Non-suckling starvation of neonatal mice promotes primordial follicle formation with activation of ovarian autophagy

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

BACKGROUND

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

MAIN TEXT

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

CONCLUSION

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

The constructive and destructive impact of autophagy on both genders' reproducibility, a comprehensive review

Reproduction is characterized by a series of massive renovations at molecular, cellular, and tissue levels. Recent studies have strongly tended to reveal the involvement of basic molecular pathways such as autophagy, a highly conserved eukaryotic cellular recycling, during reproductive processes. This review comprehensively describes the current knowledge, updated to September 2022, of autophagy contribution during reproductive processes in males including spermatogenesis, sperm motility and viability, and male sex hormones and females including germ cells and oocytes viability, ovulation, implantation, fertilization, and female sex hormones. Furthermore, the consequences of disruption in autophagic flux on the reproductive disorders including oligospermia, azoospermia, asthenozoospermia, teratozoospermia, globozoospermia, premature ovarian insufficiency, polycystic ovarian syndrome, endometriosis, and other disorders related to infertility are discussed as well.Abbreviations: AKT/protein kinase B: AKT serine/threonine kinase; AMPK: AMP-activated protein kinase; ATG: autophagy related; E2: estrogen; EDs: endocrine disruptors; ER: endoplasmic reticulum; FSH: follicle stimulating hormone; FOX: forkhead box; GCs: granulosa cells; HIF: hypoxia inducible factor; IVF: in vitro fertilization; IVM: in vitro maturation; LCs: Leydig cells; LDs: lipid droplets; LH: luteinizing hormone; LRWD1: leucine rich repeats and WD repeat domain containing 1; MAP1LC3: microtubule associated protein 1 light chain 3; MAPK: mitogen-activated protein kinase; MTOR: mechanistic target of rapamycin kinase; NFKB/NF-kB: nuclear factor kappa B; P4: progesterone; PCOS: polycystic ovarian syndrome; PDLIM1: PDZ and LIM domain 1; PI3K: phosphoinositide 3-kinase; PtdIns3P: phosphatidylinositol-3-phosphate; PtdIns3K: class III phosphatidylinositol 3-kinase; POI: premature ovarian insufficiency; ROS: reactive oxygen species; SCs: Sertoli cells; SQSTM1/p62: sequestosome 1; TSGA10: testis specific 10; TST: testosterone; VCP: vasolin containing protein.

Making a good egg: human oocyte health, aging, and in vitro development

Mammalian eggs (oocytes) are formed during fetal life and establish associations with somatic cells to form primordial follicles that create a store of germ cells (the primordial pool). The size of this pool is influenced by key events during the formation of germ cells and by factors that influence the subsequent activation of follicle growth. These regulatory pathways must ensure that the reserve of oocytes within primordial follicles in humans lasts for up to 50 years, yet only approximately 0.1% will ever be ovulated with the rest undergoing degeneration. This review outlines the mechanisms and regulatory pathways that govern the processes of oocyte and follicle formation and later growth, within the ovarian stroma, through to ovulation with particular reference to human oocytes/follicles. In addition, the effects of aging on female reproductive capacity through changes in oocyte number and quality are emphasized, with both the cellular mechanisms and clinical implications discussed. Finally, the details of current developments in culture systems that support all stages of follicle growth to generate mature oocytes in vitro and emerging prospects for making new oocytes from stem cells are outlined.

Prolonged fasting induces significant germ cell loss in chickens after hatching

Germ cell loss is a crucial biological event during germ cell development. The number of female germ cells determines the reproductive performance and egg production of hens. Various intrinsic and extrinsic factors affect germ cell loss, such as germ cell nest breakdown in early life and nutritional deficiencies during daily husbandry. Here, we examined the effect of fasting on the germ cell number of chicks. The results showed that 72 h fasting resulted in a higher germ cell loss than that by 24 h fasting in chicks. The RNA-seq analysis revealed that the genes of ribosome pathway were down-regulated and the biological processes of protein processing in endoplasmic reticulum were inhibited in starved chicks. Furthermore, in female chicks treated with 72 h fasting, the qPCR of ovaries showed down-regulation of ribosome-related genes, and transmission electron microscopy imaging of ovaries showed fewer ribosomes. The blood biochemical indices indicated that 72 h fasting reduced the liver functions and affected the glucose metabolism, lipid metabolites and ion metabolites. In summary, the present results concluded negative impacts on the germ cell pool by prolonged fasting in the early life of chicks and manifested that adequate management should be cared for fasted time for breeding.

