Imprinting in the germ line

The Insulin/IGF System in Mammalian Sexual Development and Reproduction

Persistent research over the past few decades has clearly established that the insulin-like family of growth factors, which is composed of insulin and insulin-like growth factors 1 (IGF1) and 2 (IGF2), plays essential roles in sexual development and reproduction of both males and females. Within the male and female reproductive organs, ligands of the family act in an autocrine/paracrine manner, in order to guide different aspects of gonadogenesis, sex determination, sex-specific development or reproductive performance. Although our knowledge has greatly improved over the last years, there are still several facets that remain to be deciphered. In this review, we first briefly outline the principles of sexual development and insulin/IGF signaling, and then present our current knowledge, both in rodents and humans, about the involvement of insulin/IGFs in sexual development and reproductive functions. We conclude by highlighting some interesting remarks and delineating certain unanswered questions that need to be addressed in future studies.

Transgenerational sperm DNA methylation epimutation developmental origins following ancestral vinclozolin exposure

A number of environmental factors from nutrition to toxicants have been shown to promote the epigenetic transgenerational inheritance of disease and phenotypic variation. This requires alterations in the germline (sperm or egg) epigenome. Previously, the agricultural fungicide vinclozolin was found to promote the transgenerational inheritance of sperm differential DNA methylation regions (DMRs) termed epimutations that help mediate this epigenetic inheritance. The current study was designed to investigate the developmental origins of the transgenerational DMRs during gametogenesis. Male control and vinclozolin lineage F3 generation rats were used as a source of embryonic day 13 (E13) primordial germ cells, embryonic day 16 (E16) prospermatogonia, postnatal day 10 (P10) spermatogonia, adult pachytene spermatocytes, round spermatids, caput epididymal spermatozoa, and caudal sperm. The DMRs between the control versus vinclozolin lineage samples were determined for each developmental stage. The top 100 statistically significant DMRs for each stage were compared. The developmental origins of the caudal epididymal sperm DMRs were assessed. The chromosomal locations and genomic features of the different stage DMRs were investigated. In addition, the DMR associated genes were identified. Previous studies have demonstrated alterations in the DMRs of primordial germ cells (PGCs). Interestingly, the majority of the DMRs identified in the current study for the caudal sperm originated during the spermatogenic process in the testis. A cascade of epigenetic alterations initiated in the PGCs appears to be required to alter the epigenetic programming during spermatogenesis to modify the sperm epigenome involved in the transgenerational epigenetic inheritance phenomenon.

Developmental origins of transgenerational sperm DNA methylation epimutations following ancestral DDT exposure

Epigenetic alterations in the germline can be triggered by a number of different environmental factors from diet to toxicants. These environmentally induced germline changes can promote the epigenetic transgenerational inheritance of disease and phenotypic variation. In previous studies, the pesticide DDT was shown to promote the transgenerational inheritance of sperm differential DNA methylation regions (DMRs), also called epimutations, which can in part mediate this epigenetic inheritance. In the current study, the developmental origins of the transgenerational DMRs during gametogenesis have been investigated. Male control and DDT lineage F3 generation rats were used to isolate embryonic day 16 (E16) prospermatogonia, postnatal day 10 (P10) spermatogonia, adult pachytene spermatocytes, round spermatids, caput epididymal spermatozoa, and caudal sperm. The DMRs between the control versus DDT lineage samples were determined at each developmental stage. The top 100 statistically significant DMRs at each stage were compared and the developmental origins of the caudal epididymal sperm DMRs were assessed. The chromosomal locations and genomic features of the different stage DMRs were analyzed. Although previous studies have demonstrated alterations in the DMRs of primordial germ cells (PGCs), the majority of the DMRs identified in the caudal sperm originated during the spermatogonia stages in the testis. Interestingly, a cascade of epigenetic alterations initiated in the PGCs is required to alter the epigenetic programming during spermatogenesis to obtain the sperm epigenetics involved in the epigenetic transgenerational inheritance phenomenon.

