prognosis and elevated m⁶A modification levels. Consistently, ectopic FBW7 inhibits ovarian cancer cell survival and proliferation in vitro and in vivo, while ablation of FBW7 empowers propagation of ovarian cancer cells. In addition, the m⁶A reader protein, YTHDF2, is identified as a novel substrate for FBW7. FBW7 counteracts the tumor-promoting effect of YTHDF2 by inducing proteasomal degradation of the latter in ovarian cancer. Furthermore, YTHDF2 globally regulates the turnover of m⁶A-modified mRNAs, including the pro-apoptotic gene BMF.

Conclusions

Our study has demonstrated that FBW7 suppresses tumor growth and progression via antagonizing YTHDF2-mediated BMF mRNA decay in ovarian cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12943-021-01340-8.

The Inflammasome Contributes to Depletion of the Ovarian Reserve During Aging in Mice

Ovarian aging is a natural process characterized by follicular depletion and a reduction in oocyte quality, resulting in loss of ovarian function, cycle irregularity and eventually infertility and menopause. The factors that contribute to ovarian aging have not been fully characterized. Activation of the NLRP3 inflammasome has been implicated in age-associated inflammation and diminished function in several organs. In this study, we used Asc^−/− and Nlrp3^−/− mice to investigate the possibility that chronic low-grade systemic inflammation mediated by the inflammasome contributes to diminished ovarian reserves as females age. Pro-inflammatory cytokines, IL-6, IL-18, and TNF-α, were decreased in the serum of aging Asc^−/− mice compared to WT. Within the ovary of reproductively aged Asc^−/− mice, mRNA levels of major pro-inflammatory genes Tnfa, Il1a, and Il1b were decreased, and macrophage infiltration was reduced compared to age-matched WT controls. Notably, suppression of the inflammatory phenotype in Asc^−/− mice was associated with retention of follicular reserves during reproductive aging. Similarly, the expression of intra-ovarian pro-inflammatory cytokines was reduced, and follicle numbers were significantly elevated, in aging Nlrp3^−/− mice compared to WT controls. These data suggest that inflammasome-dependent inflammation contributes to the age-associated depletion of follicles and raises the possibility that ovarian aging could be delayed, and fertile window prolonged, by suppressing inflammatory processes in the ovary.

Evaluation of inflammation and follicle depletion during ovarian ageing in mice

Reproductive ageing in females is defined by a progressive decline in follicle number and oocyte quality. This is a natural process that leads to the loss of fertility and ovarian function, cycle irregularity and eventually menopause or reproductive senescence. The factors that underlie the natural depletion of follicles throughout reproductive life are poorly characterised. It has been proposed that inflammatory processes and fibrosis might contribute to ovarian ageing. To further investigate this possibility, we evaluated key markers of inflammation and immune cell populations in the ovaries of 2, 6, 12 and 18-month-old C57BL/6 female mice. We report that the decrease in follicle numbers over the reproductive lifespan was associated with an increase in the intra-ovarian percentage of CD4 + T cells, B cells and macrophages. Serum concentration and intra-ovarian mRNA levels of several pro-inflammatory cytokines, including IL-1α/β, TNF-α, IL-6, and inflammasome genes ASC and NLRP3, were significantly increased with age. Fibrosis levels, as determined by picrosirius red staining for collagen I and III, were unchanged up to 18 months of age. Collectively, these data suggest that inflammation could be one of the mechanisms responsible for the age-related regulation of follicle number, but the role of fibrosis is unclear. Further studies are now required to determine if there is a causative relationship between inflammation and follicle depletion as females age.

Cisplatin- and cyclophosphamide-induced primordial follicle depletion is caused by direct damage to oocytes

It is well established that DNA-damaging chemotherapies can cause infertility and ovarian endocrine failure by depleting the ovarian reserve of primordial follicles. Currently, no effective pharmacological therapies exist for the preservation of long-term fertility and ovarian function in female cancer patients, due to a limited understanding of the mechanisms of chemotherapy-induced follicle depletion. This study investigated the cellular targets, molecular mechanisms, and temporal course of ovarian reserve depletion following treatment with commonly used chemotherapeutic drugs. Adult female C57BL/6 mice were injected i.p. with saline, cisplatin (5mg/kg), or cyclophosphamide (300mg/kg); ovaries were harvested after 8 or 24 hours. Follicle counts showed depletion of all follicular stages 24 hours after administration of cisplatin or cyclophosphamide. Eight hours post-treatment, H2A histone family member X (γH2AX) immunofluorescence showed DNA double-stranded breaks at all follicular stages, including within primordial follicle oocytes. This staining was resolving by 24 hours, indicating that primordial follicle oocytes begin to undergo either apoptosis or repair in this timeframe. γH2AX-positive follicles were further examined to identify the specific cell types damaged. In primordial, transitional, and primary follicles, only oocytes sustained DNA damage, whereas in secondary and antral follicles, only somatic cells were affected. TUNEL staining confirmed that apoptosis occurs in these targeted cell types. Whilst multi-drug and multi-dose regimens were not examined, this study conclusively shows that cyclophosphamide and cisplatin cause direct damage to primordial follicle oocytes, which then undergo apoptosis. Therefore, future pharmacological strategies to prevent chemotherapy-induced infertility in females must specifically prevent primordial follicle oocyte death.

