Germ cell-specific transcriptional regulator sohlh2 is essential for early mouse folliculogenesis and oocyte-specific gene expression

Transcriptional profiling of five isolated size-matched stages of human preantral follicles

Little is known of the early stages of human follicular development and the complex processes that regulate follicular growth. To identify genes of potential importance, we analysed follicle-related transcripts in five populations of isolated size-matched human preantral follicles by microarray analysis. Oocyte-specific genes were found to be the most abundant and differentially expressed transcripts and included germ cell transcription factors LHX8 and SOHLH2 which were significantly down-regulated during preantral follicle development. Differentially expressed genes also included transcription factors of NOTCH signalling, IGF2, orphan nuclear receptor LRH-1, and homeobox gene HOXA7, indicating potentially important regulatory roles for these genes during early human folliculogenesis. We also found that FSHR mRNA and protein were present in the earliest stages of preantral follicles, whereas LHR was absent. In conclusion, our data identify specific oocyte and somatic genes in small human follicles that impact early follicle growth, and provide foundation for further analysis of the signalling pathways involved in early human folliculogenesis.

Transcription factor SOHLH1 potentially associated with primary ovarian insufficiency

OBJECTIVE

To investigate whether gene variants of SOHLH1 exist in Chinese and Serbian patients with primary ovarian insufficiency (POI).

DESIGN

Case-control genetic study.

SETTING

University hospitals.

PATIENT(S)

A total of 364 Han Chinese and 197 Serbian women with nonsyndromic POI and ethnically matched controls.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

SOHLH1 gene sequencing.

RESULT(S)

We found 10 novel heterozygous variants in our cohorts of 561 women with POI but none in the 600 ethnically matched controls. Statistical and bioinformatic analyses indicated that three of the eight variants in Chinese POI cases are potentially disease causing. They comprise two missense variants (p.Ser317Phe and p.Glu376Lys) that might each change activity of the SOHLH1 protein as a transcription factor and one variant (c.*118C>T) located in the 3' untranslated region of the SOHLH1 gene, which might generate a new binding site for the microRNA hsa-miR-888-5p. Of the two variants in the Serbian POI cases, both were synonymous, and no missense variant was identified. The allele frequencies of some known single-nucleotide polymorphisms were statistically significantly different between patients and controls in both the Chinese and Serbian groups.

CONCLUSION(S)

Our results suggest that SOHLH1 may be regarded as a new candidate gene for POI.

Association of genetic variants in SOHLH1 and SOHLH2 with non-obstructive azoospermia risk in the Chinese population

OBJECTIVE

Spermatogenesis and oogenesis specific basic helix-loop-helix 1 (SOHLH1) and spermatogenesis and oogenesis specific basic helix-loop-helix 2 (SOHLH2) play essential roles for both spermatogenesis and oogenesis. The aim of this study was to evaluate the association of SOHLH1 and SOHLH2 single nucleotide polymorphisms (SNPs) with non-obstructive azoospermia (NOA) in the Chinese population.

STUDY DESIGN

In this study, we assessed 7 single nucleotide polymorphisms (SNPs) of SOHLH1 and SOHLH2 with Sequenom iplex technology in 361 NOA cases and 368 fertile controls.

RESULTS

We found that the SNPs rs1328626 and rs6563386 of SOHLH2 were significantly associated with NOA risk, of which, a protective effect of minor allele T of rs1328626 on NOA (P = 0.038, odds ratio [OR] = 0.799, 95% confidence interval [CI] = 0.645-0.988) and a significantly increased risk of the SNP rs6563386 with the minor allele G to NOA (P = 0.029, OR = 1.402, 95% CI = 1.034-1.9) were observed, respectively. Our data indicated that the haplotype GC of the variants rs1328626 and rs6563386 conferred a significantly increased risk of NOA (P = 0.031, OR = 1.397, 95% CI = 1.031-1.895). Moreover, we found the genotype distribution of rs1328641 was significantly associated with testes volume in the NOA patients (P = 0.022).

CONCLUSIONS

The polymorphisms rs1328626 and rs6563386 of the SOHLH2 gene would be the genetic risk factors for NOA in the Chinese population. The SNP rs1328641 might influence testes development in the NOA patients.

A comprehensive transcriptomic analysis of infant and adult mouse ovary

Ovary development is a complex process involving numerous genes. A well-developed ovary is essential for females to keep fertility and reproduce offspring. In order to gain a better insight into the molecular mechanisms related to the process of mammalian ovary development, we performed a comparative transcriptomic analysis on ovaries isolated from infant and adult mice by using next-generation sequencing technology (SOLiD). We identified 15,454 and 16,646 transcriptionally active genes at the infant and adult stage, respectively. Among these genes, we also identified 7021 differentially expressed genes. Our analysis suggests that, in general, the adult ovary has a higher level of transcriptomic activity. However, it appears that genes related to primordial follicle development, such as those encoding Figla and Nobox, are more active in the infant ovary, whereas expression of genes vital for follicle development, such as Gdf9, Bmp4 and Bmp15, is upregulated in the adult. These data suggest a dynamic shift in gene expression during ovary development and it is apparent that these changes function to facilitate follicle maturation, when additional functional gene studies are considered. Furthermore, our investigation has also revealed several important functional pathways, such as apoptosis, MAPK and steroid biosynthesis, that appear to be much more active in the adult ovary compared to those of the infant. These findings will provide a solid foundation for future studies on ovary development in mice and other mammals and help to expand our understanding of the complex molecular and cellular events that occur during postnatal ovary development.