Sperm epigenetics and aging

Advanced paternal age has very real consequences in fertility, embryogenesis, and even offspring health. Specifically, advanced paternal age has been linked to delayed time to pregnancy and in some studies even appears to be linked to a decreased likelihood of achieving a pregnancy. Epidemiological and animal model evidence also suggests that the offspring of older fathers are at an elevated risk for neuropsychiatric disease. For these reasons it is essential that we have a comprehensive understanding of what actually occurs in the gametes of the aging male. Available data suggest that there are very clear patterns of aging in the sperm epigenome that can be directly detected in DNA methylation patterns. Importantly, these alterations are so consistent that a predictive model has been successfully generated to predict an individual's age based only on sperm DNA methylation signatures. Because this metric is the most direct way to detect aging in sperm, it is logical that these signatures may offer predictive value for the offspring abnormalities that are also correlated with advanced paternal age and as such may offer a unique opportunity to generate diagnostic tools that can identify personalized risks for each couple hoping to achieve a pregnancy. While a great deal of work still needs to be performed to understand the real diagnostic utility of sperm epigenetic marks, the potential is real and warrants further investigation particularly in the context of advanced paternal age.

Aberrant DNA methylation of imprinted loci in human spontaneous abortions after assisted reproduction techniques and natural conception

STUDY QUESTION

Do assisted reproduction techniques (ARTs) affect DNA methylation of imprinted genes and does aberrant methylation of imprinted genes account for the incidence of human spontaneous abortion (SA)?

SUMMARY ANSWER

Our results show that imprinting errors of imprinted genes may contribute to human SA, and the occurrence of aberrant methylation of imprinted genes in ART pregnancies was comparable with that in natural pregnancies.

WHAT IS KNOWN ALREADY

Animal data and human studies demonstrated that in vitro culture of embryos can cause methylation defects in individual genes, which might affect subsequent embryonic development and contribute to SA. However, our previous studies showed an abnormal methylation pattern of PEG1 in human aborted chrionic villus samples (CVS) but an increased occurrence of aberrant methylation in CVS from ART-derived pregnancies was not observed.

STUDY DESIGN, SIZE AND DURATION

CVS were collected from women who underwent abortion procedures in the Department of Gynecology and Obstetrics in Nanfang Hospital from May 2008 to July 2011. Muscle samples (MS) were obtained from aborted fetuses and stillbirths. The samples were divided into four experimental groups: (A) SA/stillbirth after ART (n = 75), (B) multi-fetal reduction after ART (n = 73), (C) SA/stillbirth of natural pregnancies (n = 90) and (D) induced abortion (IA) of natural pregnancies (n = 82).

PARTICIPANTS/MATERIALS, SETTING AND METHODS

The mean ± SD age of patients was 31.0 ± 4.1 (range: 18-45 years). The DNA methylation patterns of one paternally methylated (H19) and two maternally methylated (LIT1 and SNRPN) genes were analyzed in CVS and MS using pyrosequencing and bisulfite sequencing PCR.

MAIN RESULTS AND THE ROLE OF CHANCE

Clear hypo-methylation (<10%) or hyper-methylation (>90%) were not detected in LIT1 and SNRPN but two regions of hyper-methylation (91.7 and 91.4%) were observed in H19. The mean percentage of methylation in the SA samples (groups A and C) was higher than that in the IA samples (groups B and D; P<0.05). Box plot analyses showed that in the 165 SA samples, methylation values for 40/495 (8.1%) differentially methylated regions of the three genes represented outliers. The incidence of outlier was highest for LIT1 (13.3%, 22/165). In contrast, no outliers were found in the 155 IA samples. The receiver operating characteristic curve analyses showed a positive correlation between percentage methylation of all three genes and incidence of SA (P<0.05). In addition, the conception modes (natural versus ART) and the fertilization methods used in ART (IVF and ICSI) did not affect the methylation patterns of the imprinted genes. No increase in the rate of abnormal methylation was found in the ART samples.

LIMITATIONS AND REASONS FOR CAUTION

The studied loci represent only a small fraction of developmentally important genes. Further studies are needed to evaluate changes in the expression and the methylation status of other genes that may lead to SA.

WIDER IMPLICATIONS OF THE FINDINGS

The findings provide new insights into the etiology of human SA. The possibility that the abnormal methylation seen is a consequence of the defect that led to the SA cannot be excluded.

