Expression of DNA methyltransferase (Dnmt1) in testicular germ cells during development of mouse embryo

Global changes in epigenomes during mouse spermatogenesis: possible relation to germ cell apoptosis

Mammalian spermatogenesis is characterized by disproportionate germ cell apoptosis. The high frequency of apoptosis is considered a safety mechanism that serves to avoid unfavorable transmission of paternal aberrant genetic information to the offspring as well as elimination mechanism for removal of overproduced immature or damaged spermatogenic cells. The molecular mechanisms involved in the induction of germ cell apoptosis include both intrinsic mitochondrial Bcl-2/Bax and extrinsic Fas/FasL pathways. However, little is known about the nuclear trigger of those systems. Recent studies indicate that epigenomes are essential in the regulation of gene expression through remodeling of the chromatin structure, and are genome-like transmission materials that reflect the effects of various environmental factors. In spermatogenesis, epigenetic errors can act as the trigger for elimination of germ cells with abnormal chromatin structure, abnormal gene expression and/or morphological defects (disordered differentiation). In this review, we focus on the relationship between global changes in epigenetic parameters and germ cell apoptosis in mice and other mammals.

BAH domains and a histone-like motif in DNA methyltransferase 1 (DNMT1) regulate de novo and maintenance methylation in vivo

DNA methyltransferase 1 (DNMT1) is a multidomain protein believed to be involved only in the passive transmission of genomic methylation patterns via maintenance methylation. The mechanisms that regulate DNMT1 activity and targeting are complex and poorly understood. We used embryonic stem (ES) cells to investigate the function of the uncharacterized bromo-adjacent homology (BAH) domains and the glycine-lysine (GK) repeats that join the regulatory and catalytic domains of DNMT1. We removed the BAH domains by means of a CRISPR/Cas9-mediated deletion within the endogenous Dnmt1 locus. The internally deleted protein failed to associate with replication foci during S phase in vivo and lost the ability to mediate maintenance methylation. The data indicate that ablation of the BAH domains causes DNMT1 to be excluded from replication foci even in the presence of the replication focus-targeting sequence (RFTS). The GK repeats resemble the N-terminal tails of histones H2A and H4 and are normally acetylated. Substitution of lysines within the GK repeats with arginines to prevent acetylation did not alter the maintenance activity of DNMT1 but unexpectedly activated de novo methylation of paternal imprinting control regions (ICRs) in mouse ES cells; maternal ICRs remained unmethylated. We propose a model under which DNMT1 deposits paternal imprints in male germ cells in an acetylation-dependent manner. These data reveal that DNMT1 responds to multiple regulatory inputs that control its localization as well as its activity and is not purely a maintenance methyltransferase but can participate in the de novo methylation of a small but essential compartment of the genome.

An Initial Investigation of an Alternative Model to Study rat Primordial Germ Cell Epigenetic Reprogramming

BACKGROUND

Primordial germ cells (PGC) are the precursors of the gametes. During pre-natal development, PGC undergo an epigenetic reprogramming when bulk DNA demethylation occurs and is followed by sex-specific de novo methylation. The de novo methylation and the maintenance of the methylation patterns depend on DNA methyltransferases (DNMTs). PGC reprogramming has been widely studied in mice but not in rats. We have previously shown that the rat might be an interesting model to study germ cell development. In face of the difficulties of getting enough PGC for molecular studies, the aim of this study was to propose an alternative method to study rat PGC DNA methylation. Rat embryos were collected at 14, 15 and 19 days post-coitus (dpc) for the analysis of 5mC, 5hmC, DNMT1, DNMT3a and DNMT3b expression or at 16dpc for treatment 5-Aza-CdR, a DNMT inhibitor, in vitro.

METHODS

Once collected, the gonads were placed in 24-well plates previously containing 45μm pore membrane and medium with or without 5-Aza-CdR. The culture was kept for five days and medium was changed daily. The gonads were either fixed or submitted to RNA extraction.