Rapamycin-sensitive mTORC1 signaling is involved in physiological primordial follicle activation in mouse ovary

In mammals, resting female oocytes reside in primordial ovarian follicles. An individual primordial follicle may stay quiescent for a protracted period of time before initiating follicular growth, which is also termed “activation.” Female reproductive capacity is sustained by the gradual, streamlined activation of the entire population of primordial follicles, but this process also results in reproductive senescence in older animals. Based on the recent findings that genetically triggered, excessive mammalian target of rapamycin complex 1 (mTORC1) activation in mouse oocytes leads to accelerated primordial follicle activation, we examined the necessity of mTORC1 signaling in physiological primordial follicle activation. We found that induction of oocyte mTORC1 activity is associated with early follicular growth in neonatal mouse ovaries. Pharmacological inhibition of mTORC1 activity in vivo by rapamycin treatment leads to a marked, but partial, suppression of primordial follicle activation. The suppressive effect of rapamycin on primordial follicle activation was reproduced in cultured ovaries. While rapamycin did not apparently affect several plausible cellular targets in neonatal mouse ovaries, such as mTORC2, AKT, or cyclin-dependent kinase (CDK) inhibitor p27-KIP1, its inhibitory effect on Cyclin A2 gene expression implies that mTORC1 signaling in oocytes may engage a Cyclin A/CDK regulatory network that promotes primordial follicle activation. The current work strengthens the concept that mTORC1-dependent events in the oocytes of primordial follicles may represent potential targets for intervention in humans to slow the depletion of the ovarian reserve.

Auto-regulation of the Sohlh1 gene by the SOHLH2/SOHLH1/SP1 complex: implications for early spermatogenesis and oogenesis

Tissue-specific basic helix-loop-helix (bHLH) transcription factor proteins often play essential roles in cellular differentiation. The bHLH proteins SOHLH2 and SOHLH1 are expressed specifically in spermatogonia and oocytes and are required for early spermatogonial and oocyte differentiation. We previously reported that knocking out Sohlh2 causes defects in spermatogenesis and oogenesis similar to those in Sohlh1-null mice, and that Sohlh1 is downregulated in the gonads of Sohlh2-null mice. We also demonstrated that SOHLH2 and SOHLH1 can form a heterodimer. These observations led us to hypothesize that the SOHLH2/SOHLH1 heterodimer regulates the Sohlh1 promoter. Here, we show that SOHLH2 and SOHLH1 synergistically upregulate the Sohlh1 gene through E-boxes upstream of the Sohlh1 promoter. Interestingly, we identified an SP1-binding sequence, called a GC-box, adjacent to these E-boxes, and found that SOHLH1 could bind to SP1. Furthermore, chromatin-immunoprecipitation analysis using testes from mice on postnatal day 8 showed that SOHLH1 and SP1 bind to the Sohlh1 promoter region in vivo. Our findings suggest that an SOHLH2/SOHLH1/SP1 ternary complex autonomously and cooperatively regulates Sohlh1 gene transcription through juxtaposed E- and GC-boxes during early spermatogenesis and oogenesis.

The grapevine basic helix-loop-helix (bHLH) transcription factor positively modulates CBF-pathway and confers tolerance to cold-stress in Arabidopsis

Basic helix-loop-helix (bHLH)-type transcription factors play diverse roles in plant physiological response and stress-adaptive regulation network. Here, we identified one grapevine bHLH transcription factor from a cold-tolerant accession 'Heilongjiang seedling' of Chinese wild Vitis amurensis (VabHLH1) as a transcriptional activator involved in cold stress. We also compared with its counterpart from a cold-sensitive Vitis vinifera cv. Cabernet Sauvignon (VvbHLH1). These two putative proteins are characterized by the presence of the identically conserved regions of 54 amino acid residues of bHLH signature domain, and shared 99.1% amino acid identity, whereas several stress-related cis-regulatory elements located in both promoter regions differed in types and positions. Expressions of two bHLHs in grapevine leaves were induced by cold stress, but evidently differ between two grapevine genotypes upon cold exposure. Two grapevine bHLH proteins were exclusively localized to the nucleus and exhibited strong transcriptional activation activities in yeast cells. Overexpression of either VabHLH1 or VvbHLH1 transcription factor did not affect the growth and development of transgenic Arabidopsis plants, but enhanced tolerance to cold stress. The improved tolerance in VabHLH1- or VvbHLH1-overexpressing Arabidopsis plants is associated with multiple physiological and biochemical changes that occurred during the time-course cold stress. These most common changes include the evaluated levels of proline, decreased amounts of malondialdehyde and reduced membrane injury as reflected by electrolyte leakage. VabHLH1 and VvbHLH1 displayed overlapping, but not identical, roles in activating the corresponding CBF cold signaling pathway, especially in regulating the expression of CBF3 and RD29A. Our findings demonstrated that two grapevine bHLHs act as positive regulators of the cold stress response, modulating the level of COR gene expression, which in turn confer tolerance to cold stress.

