Testicular Dnmt3 expression and global DNA methylation are down-regulated by gonadotropin releasing hormones in the ricefield eel Monopterus albus

Lemming and Vole Cycles: A New Intrinsic Model

It is 100 years since the first paper described the multiannual cycles in Arctic rodents and lagomorphs. The mechanisms driving population cycles in animals like lemmings and voles are complex, often attributed to extrinsic factors, such as food availability and quality, pathogens, parasites and/or predators. While extrinsic factors provide insights into population cycles, none fully explain the phenomenon. We propose an underlying innate, intrinsic mechanism, based on epigenetic regulation, that drives population cycles under harsh arctic conditions. We propose that epigenetically driven phenotypic changes associated with sexual development, growth and behaviour accumulate over time in offspring, eventually producing a phase change from rising population density to eventual population collapse. Under this hypothesis, and unlike previous hypotheses, extrinsic factors modify population cycles but would not be primary drivers. The interaction between our intrinsic cycle and extrinsic factors explains established phenomena like delayed-density dependence, whereby population growth is controlled by time-dependent negative feedback. We advocate integrating a century of field research with the latest epigenetic analysis to better understand the drivers of population cycles.

The ontogenic gonadal transcriptomes provide insights into sex change in the ricefield eel Monopterus albus

BACKGROUND

The ricefield eel is a freshwater protogynous hermaphrodite fish and has become an important aquaculture species in China. The sex change of ricefield eel is impeding its aquaculture practice, particularly the large-scale artificial breeding. Many studies including transcriptomes of mixed gonadal samples from different individuals have been aimed to elucidate mechanisms underlying the sex change. However, the key physiological factors involved in the initiation of sex change remain to be identified.  RESULTS: The present study performed transcriptomic analysis on gonadal samples of different sexual stages obtained through biopsy from the same fish undergoing sex change. A total of 539,764,816 high-quality reads were generated from twelve cDNA libraries of gonadal tissues at female (F), early intersexual (EI), mid-intersexual (MI), and late intersexual (LI) stages of three individual sex-changing fish. Pairwise comparisons between EI and F, MI and EI, and LI and MI identified 886, 319, and 10,767 differentially expressed genes (DEGs), respectively. Realtime quantitative PCR analysis of 12 representative DEGs showed similar expression profiles to those inferred from transcriptome data, suggesting the reliability of RNA-seq data for gene expression analysis. The expression of apoeb, csl2, and enpp2 was dramatically increased and peaked at EI while that of cyp19a1a, wnt4a, fgf16, and foxl2a significantly downregulated from F to EI and remained at very low levels during subsequent development until LI, which suggests that apoeb, csl2, enpp2, cyp19a1a, wnt4a, fgf16, and foxl2a may be closely associated with the initiation of sex change of ricefield eels.

CONCLUSIONS

Collectively, results of the present study confirmed that the down-regulation of female-related genes, such as cyp19a1a, wnt4a, fgf16, and foxl2a, is important for the sex change of ricefield eels. More importantly, some novel genes, including apoeb, csl2, and enpp2, were shown to be expressed with peak values at EI, which are potentially involved in the initiation of sex change. The present transcriptomic data may provide an important research resource for further unraveling the mechanisms underlying the sex change and testicular development in ricefield eels as well as other teleosts.

Decoding genome recombination and sex reversal

Over the past 440 years since the discovery of the medicinal value of swamp eels, much progress has been made in the study of their biology. The fish is emerging as an important model animal in sexual development, in addition to economic and pharmaceutical implications. Tracing genomic history that shapes speciation of the fish has led to discovery of the whole genome-wide chromosome fission/fusion events. Natural intersex differentiation is a compelling feature for sexual development research. Notably, identification of progenitors of germline stem cells that have bipotential to differentiate into either male or female germline stem cells provides new insight into sex reversal. Here, we review these advances that have propelled the field forwards and present unsolved issues that will guide future investigations to finally elucidate vertebrate sexual development using the new model.

Gene expression profiling of DNA methyltransferase genes in Siniperca chuatsi based on transcriptome sequencing

The mandarin fish (Siniperca chuatsi) DNA methyltransferase gene 1 (dnmt1) was highly expressed in the mesonephros, head kidney and gonad, whereas dnmt2 was expressed in most tissues. dnmt3a was highly expressed in the brain and spleen, but dnmt3b was mainly expressed in the brain and head kidney. The genes dnmt1 and dnmt2 were highly expressed in the early stages of embryonic development, and dnmt3a and dnmt3b were expressed later. These genes also showed certain changes after artificial diet acclimation, salinity adaptation and immune stress.

