Germline stem cells are critical for sexual fate decision of germ cells

Zuogui pills ameliorate chemotherapy-induced ovarian aging by improving stemness, regulating cell cycle and reducing apoptosis of oogonial stem cells via the Notch1/Nrf2 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Zuogui Pills (ZGP) is a classic traditional Chinese herbal formula originating from the Ming Dynasty. It has been widely used in the treatment of kidney deficiency-related diseases, including ovarian aging.

AIM OF THE STUDY

To investigate the effects and potential mechanisms of ZGP on ovarian aging induced by the chemotherapeutic agent cyclophosphamide (CTX), as well as its impact on the therapeutic target, oogonial stem cells (OSCs), involving the Notch1/nuclear factor erythroid 2-related factor 2 (Nrf2) pathway.

MATERIALS AND METHODS

This study utilized High-Performance Liquid Chromatography (HPLC) to analyze the active components of Zuogui Pills (ZGP). In vivo experiments involved the establishment of an ovarian aging model in female rats through intraperitoneal injection of CTX, followed by an 8-week treatment with ZGP and dehydroepiandrosterone (DHEA). The Notch pathway inhibitor DAPT was administered via intraperitoneal injection, followed by ZGP intervention to validate its therapeutic effects. Transcriptomic sequencing was used to analyze the differential genes before and after ZGP treatment of CTX-induced ovarian aging, and KEGG and GO analyses were applied to assess the changes in relevant signaling pathways and biological processes. In vitro experiments included the extraction, separation, and purification of ovarian germ stem cells, followed by transfection with a Notch1 overexpression plasmid. The CTX active component 4-Hydroxycyclophosphamide (4HC) was used for model intervention, and ZGP, DHEA-containing serum, and DAPT were applied to intervene with the oogonial stem cells. The effects of CTX modeling, the therapeutic efficacy of ZGP, and the general condition of the rats were observed. H&E staining was employed to assess ovarian morphology and follicle counting at various stages. Serum hormone levels were measured using ELISA, while qPCR, Western blot, flow cytometry, immunofluorescence, and IHC were utilized to analyze the expression of the Notch1/Nrf2 pathway, cell cycle proteins, and stemness-related indicators. Flow cytometry, TUNEL fluorescence, and CCK8 assays were conducted to evaluate changes in cell cycle composition, apoptosis, and proliferation. Finally, ChIP-qPCR was employed to validate the transcriptional regulation of the target gene NFE2L2 by Notch1.

RESULTS

ZGP improved serum sex hormones in ovarian aging rats, enhanced ovarian index, and optimized ovarian and uterine morphology, as well as follicle quantity composition. After transcriptome sequencing, KEGG analysis enriched the Notch signaling pathway and cell cycle, while GO analysis highlighted enrichment in the Notch pathway and stem cell population maintenance. Various experiments validated that ZGP significantly improved the expression of cell cycle-related proteins Cyclin D1 (CCND1), Cyclin E1 (CCNE1), cyclin-dependent kinase inhibitor 1a (CDKN1A), stemness markers Mouse Vasa Homolog (MVH), Octamer-binding Transcription Factor 4 (Oct4), Fragilis, 5-Bromo-2'-deoxyuridine (BrdU), as well as Notch1 and Nrf2 in aging ovarian tissues and OSCs. Additionally, ZGP promoted the proliferation of 4HC-damaged OSCs, optimized OSCs cell cycle composition, reduced G0/G1 phase arrest, and decreased early and late apoptosis. ZGP could reverse the detrimental effects on stemness and cell cycle of OSCs caused by blocking the Notch pathway. Furthermore, ZGP may activate the regulation of its target gene NFE2L2 by upregulating Notch1 expression in OSCs, thereby exerting therapeutic effects.

CONCLUSION

ZGP protects ovarian function in CTX-induced ovarian aging rats by regulating the Notch1/Nrf2 pathway. It restores serum sex hormone levels, maintains normal follicle development, promotes the proliferation of aged OSCs, optimizes the cell cycle, reduces apoptosis, and preserves stemness, thereby alleviating ovarian aging.