Di-(2-ethylhexyl) phthalate and bisphenol A exposure impairs mouse primordial follicle assembly in vitro

Bisphenol-A (BPA) and diethylhexyl phthalate (DEHP) are estrogenic compounds widely used in commercial plastic products. Previous studies have shown that exposure to such compounds have adverse effects on various aspects of mammalian reproduction including folliculogenesis. The objective of this study was to examine the effects of BPA and DEHP exposure on primordial follicle formation. We found that germ cell nest breakdown and primordial follicle assembly were significantly reduced when newborn mouse ovaries were exposed to 10 or 100 μM BPA and DEHP in vitro. Moreover, BPA and DEHP exposure increased the number of TUNEL positive oocytes and the mRNA level of the pro-apoptotic gene Bax in oocytes. These effects were associated with decreased expression of oocyte specific genes such as LIM homeobox 8 (Lhx8), factor in the germline alpha (Figla), spermatogenesis and oogenesis helix-loop-helix (Sohlh2), and newborn ovary homeobox (Nobox). Interestingly, BPA and DEHP exposure also prevented DNA demethylation of CpG sites of the Lhx8 gene in oocytes, a process normally associated with folliculogenesis. Finally, folliculogenesis was severely impaired in BPA and DEHP exposed ovaries after transplantation into the kidney capsules of immunodeficient mice. In conclusion, BPA and DEHP exposures impair mouse primordial follicle assembly in vitro.

FOXL2: a central transcription factor of the ovary

Forkhead box L2 (FOXL2) is a gene encoding a forkhead transcription factor preferentially expressed in the ovary, the eyelids and the pituitary gland. Its germline mutations are responsible for the blepharophimosis ptosis epicanthus inversus syndrome, which includes eyelid and mild craniofacial defects associated with primary ovarian insufficiency. Recent studies have shown the involvement of FOXL2 in virtually all stages of ovarian development and function, as well as in granulosa cell (GC)-related pathologies. A central role of FOXL2 is the lifetime maintenance of GC identity through the repression of testis-specific genes. Recently, a highly recurrent somatic FOXL2 mutation leading to the p.C134W subtitution has been linked to the development of GC tumours in the adult, which account for up to 5% of ovarian malignancies. In this review, we summarise data on FOXL2 modulators, targets, partners and post-translational modifications. Despite the progresses made thus far, a better understanding of the impact of FOXL2 mutations and of the molecular aspects of its function is required to rationalise its implication in various pathophysiological processes.

CRL4 complex regulates mammalian oocyte survival and reprogramming by activation of TET proteins

The duration of a woman's reproductive period is determined by the size and persistence of a dormant oocyte pool. Specific oocyte genes are essential for follicle maintenance and female fertility. The mechanisms that regulate the expression of these genes are poorly understood. We found that a cullin-ring finger ligase-4 (CRL4) complex was crucial in this process. Oocyte-specific deletion of the CRL4 linker protein DDB1 or its substrate adaptor VPRBP (also known as DCAF1) caused rapid oocyte loss, premature ovarian insufficiency, and silencing of fertility maintaining genes. CRL4(VPRBP) activates the TET methylcytosine dioxygenases, which are involved in female germ cell development and zygote genome reprogramming. Hence, CRL4(VPRBP) ubiquitin ligase is a guardian of female reproductive life in germ cells and a maternal reprogramming factor after fertilization.

Defining the neighborhoods that escort the oocyte through its early life events and into a functional follicle

The ovary functions to chaperone the most precious cargo for female individuals, the oocyte, thereby allowing the passage of genetic material to subsequent generations. Within the ovary, single oocytes are surrounded by a legion of granulosa cells inside each follicle. These two cell types depend upon one another to support follicle formation and oocyte survival. The infrastructure and events that work together to ultimately form these functional follicles within the ovary are unprecedented, given that the oocyte originates as a cell like all other neighboring cells within the embryo prior to gastrulation. This review discusses the journey of the germ cell in the context of the developing female mouse embryo, with a focus on specific signaling events and cell-cell interactions that escort the primordial germ cell as it is specified into the germ cell fate, migrates through the hindgut into the gonad, differentiates into an oocyte, and culminates upon formation of the primordial and then primary follicle.

Mouse HORMAD1 is a meiosis i checkpoint protein that modulates DNA double- strand break repair during female meiosis

Oocytes in embryonic ovaries enter meiosis I and arrest in the diplonema stage. Perturbations in meiosis I, such as abnormal double-strand break (DSB) formation and repair, adversely affect oocyte survival. We previously discovered that HORMAD1 is a critical component of the synaptonemal complex but not essential for oocyte survival. No significant differences were observed in the number of primordial, primary, secondary, and developing follicles between wild-type and Hormad1(−/−)newborn, 8-day, and 80-day ovaries. Meiosis I progression in Hormad1(−/−) embryonic ovaries was normal through the zygotene stage and in oocytes arrested in diplonema; however, we did not visualize oocytes with completely synapsed chromosomes. We investigated effects of HORMAD1 deficiency on the kinetics of DNA DSB formation and repair in the mouse ovary. We irradiated Embryonic Day 16.5 wild-type and Hormad1(−/−) ovaries and monitored DSB repair using gammaH2AX, RAD51, and DMC1 immunofluorescence. Our results showed a significant drop in unrepaired DSBs in the irradiated Hormad1(−/−) zygotene oocytes as compared to the wild-type oocytes. Moreover, Hormad1 deficiency rescued Dmc1(−/−) oocytes. These results indicate that Hormad1 deficiency promotes DMC1-independent DSB repairs, which in turn helps asynaptic Hormad1(−/−) oocytes resist perinatal loss.