STUDY FUNDING/COMPETING INTEREST(S)

None of the authors has any competing interest. This study was supported by National Natural Science Foundation of China (81170574), The National Key Basic Research Development Plan of China (973 Program) (2007CB948104), Comprehensive strategic sciences cooperation projects of Guangdong Province and Chinese Academy (04020416) and Guangzhou Science and Technology Program key projects (11C22120737).

Expression and epigenetic dynamics of transcription regulator Lhx8 during mouse oogenesis

The spatial and temporal specific activation and inhibition of numerous genes are required for successful oogenesis which is precisely regulated by germ cell-related transcription factors, and appropriate epigenetic modifications, including DNA methylation, histone modification and other mechanisms that closely regulate the functional exertion of these transcription factors. In this study, we characterized the correlation between the expression and epigenetic dynamics of Lhx8, a germ cell specific transcription factor during mouse oogenesis. Immunohistochemistry, quantitative PCR and western blots were performed to localize and quantify the expressional characteristics of Lhx8 in oocytes of 13.5 dpc (day post coitum), 17.5 dpc, 0 dpp (day post partum), 3 dpp, 7 dpp and 14 dpp. The results showed that LHX8 protein was located in the nucleus of oocytes, and increasingly expressed during primordial follicle activation. Sequencing of bisulfite-converted genomic DNAs revealed that the methylation dynamics of Lhx8-3' was highly changeable but almost no change occurred in Lhx8-5'. ChIP-QPCR analysis showed that histone H3 acetylation of Lhx8 was also increased during primordial follicle assembly and activation. In conclusion, Lhx8 expression is related with the activation of primordial follicles, which is highly correlated with the demethylation of Lhx8-3' untranslated region and the high acetylation of histone H3.

Pluripotent and multipotent stem cells in adult tissues

One of the most intriguing questions in stem cell biology is whether pluripotent stem cells exist in adult tissues. Several groups of investigators employing i) various isolation protocols, ii) detection of surface markers, and iii) experimental in vitro and in vivo models, have reported the presence of cells that possess a pluripotent character in adult tissues. Such cells were assigned various operational abbreviations and names in the literature that added confusion to the field and raised the basic question of whether these are truly distinct or overlapping populations of the same primitive stem cells. Unfortunately, these cells were never characterized side-by-side to address this important issue. Nevertheless, taking into consideration their common features described in the literature, it is very likely that various investigators have described overlapping populations of developmentally early stem cells that are closely related. These different populations of stem cells will be reviewed in this paper.

A comparison of stem cells for therapeutic use

A critical comparison of the attributes of several types of stem cells is presented, with particular emphasis on properties that are critical for the application of these cells for therapeutic purposes. The importance of an autologous source of pluripotent stem cells is stressed. It is apparent that two sources currently exist for non-embryonic pluripotent stem cells--very small embryonic-like stem cells (VSELs) and induced pluripotent stem cells (iPS). The impact of the emerging iPS research on therapy is considered.

Epigenetic epidemiology in psychiatry: A translational neuroscience perspective

Accumulating evidence from the field of neuroscience indicates a crucial role for epigenetic regulation of gene expression in development and aging of nervous system and suggests that aberrations in the epigenetic machinery are involved in the etiology of psychiatric disorders. Epidemiologic evidence on epigenetics in psychiatry, however, is currently very sparsely available, but is consistent with a mediating role for epigenetic mechanisms in bringing together inherited and acquired risk factors into a neurodevelopmental etiological model of psychiatric disorders. Here, we review evidence from the epidemiological and neuroscience literature, and aim to converge the evidence into an etiological model of psychiatric disorders that encompasses environmental, genetic and epigenetic contributions. Given the dynamic nature of the epigenetic machinery and the potential reversibility of epigenetic modifications, future well-designed interdisciplinary and translational studies will be of key importance in order to identify new targets for prevention and therapeutic strategies.

