Cloning, Identification, and Functional Analysis of the Foxl2 Gene in Procambarus clarkii

Procambarus clarkii is the most widely distributed freshwater shrimp in China, with important economic value and great potential for development. The forkheadboxL2 (Foxl2) gene has been found to be involved in the reproductive development of many crustaceans. To understand the role of the Foxl2 gene in the gonad development of P. clarkii, we designed CDS-specific primers for the P. clarkii Foxl2 (PcFoxl2) gene and cloned its CDS sequence using RT-PCR. The nucleotide and protein sequence information was then analyzed through bioinformatics analysis. The expression and subcellular localization of PcFoxl2 in various tissues were detected using qRT-PCR and in situ hybridization. The effects of PcFoxl2 knockdown on gonad development were investigated using RNA interference. The results showed that the CDS length of the PcFoxl2 gene was 1614 bp and encoded 537 amino acids. Protein sequence comparison and phylogenetic analysis showed that PcFoxl2 was the closest relative to Crayfish. qRT-PCR analysis indicated that the expression level of PcFoxl2 in the testis was significantly higher (>40 fold) than that in the ovary (p < 0.01). The in situ hybridization results showed that PcFoxl2 was expressed in both the cytoplasm and the nucleus of egg cells, and that the expression was strongest in egg cells at the early stage of yolk synthesis, while weak in the secondary oocytes. The positive signal was strongest in the spermatocyte nucleolus, while only a trace signal was observed in the cytoplasm. After interfering with the PcFoxl2 gene using dsRNA, the expression of PcFoxl2 in the RNA interference group was significantly lower than that in the control group, and this interference effect lasted for one week. Moreover, the gonad index of the experimental group was significantly lower than that of the control group (p < 0.05) after 10 days of P. clarkii cultivation following PcFoxl2 knockdown. The expression levels of the nanos and S3a genes, which are related to gonad development, decreased significantly after PcFoxl2 gene interference. The results suggest that the Foxl2 gene is involved in the growth and development of gonads, particularly in the development of testis, and is related to the early development of oocytes. This study provides a theoretical basis for the artificial breeding of P. clarkii.

Circular RNA expression profiles and CircSnd1-miR-135b/c-foxl2 axis analysis in gonadal differentiation of protogynous hermaphroditic ricefield eel Monopterus albus

BACKGROUND

The expression and biological functions of circular RNAs (circRNAs) in reproductive organs have been extensively reported. However, it is still unclear whether circRNAs are involved in sex change. To this end, RNA sequencing (RNA-seq) was performed in gonads at 5 sexual stages (ovary, early intersexual stage gonad, middle intersexual stage gonad, late intersexual stage gonad, and testis) of ricefield eel, and the expression profiles and potential functions of circRNAs were studied.

RESULTS

Seven hundred twenty-one circRNAs were identified, and the expression levels of 10 circRNAs were verified by quantitative real-time PCR (qRT-PCR) and found to be in accordance with the RNA-seq data, suggesting that the RNA-seq data were reliable. Then, the sequence length, category, sequence composition and the relationship between the parent genes of the circRNAs were explored. A total of 147 circRNAs were differentially expressed in the sex change process, and GO and KEGG analyses revealed that some differentially expressed (such as novel_circ_0000659, novel_circ_0004005 and novel_circ_0005865) circRNAs were closely involved in sex change. Furthermore, expression pattern analysis demonstrated that both circSnd1 and foxl2 were downregulated in the process of sex change, which was contrary to mal-miR-135b. Finally, dual-luciferase reporter assay and RNA immunoprecipitation showed that circSnd1 and foxl2 can combine with mal-miR-135b and mal-miR-135c. These data revealed that circSnd1 regulates foxl2 expression in the sex change of ricefield eel by acting as a sponge of mal-miR-135b/c.

CONCLUSION

Our results are the first to demonstrate that circRNAs have potential effects on sex change in ricefield eel; and circSnd1 could regulate foxl2 expression in the sex change of ricefield eel by acting as a sponge of mal-miR-135b/c. These data will be useful for enhancing our understanding of sequential hermaphroditism and sex change in ricefield eel or other teleosts.