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

BACKGROUND

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

OBJECTIVE AND RATIONALE

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

SEARCH METHODS

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

OUTCOMES

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

WIDER IMPLICATIONS

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

BNIP3-mediated autophagy via the mTOR/ULK1 pathway induces primordial follicle loss after ovarian tissue transplantation

PURPOSE

To explore the underlying mechanism of primordial follicle loss in the early period following ovarian tissue transplantation (OTT).

METHODS

BNIP3 was selected through bioinformatic protocols, as the hub gene related to autophagy during OTT. BNIP3 and autophagy in mice ovarian grafts and in hypoxia-mimicking KGN cells were detected using immunohistochemistry, transmission electron microscopy (TEM), western blotting, qPCR, and fluorescence staining. The regulatory role played by BNIP3 overexpression and the silencing of KGN cells in autophagy via the mTOR/ULK1 pathway was investigated.

RESULTS

Ultrastructure examination showed that autophagic vacuoles increased after mice ovarian auto-transplantation. The BNIP3 and autophagy-related proteins (Beclin-1, LC3B, and SQSTM1/p62) in mice ovarian granulosa cells of primordial follicle from ovarian grafts were altered compared with the control. Administration of an autophagy inhibitor in mice decreased the depletion of primordial follicles. In vitro experiments indicated that BNIP3 and autophagy activity were upregulated in KGN cells treated with cobalt chloride (CoCl2). The overexpression of BNIP3 activated autophagy, whereas the silencing of BNIP3 suppressed it and reversed the autophagy induced by CoCl2 in KGN cells. Western blotting analysis showed the inhibition of mTOR and activation of ULK1 in KGN cells treated with CoCl2 and in the overexpression of BNIP3, and the opposite results following BNIP3 silencing. The activation of mTOR reversed the autophagy induced by BNIP3 overexpression.

CONCLUSIONS

BNIP3-induced autophagy is crucial in primordial follicle loss during OTT procedure, and BNIP3 is a potential therapeutic target for primordial follicle loss after OTT.

Mechanism of Mitochondrial Homeostasis Controlling Ovarian Physiology

Ovarian cells, including oocytes, granulosa/cumulus cells, theca cells, and stromal cells, contain abundant mitochondria, which play indispensable roles in the processes of ovarian follicle development. Ovarian function is closely controlled by mitochondrial proteostasis and mitostasis. While mitochondrial proteostasis and mitostasis are disturbed by several factors, leading to dysfunction of ovarian function and initiating the mitochondrial unfolded protein response (UPRmt) and mitophagy to maintain or recover ovarian function and mitochondrial function, clear interactions between the 2 pathways in the ovary have not been fully elucidated. Here, we comprehensively summarize the molecular networks or regulatory mechanisms behind further mitochondrial research in the ovary. This review provides novel insights into the interactions between the UPRmt and mitophagy in ovarian functions.

Retention of higher fertility depending on ovarian follicle reserve in cystine-glutamate transporter gene-deficient mice

The cystine-glutamate transporter (xCT) is responsible for the transport of cystine into cells. We recently found that xCT-deficient (xCTKO) aged mice maintained a higher rate of ovulation and ovarian weight compared with wild-type (WT) mice. It has been reported that a xCT deficiency in cultured cells induces autophagy through the suppression of mTOR survival pathways. We have previously reported that starvation in neonatal mice increases the number of primordial follicles with concomitant autophagy activation. Therefore, we investigated age-related changes in follicle reserve and fertility in xCTKO mice and clarified whether the PI3K/AKT/mTOR signaling pathway contributes to this. The numbers of offspring in the xCTKO mice aged 10 and 12 months were significantly higher than those in the WT mice. The primordial follicle numbers in xCTKO neonatal mice tended to be higher than WT mice during all times evaluated. In contrast, the primary follicle number was significantly lower in the xCTKO mice at 60 h after birth. The expression of p-AKT, which promotes follicle development, was significantly lower in xCTKO mice than that in WT mice, whereas the expression ratios of LC3-II/LC3-I were significantly higher. The xCTKO mice had significantly more primordial follicles than WT mice at 2 months of age and showed a similar trend at 13-15 months of age. These results suggest that the maintenance of fertility in aged xCTKO mice can be attributed to high follicle reserve after puberty by suppression of follicle activation during the neonatal period.