Vincristine Chemotherapy Induces Atresia of Growing Ovarian Follicles in Mice

With great advances in cancer detection and treatment, patient survival rates have improved substantially. Subsequently, significant efforts are now focused on improving the long-term sequelae of anticancer therapies in survivors, which includes fertility. Vincristine is a microtubule destabilizing antimitotic chemotherapeutic agent commonly administered for the treatment of cancers or autoimmune disorders prevalent in girls and women of reproductive age. The potential off-target effects of vincristine on the ovary have not been directly examined. Eight-week and 6-month-old C57BL/6J mice were administered with vincristine (1 mg/kg/bw/day) or saline on day (d)1, d4, and d8, then sacrificed after 24 hours (h), or 14 days (n = 4-6/group). We assessed the impact of vincristine on the ovarian reserve of quiescent primordial follicles, as well as growing follicles, which produce mature ovulatory oocytes. This study clearly demonstrated that multidose vincristine administration caused acute atresia and loss of growing follicles and reduced corpora luteua counts 24 h following final treatment. Treatment also disrupted estrous cycling and reduced serum anti-Müllerian hormone levels. However, primordial follicle numbers were unaffected, and growing follicle populations were restored to control levels 14 days after final treatment. Vincristine exerted similar effects on ovarian follicle populations in both 8-week-old reproductively young mice and reproductively older 6-month-old mice. This study suggests that vincristine, administrated at the current dose, is toxic to growing follicles but does not deplete primordial follicles in mice. Further studies should be performed before extrapolating these data to infer the consequences of vincristine on the ovary in humans.

Single xenotransplant of rat brown adipose tissue prolonged the ovarian lifespan of aging mice by improving follicle survival

Prolonging the ovarian lifespan is attractive and challenging. An optimal clinical strategy must be safe, long-acting, simple, and economical. Allotransplantation of brown adipose tissue (BAT), which is most abundant and robust in infants, has been utilized to treat various mouse models of human disease. Could we use BAT to prolong the ovarian lifespan of aging mice? Could we try BAT xenotransplantation to alleviate the clinical need for allogeneic BAT due to the lack of voluntary infant donors? In the current study, we found that a single rat-to-mouse (RTM) BAT xenotransplantation did not cause systemic immune rejection but did significantly increase the fertility of mice and was effective for more than 5 months (equivalent to 10 years in humans). Next, we did a series of analysis including follicle counting; AMH level; estrous cycle; mTOR activity; GDF9, BMP15, LHR, Sirt1, and Cyp19a level; ROS and annexin V level; IL6 and adiponectin level; biochemical blood indices; body temperature; transcriptome; and DNA methylation studies. From these, we proposed that rat BAT xenotransplantation rescued multiple indices indicative of follicle and oocyte quality; rat BAT also improved the metabolism and general health of the aging mice; and transcriptional and epigenetic (DNA methylation) improvement in F0 mice could benefit F1 mice; and multiple KEGG pathways and GO classified biological processes the differentially expressed genes (DEGs) or differentially methylated regions (DMRs) involved were identical between F0 and F1. This study could be a helpful reference for clinical BAT xenotransplantation from close human relatives to the woman.

Melatonin Promotes Ubiquitination of Phosphorylated Pro-Apoptotic Protein Bcl-2-Interacting Mediator of Cell Death-Extra Long (BimEL) in Porcine Granulosa Cells