Screening and Identification of Transcription Factors Potentially Regulating Foxl2 Expression in Chlamys farreri Ovary

Foxl2 is an evolutionarily conserved female sex gene, which is specifically expressed in the ovary and mainly involved in oogenesis and ovarian function maintenance. However, little is known about the mechanism that regulates Foxl2 specific expression during the ovary development. In the present study, we constructed the gonadal yeast one-hybrid (Y1H) library of Chlamysfarreri with ovaries and testes at different developmental stages using the Gateway technology. The library capacity was more than 1.36 × 10⁷ CFU, and the length of the inserted fragment was 0.75 Kb~2 Kb, which fully met the demand of yeast library screening. The highly transcriptional activity promoter sequence of C. farreri Foxl2 (Cf-Foxl2) was determined at -1000~-616 bp by dual-luciferase reporter (DLR) assay and was used as bait to screen possible transcription factors from the Y1H library. Eleven candidate factors, including five unannotated factors, were selected based on Y1H as well as their expressional differences between ovaries and testes and were verified for the first time to be involved in the transcriptional regulation of Cf-Foxl2 by RT-qPCR and DLR. Our findings provided valuable data for further studying the specific regulation mechanism of Foxl2 in the ovary.

Nr5a homologues in the ricefield eel Monopterus albus: Alternative splicing, tissue-specific expression, and differential roles on the activation of cyp19a1a promoter in vitro

Nr5a (Fushi tarazu factor 1, Ftz-F1) homologues belong to the nuclear receptor superfamily, and are involved in the regulation of reproduction in vertebrates. Four genes encoding Nr5a homologues were present in the genome of ricefield eel, which are designated as nr5a1a, nr5a1b, nr5a2, and nr5a5 in the present study. Alternatively spliced transcripts were identified for nr5a1a and nr5a1b genes. Sequence analysis indicated that nr5a5 is possibly a paralog of nr5a2, and nr5a1b is lost during evolution in some teleosts including tilapia and medaka. Ricefield eel nr5a genes exhibit tissue-specific expression patterns, with nr5a1a and nr5a1b resembling that of the SF-1/Ad4BP (NR5A1) subfamily, and nr5a2 and nr5a5 resembling that of the NR5A2/LRH/FTF subfamily. Transcriptomic analysis revealed parallel expression profiles of nr5a1a, foxl2, and cyp19a1a in ovarian follicles during vitellogenesis, with peak values at the late vitellogenic stage. Real-time PCR indicated that the expression levels of nr5a1a and foxl2 in gonads were decreased significantly during the sexual transition from female to the late intersexual stage. In vitro transient transfection assay showed that Nr5a1a up-regulated ricefield eel cyp19a1a promoter activities synergistically with Foxl2. However, Nr5a1b, Nr5a2, and Nr5a5 could neither activate ricefield eel cyp19a1a promoter alone nor enhance the stimulatory effects of Foxl2 on cyp19a1a promoter activities. Collectively, the above data suggest that Nr5a homologues may have diverse and differential roles in the tissues of ricefield eels. The up-regulation of gonadal nr5a1a and foxl2 during vitellogenesis may be important for the ovarian development whereas their down-regulation during the sexual transition period may be important for the sex change process of ricefield eels, possibly through the regulation of cyp19a1a gene expression.

Oocytes suppress FOXL2 expression in cumulus cells in mice†

Cumulus cells and mural granulosa cells (MGCs) play distinct roles during follicular development, and normal development of these cell lineages is critical for the female fertility. Transcriptomic diversification between the two cell lineages is obviously a critical mechanism for their functional diversification; however, the transcriptional regulators responsible for this event have not been fully defined. In this study, we sought to identify key transcriptional regulators responsible for the differential gene expression between the two cell lineages. In silico analysis of transcriptomic comparison between cumulus cells and MGCs identified several candidate regulators responsible for the diversification of the two cell lineages. Among them, we herein focused on forkhead box L2 (FOXL2) and showed that expressions of FOXL2 as well as its target transcripts were differentially regulated between cumulus cells and MGCs. The lower expression of FOXL2 in cumulus cells seemed to be due to the suppression by oocyte-derived paracrine signals. These results suggest that FOXL2 is one of the critical transcription factors that determine cumulus cell and MGC lineages under the control of oocytes.