RESULTS

5mC and DNMTs labelling suggests that PGC are undergoing epigenetic reprogramming around 14/15dpc. The in vitro treatment of rat embryonic gonads with 1 μM of 5-Aza-CdR lead to a loss of 5mC labelling and to the activation of Pax6 expression in PGC, but not in somatic cells, suggesting that 5-Aza-CdR promoted a PGC-specific global DNA hypomethylation.

CONCLUSIONS

This study suggests that the protocol used here can be a potential method to study the wide DNA demethylation that takes place during PGC reprogramming.

Antibodies specific for nucleic acid modifications

Nucleotide modifications constitute marks in RNA and DNA that contribute to gene regulation, development and other cellular processes. The understanding of their intricate molecular roles has been hampered by the high number of different modifications, the lack of effective methods and tools for their detection and quantification as well as by their complex structure-function relationship. The recent development of RNA and DNA immunoprecipitation followed by high-throughput sequencing (RIP- and DIP-seq) initiated detailed transcriptome- and genome-wide studies. Both techniques depend on highly specific and sensitive antibodies to specifically enrich the targeted modified nucleotides without background or potential biases. Here, we review the challenges and developments when generating and validating antibodies targeting modified nucleotides. We discuss antibody-antigen interactions, different strategies of antigen generation and compare different binder formats suitable for state-of-the-art high resolution mapping and imaging technologies.

DNA methyltransferases exhibit dynamic expression during spermatogenesis

DNA methylation is one of the epigenetic marks and plays critically important functions during spermatogenesis in mammals. DNA methylation is catalysed by DNA methyltransferase (DNMT) enzymes, which are responsible for the addition of a methyl group to the fifth carbon atom of the cytosine residues within cytosine-phosphate-guanine (CpG) and non-CpG dinucleotide sites. Structurally and functionally five different DNMT enzymes have been identified in mammals, including DNMT1, DNMT2, DNMT3A, DNMT3B and DNMT3L. These enzymes mainly play roles in two DNA methylation processes: maintenance and de novo. While DNMT1 is primarily responsible for maintenance methylation via transferring methyl groups to the hemi-methylated DNA strands following DNA replication, both DNMT3A and DNMT3B are capable of methylating unmodified cytosine residues, known as de novo methylation. However, DNMT3L indirectly participates in de novo methylation, and DNMT2 carries out methylation of the cytosine 38 in the anticodon loop of aspartic acid transfer RNA. To date, many studies have been performed to determine spatial and temporal expression levels and functional features of the DNMT in the male germ cells. This review article comprehensively discusses dynamic expression of the DNMT during spermatogenesis and their relationship with male infertility development in the light of existing investigations.

Dynamic expression profile of DNA methyltransferases in rat testis development

DNA methyltransferases (Dnmts) are unique and perform specific functions during male germ cell development. To further characterize the significance of Dnmts in the events leading to production of spermatozoa, we investigated whether the expression patterns in Dnmt1, Dnmt3a, Dnmt3b and Dnmt3l were apparent in rat testes at different time points during development. The qRT-PCR results showed that expression levels of Dnmt3a and Dnmt3l were abundant before birth and were present at the highest levels in testes tissue at 18.5 days postcoitus (dpc), and gradually decreased from day 0 postpartum (dpp) to 90 dpp. Expression of Dnmt1 and Dnmt3b reached a peak after birth (P <0.01), and then gradually reduced until adulthood. Western blotting and immunolocalization analysis of Dnmt3a and Dnmt3b further confirmed the differential expression and localization of the two proteins during rat testis development. The dynamic expression profile of Dnmts implies specific and potentially nonredundant roles for each of these enzymes in the developing rat testis.