Role of autophagy in follicular development and maintenance of primordial follicular pool in the ovary

The reproductive life span of the organism mainly depends on follicular development that maintains the primordial follicle pool in the cohort of follicles within the ovary. The total count of primordial follicles decreases with age due to ovulation and follicular atresia. Follicular atresia, a process of ovarian follicles degradation, mainly occurs via apoptosis, but recent studies also favor autophagy existence. Autophagy is a cellular and energy homeostatic response that helps to maintain the number of healthy primordial follicles, germ cell survival, and removal of corpus luteum remnants. But the excessive autophagic cell death changes both the quality and quantity of oocytes that ultimately affect female reproductive health. Autophagy regulation occurs by various autophagy-regulated genes like BECN1 and LC3-II (autophagy marker genes). Their abnormal regulation or mutation highly influences follicular development by alteration of primordial follicles formation, the decline in oocytes count, and germ cell loss. Various classical signaling pathways such as PI3K/AKT/mTOR, MAPK/ERK1/2, AMPK, and IRE1 are involved in granulosa and oocytes autophagy, while mTOR signaling is the primary mechanism. Along with basal level autophagy, chemical/hormone/stress-mediated autophagy also affects follicular development and female reproduction. In this review, we have primarily focused on granulosa cell and oocytes' autophagy, mechanism, and the role of autophagy determining marker genes in follicular development.

Activation of autophagy in early neonatal mice increases primordial follicle number and improves lifelong fertility†

The number of stockpiled primordial follicles is thought to be responsible for the fate of female fertility and reproductive lifetime. We previously reported that starvation in nonsuckling early neonatal mice increases the number of primordial follicles with concomitant autophagy activation, suggesting that autophagy may accelerate the formation of primordial follicles. In this study, we attempted to upregulate the numbers of primordial follicles by administering an autophagy inducer and evaluated the progress of primordial follicle formation and their fertility during the life of the mice. To induce autophagy, mice were intraperitoneally injected with the Tat-beclin1 D-11 peptide (0.02 mg/g body weight) at 6-54 h or 60-84 h after birth. In animals that received Tat-beclin 1 D-11 by 54 h after birth, the primordial follicle numbers were significantly increased compared with the control group at 60 h. The ratio of expressed LC3-II/LC3-I proteins was also significantly greater. The numbers of littermates from pregnant females that had been treated with Tat-beclin 1 D-11 were maintained at remarkably greater levels until 10 months old. These results were supported by an abundance of primordial follicles at even 13-15 months old.

Primordial Follicle Formation - Some Assembly Required

Formation of primordial follicles occurs when germ cell nests break apart and individual oocytes become surrounded by pregranulosa cells. Why mammalian germ cells develop in germ cell nests is not fully understood but recent work has provided evidence that some oocytes serve as nurse cells supporting other oocytes in the cyst. Headway has also been made in understanding interactions that occur between cyst cells that must change as individual oocytes separate to associate with pregranulosa cells. As germ cell nests undergo breakdown some oocytes are lost by programmed cell death that has been attributed to apoptosis, but newer studies have implicated autophagy in counteracting apoptosis to promote cell survival and maintain the ovarian reserve. Work in the past few years has added to already known pathways regulating primordial follicle formation and has identified new players including signaling molecules, transcription factors and RNA binding proteins.

miR-378-3p maintains the size of mouse primordial follicle pool by regulating cell autophagy and apoptosis

Primordial follicle pool provides all available oocytes throughout the whole reproductive life span. Abnormal regulation in primordial follicle assembly leads to abnormal size of primordial follicle pool, even causes infertility. Here, miR-378-3p was proved to regulate mouse primordial follicle assembly both in vivo and in vitro. The expression of miR-378-3p significantly increased in mice ovaries from 17.5 dpc (days post coitum) up to 3 dpp (day post partum) compared with the expression of 16.5 dpc ovaries, which suggested that miR-378-3p was involved in primordial follicle assembly. To uncover the underlying mechanism, newborn mice ovaries were cultured in vitro in the presence of rapamycin and 3-methyladenine, which showed that the expression of miR-378-3p changed together with the percentage of primordial follicle. Moreover, during the normal process of primordial follicle assembly between 17.6 dpc and 3 dpp, autophagy is activated, while, apoptosis is inhibited. The in vivo results showed that newborn mice starved for 1.5 days showing the increased miR-378-3p, activated autophagy and inhibited apoptosis in the ovaries, had more percentage of primordial follicles. Over-expression of miR-378-3p using miR-378-3p agomir caused increased percentage of primordial follicle, increased level of autophagy, and decreased level of apoptosis. Knockdown of miR-378-3p by miR-378-3p antiagomir had the opposite results. Using pmirGLO Dual-Luciferase miRNA Target Expression system, we confirmed both PDK1 and Caspase9 were targets of miR-378-3p, which suggested that miR-378-3p activated autophagy by targeting PDK1 and inhibited apoptosis by targeting Caspase9. MiR-378-3p could be used as a biomarker of diseases caused by abnormal size of primordial follicle pool for diagnosis, prevention, or therapy.