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.

Cumulus and granulosa cell markers of oocyte and embryo quality

Lack of an objective, accurate, and noninvasive embryo assessment strategy remains one of the major challenges encountered in in vitro fertilization. Cumulus and mural granulosa cells reflect the characteristics of the oocyte, providing a noninvasive means to assess oocyte quality. Specifically, transcriptomic profiling of follicular cells may help identify biomarkers of oocyte and embryo competence. Current transcriptomics technologies include quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) for analysis of individual genes and microarrays and high-throughput deep sequencing for whole genome expression profiling. Recently, using qRT-PCR and microarray technologies, a multitude of studies correlated changes in cumulus or granulosa cell gene expression with clinically relevant outcome parameters, including in vitro embryo development and pregnancy. While the initial findings are promising, a clinical benefit from the use of identified biomarker genes remains to be demonstrated in randomized controlled trials.

Role of Mael in early oogenesis and during germ-cell differentiation from embryonic stem cells in mice in vitro

In a previous study, we have identified a set of conserved spermatogenic genes whose expression is restricted to testis and ovary and that are developmentally regulated. One of these genes, the transcription factor Mael, has been reported to play an essential role in mouse spermatogenesis. Nevertheless, the role of Mael in mouse oogenesis has not been defined. In order to analyse the role of Mael in mouse oogenesis, the expression of this gene was blocked during early oogenesis in mouse in vitro using RNAi technology. In addition, the role of Mael during differentiation of embryonic stem cells (ESC) into germ cells in vitro was analysed. Results show that downregulation of Mael by a specific short interfering RNA disrupted fetal oocyte growth and differentiation in fetal ovary explants in culture and the expression of several germ-cell markers in ESC during their differentiation. These results suggest that there is an important role for Mael in early oogenesis and during germ-cell differentiation from embryonic stem cells in mouse in vitro.

Transcription factors in the maintenance and survival of primordial follicles

Primordial follicles are formed prenatally in mammalian ovaries, and at birth they are fated to be activated to primary follicles, to be dormant, or to die. During the early stage of folliclulogenesis, the oocyte undergoes dynamic alterations in expression of numerous genes, which are regulated by transcription factors. Several germ-cell specific transcriptional regulators are critical for formation and maintenance of follicles. These transcriptional regulators include: Figla, Lhx8, Nobox, Sohlh1, and Sohlh2. A subset of these transcriptional regulators is mutated in women with ovarian insufficiency and infertility. Establishment of this oocyte pool is essential for fertility. This review focuses on these transcriptional regulators of female primordial follicles.

Effects of BMP4/SMAD signaling pathway on mouse primordial follicle growth and survival via up-regulation of Sohlh2 and c-kit

Bone morphogenetic protein 4 (BMP4) is essential for the development of primordial follicles, although its underlying mechanism remains largely unknown. By using cultured ovaries, the effects of BMP4 and the potential signal transduction pathways were investigated. Ovaries from 3-day-old female mouse pups were maintained in organ culture in the absence (control) or presence of BMP4 (100 ng/ml). At different culture time, the effects of BMP4 on primordial follicle growth and survival were assayed by follicle count and TUNEL labeling. The expression of phospho-SMAD1/5/8, Sohlh2, and c-kit were measured by immunohistochemistry, RT-PCR, and Western blotting. Immunohistochemistry was also performed to determine the expression pattern of BMP4, pSMAD1/5/8, Sohlh2, and c-kit in vivo during ovarian development. The results showed treatments of ovaries with BMP4 resulted in a significant (P < 0.05) increase on the primordial-to-primary follicle transition. The oocytes of primordial follicles treated with BMP4 were also less likely to undergo apoptosis. BMP4 enhanced the phosphorylation of SMAD1/5/8 and up-regulated the expression of Sohlh2 and c-kit in primordial follicles. During ovarian development in vivo, Sohlh2, and c-kit exhibited similar expression patterns to BMP4 and pSMAD1/5/8 in primordial follicles. The present studies suggest that BMP4/SMAD signaling pathway initiate primordial follicle growth and prevented oocyte apoptosis via up-regulation of Sohlh2 and c-kit.

Oocyte-specific linker histone H1foo is an epigenomic modulator that decondenses chromatin and impairs pluripotency

Mammalian oocytes contain the histone H1foo, a distinct member with low sequence similarity to other members in the H1 histone family. Oocyte-specific H1foo exists until the second embryonic cell stage. H1foo is essential for oocyte maturation in mice; however, the molecular function of this H1 subtype is unclear. To explore the function of H1foo, we generated embryonic stem (ES) cells ectopically expressing H1foo fused to an EGFP (H1foo-ES). Interestingly, ectopic expression of H1foo prevented normal differentiation into embryoid bodies (EBs). The EB preparations from H1foo-ES cells maintained the expression of pluripotent marker genes, including Nanog, Myc and Klf9, and prevented the shift of the DNA methylation profile. Because the short hairpin RNA-mediated knockdown of H1foo-EGFP recovered the differentiation ability, H1foo was involved in preventing differentiation. Furthermore, ChIP analysis revealed that H1foo-EGFP bound selectively to a set of hypomethylated genomic loci in H1foo-ES, clearly indicating that these loci were targets of H1foo. Finally, nuclease sensitivity assay suggested that H1foo made these target loci decondensed. We concluded that H1foo has an impact on the genome-wide, locus-specific epigenetic status.