Male-mediated developmental toxicity

Male-mediated developmental toxicity has been of concern for many years. The public became aware of male-mediated developmental toxicity in the early 1990s when it was reported that men working at Sellafield might be causing leukemia in their children. Human and animal studies have contributed to our current understanding of male-mediated effects. Animal studies in the 1980s and 1990s suggested that genetic damage after radiation and chemical exposure might be transmitted to offspring. With the increasing understanding that there is histone retention and modification, protamine incorporation into the chromatin and DNA methylation in mature sperm and that spermatozoal RNA transcripts can play important roles in the epigenetic state of sperm, heritable studies began to be viewed differently. Recent reports using molecular approaches have demonstrated that DNA damage can be transmitted to babies from smoking fathers, and expanded simple tandem repeats minisatellite mutations were found in the germline of fathers who were exposed to radiation from the Chernobyl nuclear power plant disaster. In epidemiological studies, it is possible to clarify whether damage is transmitted to the sons after exposure of the fathers. Paternally transmitted damage to the offspring is now recognized as a complex issue with genetic as well as epigenetic components.

Roles of epigenome in mammalian spermatogenesis

Mammalian spermatogenesis is a successive process consisting of spermatogonial proliferation, spermatocytic meiosis, and spermiogenesis, representing the maturation of haploid spermatids. During the process, 25-75 % of the expected sperm yield is thought to be lost through apoptosis. In addition, spermatogenesis is considered to be a process undergoing successive heterochromatinization, finally reaching a complete condensed form in the sperm head. Thus, cell proliferation, differentiation and death may be strictly regulated by epigenetic factors in this process. This review describes the current understanding of the role of epigenome in spermatogenesis, especially focusing on the following aspects; DNA methylation, modification of histones, and small RNA function. These epigenetic factors affect each other and play a central role in events essential for spermatogenesis, fertilization and embryogenesis, through the regulation of gene expression, transposon activities, meiotic sex chromosome inactivation, histone remodeling and genome imprinting. Finally, a brief discussion of future avenues of study is highlighted.

The roles of fibroblast growth factors in the testicular development and tumor

Fibroblast growth factors (FGFs) are classically known as hormonal factors and recent studies have revealed that FGFs have a key role in regulating growth and development of several reproductive organs, including the testis. The testis is mainly consisted of germ cells, Sertoli cells and Leydig cells to develop and maintain the male phenotype and reproduction. This review summarizes the structure and fuctions of testis, the roles of FGFs on testicular development and potential involvement in testicular tumor and its regulatory mechanism. Among 23 members of FGFs, the FGF-1, FGF-2, FGF-4, FGF-8, FGF-9, and FGF-21 were involved and describe in details. Understanding the roles and mechanism of FGFs is the foundation to modeling testicular development and treatments in testicular disease. Therefore, in the last part, the potential therapy with FGFs for the testis of cancer and diabetes was also discussed.

Non-catalytic functions of DNMT1

Mammalian DNA methyltransferase 1 (DNMT1) is essential during early embryo development. Consistent with its key role in embryogenesis, depletion of this protein in adult somatic cells promotes severe cellular dysfunctions and cell death. DNMT1 contains a highly evolutionary conserved C-terminal catalytic DNA methyltransferase domain that is thought to be the responsible for the maintenance of CpG methylation patterns in the genome. DNMT1 has also a large N-terminal region with different functional protein-protein and protein-DNA binding domains. The multi-domain N-terminal region and the abundant molecular binding patterns suggest potential non-catalytic functions for DNMT1. However, this hypothesis remains controversial and conflicting results can be found in the literature. Here, recent results presenting a functional role for DNMT1 independent of its catalytic domain are discussed.