Sexually dimorphic dynamics of the microtubule network in medaka (Oryzias latipes) germ cells

Gametogenesis is the process through which germ cells differentiate into sexually dimorphic gametes, eggs and sperm. In the teleost fish medaka (Oryzias latipes), a germ cell-intrinsic sex determinant, foxl3, triggers germline feminization by activating two genetic pathways that regulate folliculogenesis and meiosis. Here, we identified a pathway involving a dome-shaped microtubule structure that may be the basis of oocyte polarity. This structure was first established in primordial germ cells in both sexes, but was maintained only during oogenesis and was destabilized in differentiating spermatogonia under the influence of Sertoli cells expressing dmrt1. Although foxl3 was dispensable for this pathway, dazl was involved in the persistence of the microtubule dome at the time of gonocyte development. In addition, disruption of the microtubule dome caused dispersal of bucky ball RNA, suggesting the structure may be prerequisite for the Balbiani body. Collectively, the present findings provide mechanistic insight into the establishment of sex-specific polarity through the formation of a microtubule structure in germ cells, as well as clarifying the genetic pathways implementing oocyte-specific characteristics.

Fate Decisions of Chicken Primordial Germ Cells (PGCs): Development, Integrity, Sex Determination, and Self-Renewal Mechanisms

Primordial germ cells (PGCs) are precursor cells of sperm and eggs. The fate decisions of chicken PGCs in terms of their development, integrity, and sex determination have unique features, thereby providing insights into evolutionary developmental biology. Additionally, fate decisions in the context of a self-renewal mechanism have been applied to establish culture protocols for chicken PGCs, enabling the production of genome-edited chickens and the conservation of genetic resources. Thus, studies on the fate decisions of chicken PGCs have significantly contributed to both academic and industrial development. Furthermore, studies on fate decisions have rapidly advanced owing to the recent development of essential research technologies, such as genome editing and RNA sequencing. Here, we reviewed the status of fate decisions of chicken PGCs and provided insight into other important research issues that require attention.

A Germline-Specific Regulator of Mitochondrial Fusion is Required for Maintenance and Differentiation of Germline Stem and Progenitor Cells

Maintenance and differentiation of germline stem and progenitor cells (GSPCs) is important for sexual reproduction. Here, the authors identify zebrafish pld6 as a novel germline-specific gene by cross-analyzing different RNA sequencing results, and find that pld6 knockout mutants develop exclusively into infertile males. In pld6 mutants, GSPCs fail to differentiate and undergo apoptosis, leading to masculinization and infertility. Mitochondrial fusion in pld6-depleted GSPCs is severely impaired, and the mutants exhibit defects in piRNA biogenesis and transposon suppression. Overall, this work uncovers zebrafish Pld6 as a novel germline-specific regulator of mitochondrial fusion, and highlights its essential role in the maintenance and differentiation of GSPCs as well as gonadal development and gametogenesis.

Functional Modules in Gametogenesis

Gametogenesis, the production of eggs and sperm, is a fundamental process in sexually reproducing animals. Following gametogenesis commitment and sexual fate decision, germ cells undergo several developmental processes to halve their genomic size and acquire sex-specific characteristics of gametes, including cellular size, motility, and cell polarity. However, it remains unclear how different gametogenesis processes are initially integrated. With the advantages of the teleost fish medaka (Oryzias latipes), in which germline stem cells continuously produce eggs and sperm in mature gonads and a sexual switch gene in germ cells is identified, we found that distinct pathways initiate gametogenesis cooperatively after commitment to gametogenesis. This evokes the concept of functional modules, in which functionally interlocked genes are grouped to yield distinct gamete characteristics. The various combinations of modules may allow us to explain the evolution of diverse reproductive systems, such as parthenogenesis and hermaphroditism.

Generation of offspring-producing 3D ovarian organoids derived from female germline stem cells and their application in toxicological detection

In vitro production of oocytes capable of producing offspring has exciting potential applications in reproductive medicine. Here, we generated and characterized an ovarian organoid model derived from female germline stem cells using a three-dimensional culture system. We show that this model generated normal offspring and detected drug toxicity. The ovarian organoids could produce oocytes and exhibited endocrine functions. Single-cell analysis of ovarian organoids identified six ovarian cell lineages, such as germ, granulosa and theca cells, and produced gene-expression signatures for each cell type. Investigation of the expression patterns of genes related to meiosis and gene ontogeny analysis for germ cell clusters showed that a germ cell population was maintained in the ovarian organoids. Moreover, flow cytometric analysis confirmed that the population of germ cells could be maintained on the organoids and showed that ascorbic acid treatment had a beneficial effect of germ cell population maintenance on the organoids. Furthermore, we demonstrated the successful production of offspring from oocytes derived from ovarian organoids. Finally, we showed the ovarian organoids had the potential to drug toxicological detection. For example, we found that salinomycin impaired the formation of ovarian organoids and germ cell population maintenance by inducing apoptosis. These results indicate that the female germline stem cell-derived ovarian organoids represent a valuable model system for generating oocytes that can yield offspring, and provide a novel model for drug screening and toxicological detection.