Germ cells are not the primary factor for sexual fate determination in goldfish

The presence of germ cells in the early gonad is important for sexual fate determination and gonadal development in vertebrates. Recent studies in zebrafish and medaka have shown that a lack of germ cells in the early gonad induces sex reversal in favor of a male phenotype. However, it is uncertain whether the gonadal somatic cells or the germ cells are predominant in determining gonadal fate in other vertebrate. Here, we investigated the role of germ cells in gonadal differentiation in goldfish, a gonochoristic species that possesses an XX-XY genetic sex determination system. The primordial germ cells (PGCs) of the fish were eliminated during embryogenesis by injection of a morpholino oligonucleotide against the dead end gene. Fish without germ cells showed two types of gonadal morphology: one with an ovarian cavity; the other with seminiferous tubules. Next, we tested whether function could be restored to these empty gonads by transplantation of a single PGC into each embryo, and also determined the gonadal sex of the resulting germline chimeras. Transplantation of a single GFP-labeled PGC successfully produced a germline chimera in 42.7% of the embryos. Some of the adult germline chimeras had a developed gonad on one side that contained donor derived germ cells, while the contralateral gonad lacked any early germ cell stages. Female germline chimeras possessed a normal ovary and a germ-cell free ovary-like structure on the contralateral side; this structure was similar to those seen in female morphants. Male germline chimeras possessed a testis and a contralateral empty testis that contained some sperm in the tubular lumens. Analysis of aromatase, foxl2 and amh expression in gonads of morphants and germline chimeras suggested that somatic transdifferentiation did not occur. The offspring of fertile germline chimeras all had the donor-derived phenotype, indicating that germline replacement had occurred and that the transplanted PGC had rescued both female and male gonadal function. These findings suggest that the absence of germ cells did not affect the pathway for ovary or testis development and that phenotypic sex in goldfish is determined by somatic cells under genetic sex control rather than an interaction between the germ cells and somatic cells.

Seboxplays an important role during the early mouse oogenesisin vitro

Oogenesis is a highly complex process that requires the exquisite temporal and spatial regulation of gene expression at multiple levels. Skin–embryo–brain–oocyte homeobox (Sebox) gene encodes a transcription factor that is highly expressed in germinal vesicle stage oocytes and that plays an essential role in early embryogenesis at the 2-cell stage in the mouse. AsSeboxis also expressed in mouse fetal ovaries, the aim of the present study was to study its role during the early oogenesisin vitro. Expression ofSeboxwas low in 15.5 to 17.5 days post coitum (dpc) ovaries, showed a peak at 18.5 dpc and then its expression decreased dramatically in newborn ovaries.Seboxexpression was efficiently knocked down (>80%) in fetal mouse ovary explants in culture using RNAi technology. When fetal ovary explants were transfected withSebox-specific RNAi, the number of oocytes at germinal vesicle stage and showing a diameter of 40–70 μm was decreased significantly to 75% after 7 days of culture relative to the negative control, and to 22.4% after 10 days of culture, thus indicating thatSeboxplays an important role in the early oogenesis 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.

The ovarian reserve in mammals: a functional and evolutionary perspective

The constitution and the control of the ovarian reserve is of importance in mammals and women. In particular, the number of primordial follicles at puberty is positively correlated with the number of growing follicles and their response to gonadotropin treatments. The size of this ovarian reserve depends on genes involved in germ cell proliferation and differentiation, sexual differentiation, meiosis, germ cell degeneration, formation of primordial follicles, and on a potential mechanism of self-renewal of germ stem cells. In this review, we present the state of the art of the knowledge of genes and factors involved in all these processes. We first focus on the almost 70 genes identified mainly by mouse invalidation models, then we discuss the most plausible hypothesis concerning the possibility of the existence of germ cell self-renewal by neo-oogenesis in animal species and human, with a special interest for the role of corresponding genes in evolutionary distinct model species. All of the genes pointed out here are candidates susceptible to explain fertility defects such as the premature ovarian failure in human.

New understandings on folliculogenesis/oogenesis regulation in mouse as revealed by conditional knockout

In comparison to conventional knockout technology and in vitro research methods, conditional gene knockout has remarkable advantages. In the past decade, especially during the past five years, conditional knockout approaches have been used to study the regulation of folliculogenesis, follicle growth, oocyte maturation and other major reproductive events. In this review, we summarize the recent findings about folliculogenesis/oogenesis regulation, including the functions of four signaling cascades or glycoprotein domains that have been extensively studied by conditional gene deletion. Several other still fragmented areas of related work are introduced which are awaiting clarification. We have also discussed the future potential of this technology in clarifying gene functions in reproductive biology.

[Genetics of early ovarian differentiation: recent data]

Early ovarian development has long been thought of as a default pathway switched on passively by the absence of SRY gene. Recent genetic and transcriptomic studies challenge this view and show that two master pathways simultaneously repress male-specific genes and activate female-specific genetic cascades. This antagonistic action is maintained from embryonic stages to adulthood. The differentiation of the ovarian somatic component is regulated by both the forkhead transcription factor FOXL2 (alone or in combination with oestrogens according to the species) and β-catenin pathway activated by Wnt4 and Rspo1. The sex-specific change in the fate of primordial germ cells depends on the gonad environment. Female gonocytes actively proliferate by mitosis then enter meiosis I until the diplotene stage. Primordial follicle formation occurs when oocytes are individually surrounded with pre-granulosa cells. In mammals, the population of primordial follicles serves as a resting and finite pool of oocytes available during the female reproductive life span. Recent data on factors controlling these molecular processes will be presented in this review.