Regulation of SNAIL1 and E-cadherin function by DNMT1 in a DNA methylation-independent context

Mammalian DNA methyltransferase 1 (DNMT1) is essential for maintaining DNA methylation patterns after cell division. Disruption of DNMT1 catalytic activity results in whole genome cytosine demethylation of CpG dinucleotides, promoting severe dysfunctions in somatic cells and during embryonic development. While these observations indicate that DNMT1-dependent DNA methylation is required for proper cell function, the possibility that DNMT1 has a role independent of its catalytic activity is a matter of controversy. Here, we provide evidence that DNMT1 can support cell functions that do not require the C-terminal catalytic domain. We report that PCNA and DMAP1 domains in the N-terminal region of DNMT1 are sufficient to modulate E-cadherin expression in the absence of noticeable changes in DNA methylation patterns in the gene promoters involved. Changes in E-cadherin expression are directly associated with regulation of β-catenin-dependent transcription. Present evidence suggests that the DNMT1 acts on E-cadherin expression through its direct interaction with the E-cadherin transcriptional repressor SNAIL1.

Expression patterns and miRNA regulation of DNA methyltransferases in chicken primordial germ cells

DNA methylation is widespread in most species, from bacteria to mammals, and is crucial for genomic imprinting, gene expression, and embryogenesis. DNA methylation occurs via two major classes of enzymatic reactions: maintenance-type methylation catalyzed by DNA (cytosine-5-)-methyltransferase (DNMT) 1, and de novo methylation catalyzed by DNMT 3 alpha (DNMT3A) and -beta (DNMT3B). The expression pattern and regulation of DNMT genes in primordial germ cells (PGCs) and germ line cells has not been sufficiently established in birds. Therefore, we employed bioinformatics, RT-PCR, real-time PCR, and in situ hybridization analyses to examine the structural conservation and conserved expression patterns of chicken DNMT family genes. We further examined the regulation of a candidate de novo DNA methyltransferase gene, cDNMT3B by cotransfection of cDNMT3B 3'UTR- and cDNMT3B 3'UTR-specific miRNAs through a dual fluorescence reporter assay. All cDNMT family members were differentially detected during early embryonic development. Of interest, cDNMT3B expression was highly detected in early embryos and in PGCs. During germ line development and sexual maturation, cDNMT3B expression was reestablished in a female germ cell-specific manner. In the dual fluorescence reporter assay, cDNMT3B expression was significantly downregulated by four miRNAs: gga-miR-15c (25.82%), gga-miR-29b (30.01%), gga-miR-383 (30.0%), and gga-miR-222 (31.28%). Our data highlight the structural conservation and conserved expression patterns of chicken DNMTs. The miRNAs investigated in this study may induce downregulation of gene expression in chicken PGCs and germ cells.

Mouse germ cell development: from specification to sex determination

Primordial germ cells (PGCs) are embryonic progenitors for the gametes. In the gastrulating mouse embryo, a small group of cells begin expressing a unique set of genes and so commit to the germline. Over the next 3-5 days, these PGCs migrate anteriorly and increase rapidly in number via mitotic division before colonizing the newly formed gonads. PGCs then express a different set of unique genes, their inherited epigenetic imprint is erased and an individual methylation imprint is established, and for female PGCs, the silent X chromosome is reactivated. At this point, germ cells (GCs) commit to either a female or male sexual lineage, denoted by meiosis entry and mitotic arrest, respectively. This developmental program is determined by cues emanating from the somatic environment.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

The dynamic epigenetic program in male germ cells: Its role in spermatogenesis, testis cancer, and its response to the environment

Spermatogenesis is a truly remarkable process that requires exquisite control and synchronization of germ cell development. It is prone to frequent error, as paternal infertility contributes to 30-50% of all infertility cases; yet, in many cases, the mechanisms underlying its causes are unknown. Strikingly, aberrant epigenetic profiles, in the form of anomalous DNA and histone modifications, are characteristic of cancerous testis cells. Germ cell development is a critical period during which epigenetic patterns are established and maintained. The progression from diploid spermatogonia to haploid spermatozoa involves stage- and testis-specific gene expression, mitotic and meiotic division, and the histone-protamine transition. All are postulated to engender unique epigenetic controls. In support of this idea are the findings that mouse models with gene deletions for epigenetic modifiers have severely compromised fertility. Underscoring the importance of understanding how epigenetic marks are set and interpreted is evidence that abnormal epigenetic programming of gametes and embryos contributes to heritable instabilities in subsequent generations. Numerous studies have documented the existence of transgenerational consequences of maternal nutrition, or other environmental exposures, but it is only now recognized that there are sex-specific male-line transgenerational responses in humans and other species. Epigenetic events in the testis have just begun to be studied. New work on the function of specific histone modifications, chromatin modifiers, DNA methylation, and the impact of the environment on developing sperm suggests that the correct setting of the epigenome is required for male reproductive health and the prevention of paternal disease transmission.