Sex Determination and Differentiation in Teleost: Roles of Genetics, Environment, and Brain

The fish reproductive system is a complex biological system. Nonetheless, reproductive organ development is conserved, which starts with sex determination and then sex differentiation. The sex of a teleost is determined and differentiated from bipotential primordium by genetics, environmental factors, or both. These two processes are species-specific. There are several prominent genes and environmental factors involved during sex determination and differentiation. At the cellular level, most of the sex-determining genes suppress the female pathway. For environmental factors, there are temperature, density, hypoxia, pH, and social interaction. Once the sexual fate is determined, sex differentiation takes over the gonadal developmental process. Environmental factors involve activation and suppression of various male and female pathways depending on the sexual fate. Alongside these factors, the role of the brain during sex determination and differentiation remains elusive. Nonetheless, GnRH III knockout has promoted a male sex-biased population, which shows brain involvement during sex determination. During sex differentiation, LH and FSH might not affect the gonadal differentiation, but are required for regulating sex differentiation. This review discusses the role of prominent genes, environmental factors, and the brain in sex determination and differentiation across a few teleost species.

Cystic proliferation of germline stem cells is necessary to reproductive success and normal mating behavior in medaka

The production of an adequate number of gametes is necessary for normal reproduction, for which the regulation of proliferation from early gonadal development to adulthood is key in both sexes. Cystic proliferation of germline stem cells is an especially important step prior to the beginning of meiosis; however, the molecular regulators of this proliferation remain elusive in vertebrates. Here, we report that ndrg1b is an important regulator of cystic proliferation in medaka. We generated mutants of ndrg1b that led to a disruption of cystic proliferation of germ cells. This loss of cystic proliferation was observed from embryogenic to adult stages, impacting the success of gamete production and reproductive parameters such as spawning and fertilization. Interestingly, the depletion of cystic proliferation also impacted male sexual behavior, with a decrease of mating vigor. These data illustrate why it is also necessary to consider gamete production capacity in order to analyze reproductive behavior.

Sex determination, gonadal sex differentiation, and plasticity in vertebrate species

A diverse array of sex determination (SD) mechanisms, encompassing environmental to genetic, have been found to exist among vertebrates, covering a spectrum from fixed SD mechanisms (mammals) to functional sex change in fishes (sequential hermaphroditic fishes). A major landmark in vertebrate SD was the discovery of the SRY gene in 1990. Since that time, many attempts to clone an SRY ortholog from nonmammalian vertebrates remained unsuccessful, until 2002, when DMY/dmrt1by was discovered as the SD gene of a small fish, medaka. Surprisingly, however, DMY/dmrt1by was found in only 2 species among more than 20 species of medaka, suggesting a large diversity of SD genes among vertebrates. Considerable progress has been made over the last 3 decades, such that it is now possible to formulate reasonable paradigms of how SD and gonadal sex differentiation may work in some model vertebrate species. This review outlines our current understanding of vertebrate SD and gonadal sex differentiation, with a focus on the molecular and cellular mechanisms involved. An impressive number of genes and factors have been discovered that play important roles in testicular and ovarian differentiation. An antagonism between the male and female pathway genes exists in gonads during both sex differentiation and, surprisingly, even as adults, suggesting that, in addition to sex-changing fishes, gonochoristic vertebrates including mice maintain some degree of gonadal sexual plasticity into adulthood. Importantly, a review of various SD mechanisms among vertebrates suggests that this is the ideal biological event that can make us understand the evolutionary conundrums underlying speciation and species diversity.

Spatial and temporal expression pattern of sex-related genes in ovo-testis of the self-fertilizing mangrove killifish (Kryptolebias marmoratus)

In vertebrates, sex determination and differentiation comprehend a fine balance between female and male factors, leading the bipotential anlage to develop towards ovary or testis, respectively. Nevertheless, the mangrove killifish, (Kryptolebias marmoratus) a simultaneous hermaphroditic species, could overcome those antagonistic pathways and evolved to develop and maintain reproductively active ovarian and testicular tissues in the same organ. Morphological and mRNA localization analyzes of developing and adult gonads demonstrate that genes related to testis (dmrt1 and amh) and ovary differentiation (foxl2 and sox9a) follow the same expression pattern observed in gonochoristic species, thus functioning as two independent organs. In addition, Amh expression patterns make it a strong candidate for initiation of the formation and maintenance of the testicular tissue in the hermaphroditic gonad. Differently from described so far, foxl3 seems to have an important role in oogenesis as well as spermatogenesis and gonadal structure.