TOPAZ1, a novel germ cell-specific expressed gene conserved during evolution across vertebrates

BACKGROUND

We had previously reported that the Suppression Subtractive Hybridization (SSH) approach was relevant for the isolation of new mammalian genes involved in oogenesis and early follicle development. Some of these transcripts might be potential new oocyte and granulosa cell markers. We have now characterized one of them, named TOPAZ1 for the Testis and Ovary-specific PAZ domain gene.

PRINCIPAL FINDINGS

Sheep and mouse TOPAZ1 mRNA have 4,803 bp and 4,962 bp open reading frames (20 exons), respectively, and encode putative TOPAZ1 proteins containing 1,600 and 1653 amino acids. They possess PAZ and CCCH domains. In sheep, TOPAZ1 mRNA is preferentially expressed in females during fetal life with a peak during prophase I of meiosis, and in males during adulthood. In the mouse, Topaz1 is a germ cell-specific gene. TOPAZ1 protein is highly conserved in vertebrates and specifically expressed in mouse and sheep gonads. It is localized in the cytoplasm of germ cells from the sheep fetal ovary and mouse adult testis.

CONCLUSIONS

We have identified a novel PAZ-domain protein that is abundantly expressed in the gonads during germ cell meiosis. The expression pattern of TOPAZ1, and its high degree of conservation, suggests that it may play an important role in germ cell development. Further characterization of TOPAZ1 may elucidate the mechanisms involved in gametogenesis, and particularly in the RNA silencing process in the germ line.

Transcriptome profiling of sheep granulosa cells and oocytes during early follicular development obtained by laser capture microdissection

Background

Successful achievement of early folliculogenesis is crucial for female reproductive function. The process is finely regulated by cell-cell interactions and by the coordinated expression of genes in both the oocyte and in granulosa cells. Despite many studies, little is known about the cell-specific gene expression driving early folliculogenesis. The very small size of these follicles and the mixture of types of follicles within the developing ovary make the experimental study of isolated follicular components very difficult.

The recently developed laser capture microdissection (LCM) technique coupled with microarray experiments is a promising way to address the molecular profile of pure cell populations. However, one main challenge was to preserve the RNA quality during the isolation of single cells or groups of cells and also to obtain sufficient amounts of RNA.

Using a new LCM method, we describe here the separate expression profiles of oocytes and follicular cells during the first stages of sheep folliculogenesis.

Results

We developed a new tissue fixation protocol ensuring efficient single cell capture and RNA integrity during the microdissection procedure. Enrichment in specific cell types was controlled by qRT-PCR analysis of known genes: six oocyte-specific genes (SOHLH2, MAEL, MATER, VASA, GDF9, BMP15) and three granulosa cell-specific genes (KL, GATA4, AMH).

A global gene expression profile for each follicular compartment during early developmental stages was identified here for the first time, using a bovine Affymetrix chip. Most notably, the granulosa cell dataset is unique to date. The comparison of oocyte vs. follicular cell transcriptomes revealed 1050 transcripts specific to the granulosa cell and 759 specific to the oocyte.

Functional analyses allowed the characterization of the three main cellular events involved in early folliculogenesis and confirmed the relevance and potential of LCM-derived RNA.

Conclusions

The ovary is a complex mixture of different cell types. Distinct cell populations need therefore to be analyzed for a better understanding of their potential interactions. LCM and microarray analysis allowed us to identify novel gene expression patterns in follicular cells at different stages and in oocyte populations.

Premature ovarian failure in nobox-deficient mice is caused by defects in somatic cell invasion and germ cell cyst breakdown

PURPOSE

To understand the mechanism of premature ovarian failure (POF).

METHODS

The ultrastructural (electron microscopy) analysis of primordial ovarian follicles in Nobox deficient mice.

RESULTS

We studied, for the first time, the fate of oogonia in embryonic (prenatal) mouse ovaries and showed that the abolishment of the transition from germ cell cysts to primordial follicles in the ovaries of Nobox deficient mice is caused by defects in germ cell cyst breakdown, leading to the formation of syncytial follicles instead of primordial follicles.

CONCLUSIONS

These results indicate that POF syndrome in Nobox deficient mice results from the faulty signaling between somatic and germ line components during embryonic development. In addition, the extremely unusual and abnormal presence of adherens junctions between unseparated oocytes within syncytial follicles indicates that faulty communication between somatic and germ cells is involved in, or leads to, abnormalities in the cell adhesion program.

The oocyte-specific transcription factor, Nobox, regulates the expression of Pad6, a peptidylarginine deiminase in the oocyte

Nobox is an oocyte-specific transcriptional regulator. Nobox deficiency disrupts early folliculogenesis and the expression of oocyte-specific genes in mice. In the present study, we found that peptidylarginine deiminase 6 (Pad6) was downregulated in Nobox-null ovaries. Pad6 is preferentially expressed in oocytes and its transcript is detectable at embryonic day 16.5. In addition, we identified one Nobox DNA-binding element (NBE) within the mouse Pad6 promoter. The NBE includes a core sequence TAATTA. Sequence-specific binding of Nobox to the TAATTA motif was confirmed. Nobox overexpression augmented transcriptional activity of a luciferase reporter driven by mouse Pad6. Our findings indicate that Nobox is a critical regulator that orchestrates oocyte-specific genes such as Pad6 during folliculogenesis.