Autonomous regulation of sex-specific developmental programming in mouse fetal germ cells

In mice, unique events regulating epigenetic programming (e.g., genomic imprinting) and replication state (mitosis versus meiosis) occur during fetal germ cell development. To determine whether these processes are autonomously programmed in fetal germ cells or are dependent upon ongoing instructive interactions with surrounding gonadal somatic cells, we isolated male and female germ cells at 13.5 days postcoitum (dpc) and maintained them in culture for 6 days, either alone or in the presence of feeder cells or gonadal somatic cells. We examined allele-specific DNA methylation in the imprinted H19 and Snrpn genes, and we also determined whether these cells remained mitotic or entered meiosis. Our results show that isolated male germ cells are able to establish a characteristic "paternal" methylation pattern at imprinted genes in the absence of any support from somatic cells. On the other hand, cultured female germ cells maintain a hypomethylated status at these loci, characteristic of the normal "maternal" methylation pattern in endogenous female germ cells before birth. Further, the surviving female germ cells entered first meiotic prophase and reached the pachytene stage, whereas male germ cells entered mitotic arrest. These results indicate that mechanisms controlling both epigenetic programming and replication state are autonomously regulated in fetal germ cells that have been exposed to the genital ridge prior to 13.5 dpc.

Sex-specific windows for high mRNA expression of DNA methyltransferases 1 and 3A and methyl-CpG-binding domain proteins 2 and 4 in human fetal gonads

DNA methyltransferases (DNMTs) and 5-methyl-CpG-binding domain proteins (MBDs) are involved in the acquisition of parent-specific epigenetic modifications in human male and female germ cells. Reverse Northern blot analyses demonstrated sex-specific differences in mRNA expression for the maintenance DNMT1 and the de novo DNMT3A in developing testis and ovary. In fetal testis DNMT1 and DNMT3A expression peaked in mitotically arrested spermatogonia around 21 weeks gestation. In fetal ovary transcriptional upregulation of DNMT1 and DNMT3A occurred during a very brief period at 16 weeks gestation, when the oocytes proceeded through meiotic prophase. Fetal gonads showed several fold higher DNMT3A expression levels than fetal brain and adult tissues. The most abundant DNMT3A isoform in fetal testis and ovary was DNMT3A2, whereas in all other analyzed tissues DNMT3A1 predominated. The catalytically inactive DNMT3A3 isoform was also present at relatively high levels in developing gonads and may perform a regulatory function(s). In both male and female fetal gonads expression of genes for MBD2 and MBD4, which may be implicated in chromatin remodeling of methylated genomic DNA sequences, was tightly linked to DNMT expression. We propose that the sex-specific time windows for concomitant upregulation of DNMT1, DNMT3A, MBD2, and MBD4 are associated with prenatal remethylation of the human male and female germ line.