Differential alteration in reproductive toxicity of medaka fish on exposure to nanoscale zerovalent iron and its oxidation products

Nanoscale zerovalent iron (nZVI) is a redox-active nanomaterial commonly used in remediation of soil and groundwater pollution and wastewater treatment processes. A large quantity of nZVI (e.g., >100 mg/L) accidentally released from in situ sites to nearby oxygenized aquifers could be rapidly oxidized to iron oxides (e.g., Fe₃O₄ or Fe₂O₃) and ions (e.g., Fe²⁺), for acute hypoxia effects to aquatic life. However, we do not know the ecotoxicological fate of nZVI and its oxidation products at lower, environmentally concentrations in surface water receiving waterborne transportation or effluent discharge in terms of exposure to aquatic vertebrate species. This study assessed the causal effect on reproductive toxicity in medaka adults (Oryzias latipes) of carboxymethyl cellulose-stabilized nZVI (CMC-nZVI), Fe²⁺ and iron oxide nanoparticles (nFe₃O₄) with 21-day aqueous exposure at 5 and 20 mg/L (Fe-equivalent). Such concentrations did not significantly change the dissolved oxygen, oxidation-reduction potential or pH values in the 3 iron solutions during the fish exposure period. Neither CMC-nZVI nor Fe²⁺ treated adults showed altered daily egg production (fecundity) and oxidative stress responses in observed tissues, as compared to controls. However, the fecundity in nFe₃O₄ (20 mg/L)-treated pairs was significantly decreased, with increased incidence of abnormal immature oocytes in the ovary. As well, nFe₃O₄ treatment suppressed activities of the antioxidants superoxide dismutase and expression of glutathione peroxidase (gpx) in the brain and ovary. Although nFe₃O₄ or Fe²⁺ treatments inhibited mRNA expression of hepatic estrogen receptor (er-α) in females, plasma levels of sex hormones and (Na, K)-ATPase activity in gills of treated fish did not differ from controls for both sexes. Hence, oxidation products (e.g., nFe₃O₄) from nZVI at lower milligram-per-liter levels may be potent in inducing nanoparticle-specific reproductive toxicity in medaka fish by inducing oxidative stress in female gonads. MAIN FINDING: nZVI oxidation product nFe₃O₄ at lower mg/L induces nanoparticle-specific reproductive toxicity in medaka fish.

Comparison of sex determination mechanism of germ cells between birds and fish: Cloning and expression analyses of chicken forkhead box L3-like gene

Background

Birds harbor specific sex determination and differentiation mechanisms. Although the molecular mechanisms associated with sex determination in somatic cells have been elucidated, those for germ cells remain unclear.

Results

Here, we characterized the chicken forkhead box L3 (foxl3)‐like gene as a sex‐determination factor in sexually indifferent medaka germline stem cells. The foxl3‐like gene was cloned by rapid amplification of cDNA ends, and the nucleotide sequence was analyzed. The deduced amino acid sequence was compared with FOXL3 sequences from other species, revealing low identity and similarity scores. Expression analysis of foxl3‐like mRNA during gonadogenesis showed female left‐gonad‐specific temporal expression in an egg incubated from 10 to 16 days, as well as low general expression in certain hatched female chicken organs. Moreover, the amino acid sequence deduced for the FOXL3‐like protein displayed low identity with medaka FOXL3, with the FOXL3‐like protein specifically localized in the oogonia, whereas medaka FOXL3 was found in sexually indifferent germline stem cells. Furthermore, the timing of expression differed between the foxl3‐like gene and that of medaka foxl3.

Conclusions

These results suggest that chicken FOXL3‐like protein and medaka FOXL3 differ in terms of their functions as female sex‐determination factors.

The nucleotide sequence of the chicken foxl3‐like gene was determined by RACE.

The expression of chicken foxl3‐like mRNA was virtually undetectable in specific organs, including the ovary, of 2‐week‐old female chickens.