Sexual dimorphism of DNA and histone methylation profiles in the gonads of the olive flounder Paralichthys olivaceus

DNA methylation and histone methylation are two types of the most important epigenetic modifications. However, research on their differential expression in gonads of male and female fish is limited. In this study, we examined the characteristics of DNA methylation and tri-methylation of lysine 4 of histone H3 (H3K4me3) modification profiles in the gonads of the wild-type and meio-gynogenetic olive flounders Paralichthys olivaceus. Enzyme-linked immunosorbent assay (ELISA) analysis revealed that the global DNA methylation level was higher in the testis than in the ovary. Real-time quantitative PCR (qPCR) results indicated that maintenance DNA methyltransferase gene dnmt1 and de novo DNA methyltransferase gene dnmt3a are highly expressed in the ovary, while DNA demethyltransferase genes tets are highly expressed in the testis. The inconsistency of DNA methylation and methyltransferase genes in the gonads might associate with the differential distribution in the testis. 5-mC mainly located in the spermatids of the testis was shown with immunohistochemistry (IHC). Furtherly, dnmt3a and tets are mainly located in spermatocytes and oocytes with in situ hybridization (ISH) analysis. As for H3K4me3, total level is higher in the ovary detected with western blot assay. IHC results showed that the signals of H3K4me3 in Sertoli cells of the testis were stronger than those in spermatocytes and spermatids. Methyltransferase gene kmt2b and demethylase genes kdm5a and kdm5c also exhibit much higher expression in the testis with qPCR, and ISH stronger signals of kmt2b and kdm5s were detected in spermatocytes. These results implied that DNA methylation and H3K4me3 are involved in the flounder sex differences and gametogenesis.

Dnmt3aa but Not Dnmt3ab Is Required for Maintenance of Gametogenesis in Nile Tilapia (Oreochromis niloticus)

Dnmt3a, a de novo methyltransferase, is essential for mammalian germ line DNA methylation. Only one Dnmt3a is identified in mammals, and homozygous mutants of Dnmt3a are lethal, while two Dnmt3a paralogs, dnmt3aa and dnmt3ab, are identified in teleosts due to the third round of genome duplication, and homozygous mutants of dnmt3aa and dnmt3ab are viable in zebrafish. The expression patterns and roles of dnmt3aa and dnmt3ab in gonadal development remain poorly understood in teleosts. In this study, we elucidated the precise expression patterns of dnmt3aa and dnmt3ab in tilapia gonads. Dnmt3aa was highly expressed in oogonia, phase I and II oocytes and granulosa cells in ovaries and spermatogonia and spermatocytes in testes, while dnmt3ab was mainly expressed in ovarian granulosa cells and testicular spermatocytes. The mutation of dnmt3aa and dnmt3ab was achieved by CRISPR/Cas9 in tilapia. Lower gonadosomatic index (GSI), increased apoptosis of oocytes and spermatocytes and significantly reduced sperm quality were observed in dnmt3aa^−/− mutants, while normal gonadal development was observed in dnmt3ab^−/− mutants. Consistently, the expression of apoptotic genes was significantly increased in dnmt3aa^−/− mutants. In addition, the 5-methylcytosine (5-mC) level in dnmt3aa^−/− gonads was decreased significantly, compared with that of dnmt3ab^−/− and wild type (WT) gonads. Taken together, our results suggest that dnmt3aa, not dnmt3ab, plays important roles in maintaining gametogenesis in teleosts.

Effects of 2,3',4,4',5-pentachlorobiphenyl exposure during pregnancy on DNA methylation in the testis of offspring in the mouse

Polychlorinated biphenyls (PCBs) are persistent organic pollutants, and the widespread use of PCBs has had adverse effects on human and animal health. This study experiment explored the effects of 2,3',4,4',5-pentachlorobiphenyl (PCB118) on the mammalian reproductive system. PCB118 was administered to pregnant mice from 7.5 to 12.5 days of gestation; F1 mice were obtained and the reproductive system of F1 male mice was examined. PCB118 damaged the reproductive system in male F1 mice, as evidenced by negative effects on the testicular organ coefficient (testes weight/bodyweight), a decrease in the diameter of seminiferous tubules and a significant reduction in the anogenital distance in 35-day-old F1 mice. In addition, methylation levels of genomic DNA were reduced, with reductions in the expression of the DNA methyltransferases DNMT1, DNMT3A and DNMT3B, as well as that of the epigenetic regulatory factor ubiquitin like with PHD and ring finger domains 1 (Uhrf1). Together, the results of this study provide compelling evidence that exposure of pregnant mice to PCB118 during primordial germ cell migration in the fetus affects the reproductive system of the offspring and decreases global methylation levels in the testis.

DNA methylation modification is associated with gonadal differentiation in Monopterus albus

Background

Both testis and ovary can be produced sequentially in an individual with the same genome when sex reversal occurs in the teleost Monopterus albus, and epigenetic modification is supposed to be involved in gonadal differentiation. However, DNA methylation regulation mechanism underlying the gonadal differentiation remains unclear.

Results

Here, we used liquid chromatography-electrospray ionization tandem mass spectrometry (LC–ESI–MS/MS) to simultaneously determine endogenous levels of both 5-methyl-2′-deoxycytidine (m⁵dC) and 5-hydroxymethyl-2′-deoxycytidine (hm⁵dC) during gonadal differentiation. Overall DNA methylation level was upregulated from ovary to testis via ovotestis. As a de novo methylase, dnmt3aa expression was also upregulated in the process. Notably, we determined transcription factor Foxa1 for dnmt3aa gene expression. Site-specific mutations and chromatin immunoprecipitation showed that Foxa1 can bind to and activate the dnmt3aa promoter. Furthermore, DNA methylation levels of key genes foxl2 (forkhead box L2) and cyp19a1a (cytochrome P450, family 19, subfamily A, polypeptide 1a) in regulation of female hormone synthesis were consistently upregulated during gonadal differentiation.