DNMT1 and HDAC1 gene expression in impaired spermatogenesis and testicular cancer

DNA methylation catalyzed by DNA methyltransferases (DNMTs) and histone deacetylation catalyzed by histone deacetylases (HDACs) play an important role for the regulation of gene expression during carcinogenesis and spermatogenesis. We therefore studied the cell-specific expression of DNMT1 and HDAC1 for the first time in human testicular cancer and impaired human spermatogenesis. During normal spermatogenesis, DNMT1 and HDAC1 were colocalized in nuclei of spermatogonia. While HDAC1 was additionally present in nuclei of Sertoli cells, DNMT1 was restricted to germ cells exhibiting a different expression pattern of mRNA (in pachytene spermatocytes and round spermatids) and protein (in round spermatids). Interestingly, in infertile patients revealing round spermatid maturation arrest, round spermatids lack DNMT1 protein, while pachytene spermatocytes became immunopositive for DNMT1. In contrast, no changes in the expression pattern could be observed for HDAC1. This holds true also in testicular tumors, where HDAC1 has been demonstrated in embryonal carcinoma, seminoma and teratoma. Interestingly, DNMT1 was not expressed in seminoma, but upregulated in embryonal carcinoma.

Dynamic expression of DNMT3a and DNMT3b isoforms during male germ cell development in the mouse

In the male germ line, sequence-specific methylation patterns are initially acquired prenatally in diploid gonocytes and are further consolidated after birth during spermatogenesis. It is still unclear how DNA methyltransferases are involved in establishing and/or maintaining these patterns in germ cells, or how their activity is regulated. We compared the temporal expression patterns of the postulated de novo DNA methyltransferases DNMT3a and DNMT3b in murine male germ cells. Mitotic, meiotic and post-meiotic male germ cells were isolated, and expression of various transcript variants and isoforms of Dnmt3a and Dnmt3b was examined using Quantitative RT-PCR and Western blotting. We found that proliferating and differentiating male germ cells were marked by distinctive expression profiles. Dnmt3a2 and Dnmt3b transcripts were at their highest levels in type A spermatogonia, decreased dramatically in type B spermatogonia and preleptotene spermatocytes and rose again in leptotene/zygotene spermatocytes, while Dnmt3a expression was mostly constant, except in type B spermatogonia where it increased. In all cases, expression declined as pachynema progressed. At the protein level, DNMT3a was the predominant isoform in type B spermatogonia, while DNMT3a2, DNMT3b2, and DNMT3b3 were expressed throughout most of spermatogenesis, except in pachytene spermatocytes. We also detected DNMT3a2 and DNMT3b2 in round spermatids. Taken together, these data highlight the tightly regulated expression of these genes during spermatogenesis and provide evidence that DNMTs may be contributing differentially to the establishment and/or maintenance of methylation patterns in male germ cells.

Gamete imprinting: setting epigenetic patterns for the next generation

The acquisition of genomic DNA methylation patterns, including those important for development, begins in the germ line. In particular, imprinted genes are differentially marked in the developing male and female germ cells to ensure parent-of-origin-specific expression in the offspring. Abnormalities in imprints are associated with perturbations in growth, placental function, neurobehavioural processes and carcinogenesis. Based, for the most part, on data from the well-characterised mouse model, the present review will describe recent studies on the timing and mechanisms underlying the acquisition and maintenance of DNA methylation patterns in gametes and early embryos, as well as the consequences of altering these patterns.

Distinct DNA methylation activity of Dnmt3a and Dnmt3b towards naked and nucleosomal DNA

In mammals, the resetting of DNA methylation patterns in early embryos and germ cells is crucial for development. De novo type DNA methyltransferases Dnmt3a and Dnmt3b are responsible for creating DNA methylation patterns during embryogenesis and in germ cells. Although their in vitro DNA methylation properties are similar, Dnmt3a and Dnmt3b methylate different genomic DNA regions in vivo. In the present study, we have examined the DNA methylation activity of Dnmt3a and Dnmt3b towards nucleosomes reconstituted from recombinant histones and DNAs, and compared it to that of the corresponding naked DNAs. Dnmt3a showed higher DNA methylation activity than Dnmt3b towards naked DNA and the naked part of nucleosomal DNA. On the other hand, Dnmt3a scarcely methylated the DNA within the nucleosome core region, while Dnmt3b significantly did, although the activity was low. We propose that the preferential DNA methylation activity of Dnmt3a towards the naked part of nucleosomal DNA and the significant methylation activity of Dnmt3b towards the nucleosome core region contribute to their distinct methylation of genomic DNA in vivo.