Chicken FOXL3‐like protein was detected in the oogonia of an egg incubated for 14 days.

Temporal expression of chicken foxl3‐like mRNA was observed only in the oogonia of an egg incubated from 8 to 18 days during gonadogenesis, and the timing of gene expression differed from that of medaka foxl3.

Characterization of the European Sea Bass (Dicentrarchus labrax) Gonadal Transcriptome During Sexual Development

The European sea bass is one of the most important cultured fish in Europe and has a marked sexual growth dimorphism in favor of females. It is a gonochoristic species with polygenic sex determination, where a combination between still undifferentiated genetic factors and environmental temperature determines sex ratios. The molecular mechanisms responsible for gonadal sex differentiation are still unknown. Here, we sampled fish during the gonadal developmental period (110 to 350 days post fertilization, dpf), and performed a comprehensive transcriptomic study by using a species-specific microarray. This analysis uncovered sex-specific gonadal transcriptomic profiles at each stage of development, identifying larger number of differentially expressed genes in ovaries when compared to testis. The expression patterns of 54 reproduction-related genes were analyzed. We found that hsd17β10 is a reliable marker of early ovarian differentiation. Further, three genes, pdgfb, snx1, and nfy, not previously related to fish sex differentiation, were tightly associated with testis development in the sea bass. Regarding signaling pathways, lysine degradation, bladder cancer, and NOD-like receptor signaling were enriched for ovarian development while eight pathways including basal transcription factors and steroid biosynthesis were enriched for testis development. Analysis of the transcription factor abundance showed an earlier increase in females than in males. Our results show that, although many players in the sex differentiation pathways are conserved among species, there are peculiarities in gene expression worth exploring. The genes identified in this study illustrate the diversity of players involved in fish sex differentiation and can become potential biomarkers for the management of sex ratios in the European sea bass and perhaps other cultured species.

Regulation of germ cell sex identity in medaka

Germline stem cells are sexually indifferent or flexible even in the mature ovary and testis. Acquiring sex identity consistent with the sex of the body is a critical issue in germline stem cells for producing eggs or sperm. However, the molecular mechanism of the sexual fate decision in germ cells is unclear. Medaka is the first vertebrate in which germline stem cells were found in the mature ovary (Nakamura, Kobayashi, Nishimura, Higashijima, & Tanaka, 2010), and a germ cell autonomous switch gene involved in the sexual fate decision, foxl3, was identified (Nishimura et al., 2015) in vertebrates. Here, the mechanism underlying the sex identity of germ cells is described based on the current understanding of germ cell behavior during the sexual fate decision. The control of foxl3 expression in germ cells and components acting downstream of foxl3 are also described.

Systematic identification and comparison of expressed profiles of lncRNAs and circRNAs with associated co-expression and ceRNA networks in mouse germline stem cells

Accumulating evidence indicates that long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs) involve in germ cell development. However, little is known about the functions and mechanisms of lncRNAs and circRNAs in self-renewal and differentiation of germline stem cells. Therefore, we explored the expression profiles of mRNAs, lncRNAs, and circRNAs in male and female mouse germline stem cells by high-throughput sequencing. We identified 18573 novel lncRNAs and 18822 circRNAs in the germline stem cells and further confirmed the existence of these lncRNAs and circRNAs by RT-PCR. The results showed that male and female germline stem cells had similar GDNF signaling mechanism. Subsequently, 8115 mRNAs, 3996 lncRNAs, and 921 circRNAs exhibited sex-biased expression that may be associated with germline stem cell acquisition of the sex-specific properties required for differentiation into gametes. Gene Ontology (GO) and KEGG pathway enrichment analyses revealed different functions for these sex-biased lncRNAs and circRNAs. We further constructed correlated expression networks including coding–noncoding co-expression and competing endogenous RNAs with bioinformatics. Co-expression analysis showed hundreds of lncRNAs were correlated with sex differences in mouse germline stem cells, including lncRNA Gm11851, lncRNA Gm12840, lncRNA 4930405O22Rik, and lncRNA Atp10d. CeRNA network inferred that lncRNA Meg3 and cirRNA Igf1r could bind competitively with miRNA-15a-5p increasing target gene Inha, Acsl3, Kif21b, and Igfbp2 expressions. These findings provide novel perspectives on lncRNAs and circRNAs and lay a foundation for future research into the regulating mechanisms of lncRNAs and circRNAs in germline stem cells.