Identification and functional analysis of a sex-biased transcriptional factor Foxl2 in the bay scallop Argopecten irradians irradians

Transcription factor Foxl2 is an evolutionarily conserved gene playing pivotal roles in regulation of early ovarian differentiation and maintenance in animals. However, the Foxl2 gene has not been thoroughly studied in hermaphroditic scallops. In this study, we cloned and characterized a Foxl2 (designated as AiFoxl2) from the bay scallop Argopecten irradians irradians. The open reading frame of AiFoxl2 was 1122 bp encoding 373 amino acids residues and contained a conserved forkhead box domain. Quantitative real-time PCR showed that AiFoxl2 was mainly expressed in the ovary. Moreover, the highest expression of AiFoxl2 in the ovary was detected at proliferative stage and growing stage, while the lowest level was found at resting stage. During the embryonic and larval development, expression of AiFoxl2 was found first in fertilized eggs, increased significantly at the blastula stage, and reached peak value at the D-larvae stage. When AiFoxl2 was knocked down, testis development-related genes (Dmrt1, Sox7 and Sox9) were up-regulated significantly while the ovary development-related genes (Vg, HSD14, and GATA-1) were down-regulated manifestly. These findings suggested that AiFoxl2 was a female-related gene in A. i. irradians and may be involved in regulation of ovarian development and differentiation.

Variance in expression and localization of sex-related genes CgDsx, CgBHMG1 and CgFoxl2 during diploid and triploid Pacific oyster Crassostrea gigas gonad differentiation

Several evolutionarily conserved classes of transcriptional regulators were involved in diverse sex determination and differentiation pathways across taxa, whereas their roles in most mollusks is still limited. The Pacific oyster Crassostrea gigas, a dioecious bivalve with sex reversal, could be an ideal model for this issue because of its complex sexuality and potential disruption of sex differentiation in triploid individuals. Here, two mRNA splicing isoforms of a DM domain gene CgDsx and two isoforms of a novel sex-related CgBHMG1 (ortholog of BHMG1 in mammals) were identified in C. gigas. Real time PCR showed that two isoforms of CgDsx and one isoform of CgBHMG1 displayed male-specific expression in diploid oysters, opposite with the female-specific CgFoxl2 (a potential factor of female gonadic differentiation). Interestingly, the four sex-specific transcripts in diploid oyster were expressed in triploid oysters with opposite sex, triploid hermaphrodites and individuals at stage I that sex could not be determined. Subsequent in situ hybridization analysis on gonads of diploid oysters revealed predominant expression of CgDsx in spermatogonia of testes, CgBHMG1 in spermatocytes of testes and follicle cells of ovaries, and CgFoxl2 in follicle cells of ovaries and some male germ cells in testes. And aberrant co-expression of the three genes in triploid oysters was localized in gonadal tubules of gonads at stage I, ovarian follicle cells and undetermined gonial cells in nontypical hermaphroditic gonads with rare female materials. From the above, temporal and spatial expression of sex-related genes in diploid and triploid gonads indicated that CgDsx and CgFoxl2 might mainly function in C. gigas sex differentiation, and CgBHMG1 appeared as a factor involved in meiosis. This work will help to illuminate the gene network of sex differentiation in bivalves and provides new sight on this issue from comparison between diploid and triploid individuals.

Spatiotemporal Expression of Foxl2 and Dmrt1 before, during, and after Sex Determination in the Sea Turtle Lepidochelys olivacea

The sex of sea turtles is determined by temperature during egg incubation. Thus, climate change affects the sex ratio, exacerbating their vulnerability to extinction. Understanding spatiotemporal effects of temperature on sex determination at the gonadal level may facilitate the design of strategies to mitigate the effects of global warming. Here, we used qRT-PCR and immunofluorescence to analyze the spatiotemporal expression of Dmrt1 and Foxl2 in developing gonads of Lepidochelys olivacea incubated at male-producing temperature (MPT, 26°C) or female-producing temperature (FPT, 33°C). Although both transcription factors are expressed in bipotential gonads up to stage 25, the timing of their sexually dimorphic regulation differs. Whereas the dimorphic expression of Dmrt1 protein initiates at stage 24, Foxl2 protein was expressed specifically in females at stage 25. Interestingly, whereas Dmrt1 colocalizes with Sox9 in cell nuclei of primary medullary cords to form the testis cords, Foxl2 protein is first detected in Sox9-negative cells of primary medullary cords, prior to its substantial expression in the ovarian cortex. Thus, results suggest that the temperature-dependent regulation of sexual pathways is stochastic among the cells of primary medullary cords in undifferentiated bipotential gonads of the olive ridley.