Expression of Dnmt3b in mouse hematopoietic progenitor cells and spermatogonia at specific stages

Two de novo-type DNA methyltransferases, Dnmt3a and Dnmt3b, are responsible for the creation of DNA methylation patterns during development. Dnmt3b is specifically expressed in the totipotent cells of mouse early embryos and Dnmt3a, a longer form of the two isoforms, is ubiquitously expressed in mesenchyme cells after the 10 day embryo stage [Mech. Dev. 118 (2002) 187]. In the present study, we demonstrated that Dnmt3b was expressed in the nuclei of specific cells in certain tissues after the 10 day embryo stage. In fetal liver, dorsal aorta and portal vein, Dnmt3b was expressed in cells expressing CD34, indicating that the cells were hematopoietic progenitor cells. However, Dnmt3b was not expressed in the hematopoietic progenitor cells in yolk sac blood islands at 8 day embryo stage and in adult bone marrow cells. Dnmt3b was also expressed in type-A spermatogonia after birth. Dnmt3b was expressed not only in the totipotent stem cells but also in the progenitor cells the direction of differentiation of which had been already determined. On the other hand, the long form of Dnmt3a was not expressed in these hematopoietic progenitor cells in fetal liver or type-A spermatogonia, but was expressed in hepatocytes in fetal liver and type-B spermatogonia. While Dnmt3b was distributed in both the heterochromatin and euchromatin regions, Dnmt3a was specifically localized to the euchromatin region.

Extensive and orderly reprogramming of genome-wide chromatin modifications associated with specification and early development of germ cells in mice

Induction of mouse germ cells occurs from the proximal epiblast at around embryonic day (E) 7.0. These germ cells then migrate to, and enter the gonads at about E10.5 after which they undergo epigenetic reprogramming including erasure of parental imprints. However, the epigenetic properties acquired by nascent germ cells and the potential remodeling of these epigenetic marks in the subsequent migratory period have been largely unexplored. Here we have used immunohistochemistry to examine several genome-wide epigenetic modifications occurring in germ cells from their specification to their colonization of the genital ridges. We show that at around E8.0, germ cells concomitantly and significantly reduce H3-K9 dimethylation and DNA methylation, two major repressive modifications for gene expression. These events are preceded by the transient loss of all the DNA methyltransferases from their nuclei. By contrast, germ cells substantially increase the levels of H3-K27 trimethylation, another repressive modification with more plasticity, at E8.5-9.0 and maintain this state until at least E12.5. H3-K4 methylation and H3-K9 acetylation, modifications associated with transcriptionally permissive/active chromatin, are similar in germ and surrounding somatic cells but germ cells transiently increase these marks sharply upon their entry into the genital ridge. H3-K9 trimethylation, a hallmark of centromeric heterochromatin, is kept relatively constant during the periods examined. We suggest that this orderly and extensive epigenetic reprogramming in premigratory and migratory germ cells might be necessary for their reacquisition of underlying totipotency, for subsequent specific epigenetic remodeling, including the resetting of parental imprints, and for the production of gametes with an appropriate epigenotype for supporting normal development.

Co-expression of de novo DNA methyltransferases Dnmt3a2 and Dnmt3L in gonocytes of mouse embryos