Conclusions

These data suggested that dynamic change of DNA methylation modification is associated with gonadal differentiation.

Evolutionary history of DNA methylation related genes in chordates: new insights from multiple whole genome duplications

DNA methylation is an important epigenetic mechanism involved in many biological processes, i.e. gametogenesis and embryonic development. However, increased copy numbers of DNA methylation related genes (dnmt, tet and tdg) have been found during chordate evolution due to successive whole genome duplication (WGD) events. Their evolutionary history and phylogenetic relationships remain unclear. The present study is the first to clarify the evolutionary history of DNA methylation genes in chordates. In particular, our results highlight the fixation of several dnmt3-related genes following successive WGD throughout evolution. The rainbow trout genome offered a unique opportunity to study the early evolutionary fates of duplicated genes due to a recent round of WGD at the radiation of salmonids. Differences highlighted in transcriptional patterns of these genes during gametogenesis and ontogenesis in trout indicated that they might be subjected to sub- or neo-functionalisation after WDG. The fixation of multiple dnmt3 genes in genomes after WGD could contribute to the diversification and plastic adaptation of the teleost.

MicroRNA-377 Inhibits Atherosclerosis by Regulating Triglyceride Metabolism Through the DNA Methyltransferase 1 in Apolipoprotein E-Knockout Mice

BACKGROUND

Lipoprotein lipase (LPL) plays an important role in triglyceride metabolism. It is translocated across endothelial cells to reach the luminal surface of capillaries by glycosylphosphatidylinositol-anchored high-density lipoprotein binding protein 1 (GPIHBP1), where it hydrolyzes triglycerides in lipoproteins. MicroRNA 377 (miR-377) is highly associated with lipid levels. However, how miR-377 regulates triglyceride metabolism and whether it is involved in the development of atherosclerosis remain largely unexplored. Methods and Results: The clinical examination displayed that miR-377 expression was markedly lower in plasma from patients with hypertriglyceridemia compared with non-hypertriglyceridemic subjects. Bioinformatics analyses and a luciferase reporter assay showed that DNA methyltransferase 1 (DNMT1) was a target gene of miR-377. Moreover, miR-377 increased LPL binding to GPIHBP1 by directly targeting DNMT1 in human umbilical vein endothelial cells (HUVECs) and apolipoprotein E (ApoE)-knockout (KO) mice aorta endothelial cells (MAECs). In vivo, hematoxylin-eosin (H&E), Oil Red O and Masson's trichrome staining showed that ApoE-KO mice treated with miR-377 developed less atherosclerotic plaques, accompanied by reduced plasma triglyceride levels.

CONCLUSIONS

It is concluded that miR-377 upregulates GPIHBP1 expression, increases the LPL binding to GPIHBP1, and reduces plasma triglyceride levels, likely through targeting DNMT1, inhibiting atherosclerosis in ApoE-KO mice.

A Type IIb, but Not Type IIa, GnRH Receptor Mediates GnRH-Induced Release of Growth Hormone in the Ricefield Eel

Multiple gonadotropin-releasing hormone receptors (GnRHRs) are present in vertebrates, but their differential physiological relevances remain to be clarified. In the present study, we identified three GnRH ligands GnRH1 (pjGnRH), GnRH2 (cGnRH-II), and GnRH3 (sGnRH) from the brain, and two GnRH receptors GnRHR1 (GnRHR IIa) and GnRHR2 (GnRHR IIb) from the pituitary of the ricefield eel Monopterus albus. GnRH1 and GnRH3 but not GnRH2 immunoreactive neurons were detected in the pre-optic area, hypothalamus, and pituitary, suggesting that GnRH1 and GnRH3 may exert hypophysiotropic roles in ricefield eels. gnrhr1 mRNA was mainly detected in the pituitary, whereas gnrhr2 mRNA broadly in tissues of both females and males. In the pituitary, GnRHR1 and GnRHR2 immunoreactive cells were differentially distributed, with GnRHR1 immunoreactive cells mainly in peripheral areas of the adenohypophysis whereas GnRHR2 immunoreactive cells in the multicellular layers of adenohypophysis adjacent to the neurohypophysis. Dual-label fluorescent immunostaining showed that GnRHR2 but not GnRHR1 was localized to somatotropes, and all somatotropes are GnRHR2-positive cells and vice versa at all stages examined. GnRH1 and GnRH3 were shown to stimulate growth hormone (Gh) release from primary culture of pituitary cells, and to decrease Gh contents in the pituitary of ricefield eels 12 h post injection. GnRH1 and GnRH3 stimulated Gh release probably via PLC/IP₃/PKC and Ca²⁺ pathways. These results, as a whole, suggested that GnRHs may bind to GnRHR2 but not GnRHR1 to trigger Gh release in ricefield eels, and provided novel information on differential roles of multiple GnRH receptors in vertebrates.