Mammalian oocyte development: checkpoints for competence

During the lifespan of the female, biochemical changes occur in the ovarian environment. These changes are brought about by numerous endogenous and exogenous factors, including husbandry practices, production demands and disease, and can have a profound effect on ovarian oocyte quality and subsequent embryo development. Despite many investigations, there is no consensus regarding the time or period of follicular oocyte development that is particularly sensitive to insult. Here, the key molecular and morphological events that occur during oocyte and follicle growth are reviewed, with a specific focus on identifying critical checkpoints in oocyte development. The secondary follicle stage appears to be a key phase in follicular oocyte development because major events such as activation of the oocyte transcriptome, sequestration of the zona pellucida, establishment of bidirectional communication between the granulosa cells and the oocyte and cortical granule synthesis occur during this period of development. Several months later, the periovulatory period is also characterised by the occurrence of critical events, including appropriate degradation or polyadenylation of mRNA transcripts, resumption of meiosis, spindle formation, chromosome alignment and segregation, and so should also be considered as a potential checkpoint of oocyte development.

Steady-state level of kit ligand mRNA in goat ovaries and the role of kit ligand in preantral follicle survival and growth in vitro

The aims of this study were to investigate steady-state level of Kit Ligand (KL) mRNA and its effects on in vitro survival and growth of caprine preantral follicles. RT-PCR was used to analyze caprine steady-state level of KL mRNA in primordial, primary, and secondary follicles, and in small (1-3 mm) and large (3-6 mm) antral follicles. Furthermore, ovarian fragments were cultured for 1 or 7 days in Minimal Essential Medium (MEM(+)) supplemented with KL (0, 1, 10, 50, 100, or 200 ng/ml). Noncultured (control) and cultured fragments were processed for histology and transmission electron microscopy (TEM). RT-PCR demonstrated an increase in steady-state level of KL mRNA during the transition from primary to secondary follicles. Small antral follicles had higher steady-state levels of KL mRNA in granulosa and theca cells than large follicles. After 7 days, only 50 ng/ml of KL had maintained the percentage of normal follicles similar to control. After 1 day, all KL concentrations reduced the percentage of primordial follicles and increased the percentage of growing follicles. KL at 10, 50, 100, or 200 ng/ml increased primary follicles, compared to MEM(+) after 7 days. An increase in oocyte and follicular diameter was observed at 50 ng/ml of KL. TEM confirmed ultrastructural integrity of follicles after 7 days at 50 ng/ml of KL. In conclusion, the KL mRNAs were detected in all follicular categories. Furthermore, 50 ng/ml of KL maintained the integrity of caprine preantral follicle cultured for 7 days and stimulated primordial follicle activation and follicle growth.

The ovary: basic biology and clinical implications

The classical view of ovarian follicle development is that it is regulated by the hypothalamic-pituitary-ovarian axis, in which gonadotropin-releasing hormone (GnRH) controls the release of the gonadotropic hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH), and that ovarian steroids exert both negative and positive regulatory effects on GnRH secretion. More recent studies in mice and humans indicate that many other intra-ovarian signaling cascades affect follicular development and gonadotropin action in a stage- and context-specific manner. As we discuss here, mutant mouse models and clinical evidence indicate that some of the most powerful intra-ovarian regulators of follicular development include the TGF-beta/SMAD, WNT/FZD/beta-catenin, and RAS/ERK1/2 signaling pathways and the FOXO/FOXL2 transcription factors.

MicroRNA transcriptome in the newborn mouse ovaries determined by massive parallel sequencing

Small non-coding RNAs, such as microRNAs (miRNAs), are involved in diverse biological processes including organ development and tissue differentiation. Global disruption of miRNA biogenesis in Dicer knockout mice disrupts early embryogenesis and primordial germ cell formation. However, the role of miRNAs in early folliculogenesis is poorly understood. In order to identify a full transcriptome set of small RNAs expressed in the newborn (NB) ovary, we extracted small RNA fraction from mouse NB ovary tissues and subjected it to massive parallel sequencing using the Genome Analyzer from Illumina. Massive sequencing produced 4 655 992 reads of 33 bp each representing a total of 154 Mbp of sequence data. The Pash alignment algorithm mapped 50.13% of the reads to the mouse genome. Sequence reads were clustered based on overlapping mapping coordinates and intersected with known miRNAs, small nucleolar RNAs (snoRNAs), piwi-interacting RNA (piRNA) clusters and repetitive genomic regions; 25.2% of the reads mapped to known miRNAs, 25.5% to genomic repeats, 3.5% to piRNAs and 0.18% to snoRNAs. Three hundred and ninety-eight known miRNA species were among the sequenced small RNAs, and 118 isomiR sequences that are not in the miRBase database. Let-7 family was the most abundantly expressed miRNA, and mmu-mir-672, mmu-mir-322, mmu-mir-503 and mmu-mir-465 families are the most abundant X-linked miRNA detected. X-linked mmu-mir-503, mmu-mir-672 and mmu-mir-465 family showed preferential expression in testes and ovaries. We also identified four novel miRNAs that are preferentially expressed in gonads. Gonadal selective miRNAs may play important roles in ovarian development, folliculogenesis and female fertility.