In mouse male germ cells, global DNA methylation occurs in gonocytes at 16-18 days postcoitum. In the present study, we examined which de novo-type DNA methyltransferase, Dnmt3a, Dnmt3a2 or Dnmt3b is expressed in gonocytes at these stages. Immuno-histochemical and Western blot analyses revealed that Dnmt3a2 was the major DNA methyltransferase expressed in gonocytes at 14-18 day postcoitum. Dnmt3L, which is necessary for spermatogenesis, was co-expressed in gonocytes at identical stages to Dnmt3a2. On the other hand, Dnmt3a was expressed not in germ cells but in the Sertoli cells and connective tissue cells that surround gonocytes and spermatogonia. Dnmt3b2, an isoform of Dnmt3b, was expressed faintly but significantly in gonocytes at 16 days postcoitum, and increased in spermatogonia at 4 and 6 days postpartum. The expression of Dnmt3a2, Dnmt3L, and Dnmt3b2 at 14-18 dpc was confirmed by reverse transcriptase-coupled polymerase chain reaction amplification and nucleotide sequencing of the amplified fragments. The results strongly suggest that Dnmt3a2 and Dnmt3L are responsible for the global DNA methylation in mouse male germ cells.

DNA methylation in mouse gametogenesis

DNA methylation is involved in many biological processes and is particularly important for both development and germ cell differentiation. Several waves of demethylation and de novo methylation occur during both male and female germ line development. This has been found at both the gene and all genome levels, but there is no demonstrated correlation between them. During the postnatal germ line development of spermatogenesis, we found very complex and drastic DNA methylation changes that we could correlate with chromatin structure changes. Thus, detailed studies focused on localization and expression pattern of the chromatin proteins involved in both DNA methylation, histone tails modification, condensin and cohesin complex formation, should help to gain insights into the mechanisms at the origin of the deep changes occurring during this particular period.

Windows for sex-specific methylation marked by DNA methyltransferase expression profiles in mouse germ cells

The acquisition of genomic methylation in the male germ line is initiated prenatally in diploid gonocytes, while DNA methylation in the female germ line is initiated postnatally in growing oocytes. We compared the temporal expression patterns of the DNA methyltransferases, DNMT1, DNMT3a, DNMT3b, and DNMT3l in the male and female germ lines. DNMT1 expression was examined by immunocytochemistry and Northerns with an emphasis on the prenatal period. In the female, there is a gradual down-regulation of DNMT1 protein in prenatal meiotic prophase I oocytes that is not associated with the production of an untranslated transcript, as it is in the male; these results suggest that the mechanism of meiotic down-regulation differs between the sexes. In the male, DNMT1 is unlikely to play a role in the prenatal acquisition of germ line methylation patterns since it is down-regulated in gonocytes between 14.5 and 18.5 days of gestation and is absent at the time of initiation of DNA methylation. To search for candidate DNMTs that could be involved in establishing methylation patterns in both germ lines, real-time RT-PCR was used to simultaneously study the expression profiles of the three DNMT3 enzymes in developing testes and ovaries; DNMT1 expression was included as a control. Expression profiles of DNMT3a and DNMT3l provide support for an interaction of the two enzymes during prenatal germ cell development and de novo methylation in the male. DNMT3l is the predominant DNMT3 enzyme expressed at high levels in the postnatal female germ line at the time of acquisition of DNA methylation patterns. DNMT1 and DNMT3b expression levels peak concomitantly, shortly after birth in the male, consistent with a role in the maintenance of methylation patterns in proliferating spermatogonia. Together, the results provide clues to specific roles for the different DNMT family members in de novo and maintenance methylation in the developing testis and ovary.

Reproductive epigenetics

Epigenetics refers to covalent modifications of DNA and core histones that regulate gene activity without altering DNA sequence. To date, the best-characterized DNA modification associated with the modulation of gene activity is methylation of cytosine residues within CpG dinucleotides. Human disorders associated with epigenetic abnormalities include rare imprinting diseases, molar pregnancies, and childhood cancers. Germ cell development and early embryo development are critical times when epigenetic patterns are initiated or maintained. This review focuses on the epigenetic modification DNA methylation and discusses recent progress that has been made in understanding when and how epigenetic patterns are differentially established in the male and female germlines, the mouse, and human disorders associated with abnormalities in epigenetic programming in germ cells and early embryos, as well as genetic and other modulators (e.g. nutrition and drugs) of reproductive epigenetic events.