Estrogen alters gonadal soma-derived factor (Gsdf)/Foxl2 expression levels in the testes associated with testis-ova differentiation in adult medaka, Oryzias latipes

Testis-ova differentiation in sexually mature male medaka (Oryzias latipes) is easily induced by estrogenic chemicals, indicating that spermatogonia persist in sexual bipotentiality, even in mature testes in medaka. By contrast, the effects of estrogen on testicular somatic cells associated with testis-ova differentiation in medaka remain unclear. In this study, we focused on the dynamics of sex-related genes (Gsdf, Dmrt1, and Foxl2) expressed in Sertoli cells in the mature testes of adult medaka during estrogen-induced testis-ova differentiation. When mature male medaka were exposed to estradiol benzoate (EB; 800ng/L), testis-ova first appeared after EB treatment for 14days (observed as the first oocytes of the leptotene-zygotene stage). However, the testis remained structurally unchanged, even after EB treatment for 28days. Although Foxl2 is a female-specific sex gene, EB treatment for 7days induced Foxl2/FOXL2 expression in all Sertoli cell-enclosed spermatogonia before testis-ova first appeared; however, Foxl2 was not detected in somatic cells in control testes. Conversely, Sertoli-cell-specific Gsdf mRNA expression levels significantly decreased after EB treatment for 14days, and no changes were observed in DMRT1 localization following EB treatment, whereas Dmrt1 mRNA levels increased significantly. Furthermore, after EB exposure, FOXl2 and DMRT1 were co-localized in Sertoli cells during testis-ova differentiation, although FOXL2 localization was undetectable in Sertoli-cell-enclosed apoptotic testis-ova, whereas DMRT1 remained localized in Sertoli cells. These results indicated for the first time that based on the expression of female-specific sex genes, feminization of Sertoli cells precedes testis-ova differentiation induced by estrogen in mature testes in medaka; however, complete feminization of Sertoli cells was not induced in this study. Additionally, it is suggested strongly that Foxl2 and Gsdf expression constitute potential molecular markers for evaluating the effects of estrogenic chemicals on testicular somatic cells associated with estrogen-induced testis-ova differentiation in mature male medaka.

Molecular identification and expression of the Foxl2 gene during gonadal sex differentiation in northern snakehead Channa argus

Channa argus is one of the most commercially important fish species in China. Studies show that males of C. argus grow faster than females at the same age. In order to explore the sex differentiation mechanism of C. argus, we isolated the full length of the sex-related gene Foxl2 cDNA and analysed its expression patterns during gonadal sex differentiation. Alignment of known Foxl2 amino acid sequences from vertebrates confirmed the conservation of the Foxl2 open reading frame, especially the forkhead domain and C-terminal region. Quantitative RT-PCR revealed that Foxl2 is predominantly expressed in brain, pituitary, gill and ovary, with its highest level in ovary but low levels in testis and other tissues, reflecting a potential role for Foxl2 in the brain-pituitary-gonad axis in C. argus. Our ontogenetic stage data showed that C. argus Foxl2 expression was significantly upregulated from 1 to 11 days posthatching (dph) and that the initiation of expression preceded the first anatomical ovarian differentiation (27 dph), suggesting that Foxl2 might play a potential role in early gonadal sex differentiation in C. argus. In addition, the Foxl2 protein was primarily located in granulosa cells surrounding the oocytes of mature C. argus, implying that Foxl2 may have a basic function in granulosa cell differentiation and the maintenance of oocytes.

DNA methylation level of cyp19a1a and Foxl2 gene related to their expression patterns and reproduction traits during ovary development stages of Japanese flounder (Paralichthys olivaceus)

Foxl2 and cyp19a1a genes are crucial for the ovarian development, and Foxl2 could play a direct regulatory role on cyp19a1a transcription. In this study, we aimed to study DNA methylation status and mRNA expression patterns of Foxl2 and cyp19a1a genes during ovarian development of female Japanese flounder. The relative expression level of cyp19a1a and Foxl2 gene during the gonadal development stages was measured by quantitative PCR. Moreover, DNA methylation status in the promoter and coding regions of the two genes was detected by bisulfite sequencing. The estradiol-17β (E2) was measured by radioimmunoassay. The results showed low expression levels of cyp19a1a and Foxl2 genes in stages II and V, while the highest expression levels were detected in stage IV. The variation trend of the methylation level of all CpG sites in promoter and exon 1 of cyp19a1a gene and three CpG rich regions in coding region of Foxl2 gene was negatively associated with their expression levels during the ovarian development. In addition, two CpG sites in promoter and seven CpG sites in exon 1 of cyp19a1a were on the putative transcription factors binding sequence. Further studies showed that the forkhead domain, which is important for Foxl2 binding to cyp19a1a was located in the F1 and F2 region. These results provide a powerful theoretical basis for the regulatory mechanism on Foxl2 regulating cyp19a1a and promoting gonadal differentiation towards the female pathway, and further reveal that Foxl2 and cyp19a1a play a vital role in the female Japanese flounder gonad development.