Pathways coordinating oocyte attrition and abundance during mammalian ovarian reserve establishment

The mammalian ovarian reserve is comprised of a finite pool of primordial follicles, representing the lifetime reproductive capacity of females. In most mammals, the reserve is produced during embryonic and early postnatal development with oocyte numbers peaking during mid-to-late gestation, and then experiencing a dramatic decline continuing until shortly after birth. Oocytes remaining after the bulk of this attrition are subsequently surrounded by a layer of somatic pre-granulosa cells with these units then referred to as "primordial follicles." The complex and varied cell death mechanisms intrinsic to this process are not only characteristic of, but also essential for, the proper formation of this pool of follicles, and as a result must be immaculately balanced to ensure long-term fertility and reproductive health. Too few follicles can lead to Primary Ovarian Insufficiency, resulting in fertility loss and other features of aging, such as an overall shorter lifespan. On the other hand, whereas an excess of follicles might extend reproductive lifespan, this might also be the underlying etiology of other ovarian pathologies. The last decade, in particular, has vastly expanded our understanding of oocyte attrition and determinants of ovarian reserve abundance. By continuing to decipher the intricacies underlying the cell death processes and development of the initial primordial follicle pool, we may be in a much better position to understand idiopathic cases of premature follicle depletion and improve ovarian health in reproductive-age women.

Autophagy in hypoxic ovary

Oxygen deprivation affects human health by modulating system as well as cellular physiology. Hypoxia generates reactive oxygen species (ROS), causes oxidative stress and affects female reproductive health by altering ovarian as well as oocyte physiology in mammals. Hypoxic conditions lead to several degenerative changes by inducing various cell death pathways like autophagy, apoptosis and necrosis in the follicle of mammalian ovary. The encircling somatic cell death interrupts supply of nutrients to the oocyte and nutrient deprivation may result in the generation of ROS. Increased level of ROS could induce granulosa cells as well as oocyte autophagy. Although autophagy removes damaged proteins and subcellular organelles to maintain the cell survival, irreparable damages could induce cell death within intra-follicular microenvironment. Hypoxia-induced autophagy is operated through 5' AMP activated protein kinase-mammalian target of rapamycin, endoplasmic reticulum stress/unfolded protein response and protein kinase C delta-c-junN terminal kinase 1 pathways in a wide variety of somatic cell types. Similar to somatic cells, we propose that hypoxia may induce granulosa cell as well as oocyte autophagy and it could be responsible at least in part for germ cell elimination from mammalian ovary. Hypoxia-mediated germ cell depletion may cause several reproductive impairments including early menopause in mammals.

Establishment and depletion of the ovarian reserve: physiology and impact of environmental chemicals

The reproductive life span in women starts at puberty and ends at menopause, following the exhaustion of the follicle stockpile termed the ovarian reserve. Increasing data from experimental animal models and epidemiological studies indicate that exposure to a number of ubiquitously distributed reproductively toxic environmental chemicals (RTECs) can contribute to earlier menopause and even premature ovarian failure. However, the causative relationship between environmental chemical exposure and earlier menopause in women remains poorly understood. The present work, is an attempt to review the current evidence regarding the effects of RTECs on the main ovarian activities in mammals, focusing on how such compounds can affect the ovarian reserve at any stages of ovarian development. We found that in rodents, strong evidence exists that in utero, neonatal, prepubescent and even adult exposure to RTECs leads to impaired functioning of the ovary and a shortening of the reproductive lifespan. Regarding human, data from cross-sectional surveys suggest that human exposure to certain environmental chemicals can compromise a woman's reproductive health and in some cases, correlate with earlier menopause. In conclusion, evidences exist that exposure to RTECs can compromise a woman's reproductive health. However, human exposures may date back to the developmental stage, while the adverse effects are usually diagnosed decades later, thus making it difficult to determine the association between RTECs exposure and human reproductive health. Therefore, epidemiological surveys and more experimental investigation on humans, or alternatively primates, are needed to determine the direct and indirect effects caused by RTECs exposure on the ovary function, and to characterize their action mechanisms.