Melatonin (N-acetyl-5-methoxytryptamine) is found in ovarian follicular fluid, and its concentration is closely related to follicular health status. Nevertheless, the molecular mechanisms underlying melatonin function in follicles are uncertain. In this study, melatonin concentration was measured in porcine follicular fluid at different stages of health. The melatonin concentration decreased as the follicles underwent atresia, suggesting that melatonin may participate in the maintenance of follicular health. The molecular pathway through which melatonin may regulate follicular development was further investigated. The pro-apoptotic protein Bim_EL (Bcl-2-interacting mediator of cell death-Extra Long), a key protein controlling granulosa cell apoptosis during follicular atresia, was selected as the target molecule. Bim_EL was downregulated when porcine granulosa cells were cultured in medium containing 10⁻⁹ M melatonin and isolated cumulus oocyte complexes (COCs) or follicle stimulating hormone (FSH). Interestingly, ERK-mediated phosphorylation was a prerequisite for the melatonin-induced decline in Bim_EL, and melatonin only promoted the ubiquitination of phosphorylated Bim_EL, and did not affect the activities of the lysosome or the proteasome. Moreover, the melatonin-induced downregulation of Bim_EL was independent of its receptor and its antioxidant properties. In conclusion, melatonin may maintain follicular health by inducing Bim_EL ubiquitination to inhibit the apoptosis of granulosa cells.

Dacarbazine depletes the ovarian reserve in mice and depletion is enhanced with age

Dacarbazine is commonly administered for the treatment of cancers prevalent in reproductive age females. However, investigations of off-target effects of dacarbazine on the ovary are limited. We assessed the impact of dacarbazine on the ovarian reserve of primordial follicles, essential for fertility. Eight week and 6 month old C57BL/6 J mice were administered with dacarbazine or saline on day (d)0 and d7, then sacrificed after 12 hours (h), or 14d (n = 4-5/group). Follicle numbers, follicle density, serum AMH and corpora lutea were quantified and estrous cyclicity monitored. In reproductively young mice, dacarbazine did not affect primordial follicle numbers at 12 h, but resulted in a 36% reduction at 14d (p < 0.05). Dacarbazine-mediated primordial follicle depletion was accelerated with age, with a 24% (p < 0.05) and 36% (p < 0.01) reduction at 12 h and 14d. Follicle density remained unchanged between treatment groups at either age. Dacarbazine depleted antral follicles at 14d (p < 0.05), at both ages. Despite partial reduction of antral follicles, serum AMH, estrous cyclicity and corpora lutea (indicative of ovulation) remained unchanged between treatment groups, at both ages. Importantly, diminished ovarian reserve can result in premature ovarian insufficiency and infertility, thus, fertility preservation options should be considered for young female patients prior to dacarbazine treatment.

Examination of the ovotoxicity of 5-fluorouracil in mice

PURPOSE

Undesirable side effects of cancer treatments are common and include damage to the ovary, and depletion of the follicle reserve, which if severe enough, can lead to infertility and early menopause. Antimetabolite drugs, such as 5-fluorouracil (5-FU), are not considered to be detrimental to the ovary, but the ovotoxicity of 5-FU has not been evaluated in any detail. The purpose of this study was to evaluate the effects of 5-FU on follicle number.

METHODS

In this study, adult female C57Bl6 mice (n = 4-6 animals/group) received a single dose of saline or 5-FU (150 mg/kg) and markers of ovarian damage and follicle depletion were assessed 12 h and 7 days later.

RESULTS

Exposure to 5-FU did not alter primordial and primary follicle numbers. Atresia of secondary and antral follicles was increased significantly 12 h after 5-FU treatment, but atresia rates returned to levels similar to that of saline treated controls at 7 days. The number of corpora lutea were reduced 7 days after exposure to 5-FU, possibly as a consequence of earlier follicular atresia.

CONCLUSIONS

These findings suggest that a single dose of 5-FU is mildly ovotoxic, but any effects on ovarian function are likely transient because the primordial follicle population is not depleted. Collectively, these data support the notion that 5-FU is unlikely to impact on the long-term fertility of women.

The ovarian reserve is depleted during puberty in a hormonally driven process dependent on the pro-apoptotic protein BMF

In females, germ cells are maintained in ovarian structures called primordial follicles. The number of primordial follicles in the ovarian reserve is a critical determinant of the length of the fertile lifespan. Despite this significance, knowledge of the precise physiological mechanisms that regulate primordial follicle number is lacking. In this study we show that a wave of primordial follicle depletion occurs during the transition to adulthood in mice. We demonstrate that this sudden and dramatic loss of primordial follicles is hormonally triggered and identify the pro-apoptotic BH3-only protein, BCL-2 modifying factor (BMF), as essential for this process, implicating the intrinsic apoptotic pathway as a key mechanism. The elimination of primordial follicles during puberty is not only a striking developmental event, it is also physiologically important because it ultimately reduces the availability of primordial follicles and determines the duration of fertility. Collectively, these findings show that puberty is a critical developmental window for the regulation of the size of ovarian reserve, impacting on female fertility and reproductive longevity.

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.

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.

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

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