foxl2 and foxl3 are two ancient paralogs that remain fully functional in teleosts

FOXL2 is a forkhead transcription factor involved in mammalian development and regulation of reproduction. Two foxl2 paralogs, foxl2a and foxl2b, have been described in various teleost species and were considered as fish-specific duplicates. Here, we report the isolation and characterization of foxl2a (foxl2) and foxl2b (foxl3) in European sea bass (Dicentrarchus labrax), together with the identification of these two genes in non-teleost genomes. Phylogenetic and synteny analyses indicate that these paralogs originated from an ancient genome duplication event that happened long before the teleost specific duplication. While foxl2/foxl2a has been maintained in most vertebrate lineages, foxl2b, which we propose to rename as foxl3, was repeatedly lost in tetrapods. Gonadal expression patterns of the sea bass genes point to a strong sexual dimorphism, and the mRNA levels of foxl2 in ovary and foxl3 in testis vary significantly during the reproductive cycle. When overexpressed in cultured ovarian follicular cells, foxl2 and foxl3 produced functional transcription factors able to control the expression of reproduction-related genes. Taken together, these data suggest that Foxl2 may play a conserved role in ovarian maturation, while Foxl3 could be involved in testis physiology.

Analysis of protein expression in zebrafish during gonad differentiation by targeted proteomics

The molecular mechanisms governing sex determination and differentiation in the zebrafish (Danio rerio) are not fully understood. To gain more insights into the function of specific genes in these complex processes, the expression of multiple candidates needs to be assessed, preferably on the protein level. Here, we developed a targeted proteomics method based on selected reaction monitoring (SRM) to study the candidate sex-related proteins in zebrafish which were selected based on a global proteomics analysis of adult gonads and representational difference analysis of male and female DNA, as well as on published information on zebrafish and other vertebrates. We employed the developed SRM protocols to acquire time-resolved protein expression profiles during the gonad differentiation period in vas::EGFP transgenic zebrafish. Evidence on protein expression was obtained for the first time for several candidate genes previously studied only on the mRNA level or suggested by bioinformatic predictions. Tuba1b (tubulin alpha 1b), initially included in the study as one of the potential housekeeping proteins, was found to be preferentially expressed in the adult testis with nearly absent expression in the ovary. The revealed changes in protein expression patterns associated with gonad differentiation suggest that several of the examined proteins, especially Ilf2 and Ilf3 (interleukin enhancer-binding factors 2 and 3), Raldh3 (retinaldehyde dehydrogenase type 3), Zgc:195027 (low density lipoprotein-related receptor protein 3) and Sept5a (septin 5a), may play a specific role in the sexual differentiation in zebrafish.

MicroRNA-133b stimulates ovarian estradiol synthesis by targeting Foxl2

Forkhead L2 (Foxl2) is expressed in ovarian granulosa cells and participates in steroidogenesis by transcriptionally regulating target genes such as steroidogenic acute regulatory protein (StAR) and CYP19A1. In this study, a direct link between microRNA-133b (miR-133b) and Foxl2-mediated estradiol release in granulosa cells was established. miR-133b was involved in follicle-stimulating hormone (FSH)-induced estrogen production. Luciferase assays confirmed that miR-133b was bound to the 3' untranslated region (3'UTR) of Foxl2 mRNA. Consistent with this finding, miR-133b overexpression reduced the Foxl2 levels. Furthermore, miR-133b inhibited Foxl2 binding to the StAR and CYP19A1 promoter sequences. These results demonstrate that miR-133b down-regulates Foxl2 expression in granulosa cells by directly targeting the 3'UTR, thus inhibiting the Foxl2-mediated transcriptional repression of StAR and CYP19A1to promote estradiol production.