Oogenesis specific genes (Nobox, Oct4, Bmp15, Gdf9, Oogenesin1 and Oogenesin2) are differentially expressed during natural and gonadotropin-induced mouse follicular development

Using a semi-quantitative, single-cell sensitive RT-PCR method, we studied the expression of oogenesis specific genes (Nobox, Oct4, Bmp15, Gdf9, Oogenesin1 and Oogenesin2) in single oocytes collected from primordial, primary, secondary, preantral and antral follicles during natural and gonadotropin-induced mouse follicular development. We compared the number of transcripts of these genes, showing that they are differentially expressed, both in natural conditions and under gonadotropin-induction throughout the assessed developmental stages. Our data show a clear increase in the number of transcripts between the primordial until the preantral stages, with the exception of the Oogenesin1 transcripts under gonadotropin-induction. The number of transcripts starts decreasing at the antral stage and proceeds until the metaphase II stage, with values very similar to those obtained for the primordial oocytes in both analyzed conditions. Under exogenous gonadotropin-induction, oocyte recruitment to ovulation at the preantral stage is marked by an increase in Nobox and Oogenesin2 gene expression that is concomitant with a decrease in Oogenesin1 gene expression. Oocytes that are able to proceed into whole embryo development show a tight regulation of Nobox and Oct4 expression at the antral stage. A parallel immunocytochemical study at the protein level corroborates these findings.

New insights into ovarian function

Infertility adversely affects many couples worldwide. Conversely, the exponential increase in world population threatens our planet and its resources. Therefore, a greater understanding of the fundamental cellular and molecular events that control the size of the primordial follicle pool and follicular development is of utmost importance to develop improved in vitro fertilization as well as to design novel approaches to regulate fertility. In this review we attempt to highlight some new advances in basic research of the mammalian ovary that have occurred in recent years focusing primarily on mouse models that have contributed to our understanding of ovarian follicle formation, development, and ovulation. We hope that these new insights into ovarian function will trigger more research and translation to clinically relevant problems.

Coordinated regulation of follicle development by germ and somatic cells

The continuum of folliculogenesis begins in the fetal ovary with the differentiation of the oogonia and their isolation within the primordial follicles. Primordial follicle activation is an enigmatic process, whereby some follicles enter the growing pool to become primary follicles, thereby embarking on an irreversible progression towards ovulation or atresia. This process is under the coordinated regulation of factors from the oocyte itself, as well as from the somatic cells of the ovary, in particular the theca and granulosa cells, which are structural components of the follicle. These two influences provide the principal stimuli for the growth of the follicle to the late preantral or early antral stage of development. The endocrine effects of the gonadotrophins FSH and LH are essential to the continued progression of the follicle and most atresia can be attributed to the failure to receive or process the gonadotrophin signals. The peri-ovulatory state has received intensive investigation recently, demonstrating a coordinated role for gonadotrophins, steroids, epidermal growth factor family proteins and prostaglandins. Thus, a complex programme of coordinated interaction of governing elements from both germ and somatic cell sources is required for successful follicle development.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Molecular mechanisms underlying the activation of mammalian primordial follicles

In humans and other mammalian species, the pool of resting primordial follicles serves as the source of developing follicles and fertilizable ova for the entire length of female reproductive life. One question that has intrigued biologists is: what are the mechanisms controlling the activation of dormant primordial follicles. Studies from previous decades have laid a solid, but yet incomplete, foundation. In recent years, molecular mechanisms underlying follicular activation have become more evident, mainly through the use of genetically modified mouse models. As hypothesized in the 1990s, the pool of primordial follicles is now known to be maintained in a dormant state by various forms of inhibitory machinery, which are provided by several inhibitory signals and molecules. Several recently reported mutant mouse models have shown that a synergistic and coordinated suppression of follicular activation provided by multiple inhibitory molecules is necessary to preserve the dormant follicular pool. Loss of function of any of the inhibitory molecules for follicular activation, including PTEN (phosphatase and tensin homolog deleted on chromosome 10), Foxo3a, p27, and Foxl2, leads to premature and irreversible activation of the primordial follicle pool. Such global activation of the primordial follicle pool leads to the exhaustion of the resting follicle reserve, resulting in premature ovarian failure in mice. In this review, we summarize both historical and recent results on mammalian primordial follicular activation and focus on the up-to-date knowledge of molecular networks controlling this important physiological event. We believe that information obtained from mutant mouse models may also reflect the molecular machinery responsible for follicular activation in humans. These advances may provide a better understanding of human ovarian physiology and pathophysiology for future clinical applications.

Awakening the oocyte: controlling primordial follicle development

Oocytes are sequestered in primordial follicles before birth and remain quiescent in the ovary, often for decades, until recruited into the growing pool throughout the reproductive years. Therefore, activation of follicle growth is a major biological checkpoint that controls female reproductive potential. However, we are only just beginning to elucidate the cellular mechanisms required for either maintenance of the quiescent primordial follicle pool or initiation of follicle growth. Understanding the intracellular signalling systems that control oocyte maintenance and activation has significant implications for improving female reproductive productivity and longevity in mammals, and has application in domestic animal husbandry, feral animal population control and infertility in women.