Genome-wide mapping of imprinted differentially methylated regions by DNA methylation profiling of human placentas from triploidies

Background

Genomic imprinting is an important epigenetic process involved in regulating placental and foetal growth. Imprinted genes are typically associated with differentially methylated regions (DMRs) whereby one of the two alleles is DNA methylated depending on the parent of origin. Identifying imprinted DMRs in humans is complicated by species- and tissue-specific differences in imprinting status and the presence of multiple regulatory regions associated with a particular gene, only some of which may be imprinted. In this study, we have taken advantage of the unbalanced parental genomic constitutions in triploidies to further characterize human DMRs associated with known imprinted genes and identify novel imprinted DMRs.

Results

By comparing the promoter methylation status of over 14,000 genes in human placentas from ten diandries (extra paternal haploid set) and ten digynies (extra maternal haploid set) and using 6 complete hydatidiform moles (paternal origin) and ten chromosomally normal placentas for comparison, we identified 62 genes with apparently imprinted DMRs (false discovery rate <0.1%). Of these 62 genes, 11 have been reported previously as DMRs that act as imprinting control regions, and the observed parental methylation patterns were concordant with those previously reported. We demonstrated that novel imprinted genes, such as FAM50B, as well as novel imprinted DMRs associated with known imprinted genes (for example, CDKN1C and RASGRF1) can be identified by using this approach. Furthermore, we have demonstrated how comparison of DNA methylation for known imprinted genes (for example, GNAS and CDKN1C) between placentas of different gestations and other somatic tissues (brain, kidney, muscle and blood) provides a detailed analysis of specific CpG sites associated with tissue-specific imprinting and gestational age-specific methylation.

Conclusions

DNA methylation profiling of triploidies in different tissues and developmental ages can be a powerful and effective way to map and characterize imprinted regions in the genome.

L1 hybridization enrichment: a method for directly accessing de novo L1 insertions in the human germline

Long interspersed nuclear element 1 (L1) retrotransposons are the only autonomously mobile human transposable elements. L1 retrotransposition has shaped our genome via insertional mutagenesis, sequence transduction, pseudogene formation, and ectopic recombination. However, L1 germline retrotransposition dynamics are poorly understood because de novo insertions occur very rarely: the frequency of disease-causing retrotransposon insertions suggests that one insertion event occurs in roughly 18–180 gametes. The method described here recovers full-length L1 insertions by using hybridization enrichment to capture L1 sequences from multiplex PCR-amplified DNA. Enrichment is achieved by hybridizing L1-specific biotinylated oligonucleotides to complementary molecules, followed by capture on streptavidin-coated paramagnetic beads. We show that multiplex, long-range PCR can amplify single molecules containing full-length L1 insertions for recovery by hybridization enrichment. We screened 600 µg of sperm DNA from one donor, but no bone fide de novo L1 insertions were found, suggesting a L1 retrotransposition frequency of <1 insertion in 400 haploid genomes. This lies below the lower bound of previous estimates, and indicates that L1 insertion, at least into the loci studied, is very rare in the male germline. It is a paradox that L1 replication is ongoing in the face of such apparently low activity. Hum Mutat 32:1–11, 2011. © 2011 Wiley-Liss, Inc.

Epigenetic regulation of mesenchymal stem cells: a focus on osteogenic and adipogenic differentiation

Stem cells are characterized by their capability to self-renew and terminally differentiate into multiple cell types. Somatic or adult stem cells have a finite self-renewal capacity and are lineage-restricted. The use of adult stem cells for therapeutic purposes has been a topic of recent interest given the ethical considerations associated with embryonic stem (ES) cells. Mesenchymal stem cells (MSCs) are adult stem cells that can differentiate into osteogenic, adipogenic, chondrogenic, or myogenic lineages. Owing to their ease of isolation and unique characteristics, MSCs have been widely regarded as potential candidates for tissue engineering and repair. While various signaling molecules important to MSC differentiation have been identified, our complete understanding of this process is lacking. Recent investigations focused on the role of epigenetic regulation in lineage-specific differentiation of MSCs have shown that unique patterns of DNA methylation and histone modifications play an important role in the induction of MSC differentiation toward specific lineages. Nevertheless, MSC epigenetic profiles reflect a more restricted differentiation potential as compared to ES cells. Here we review the effect of epigenetic modifications on MSC multipotency and differentiation, with a focus on osteogenic and adipogenic differentiation. We also highlight clinical applications of MSC epigenetics and nuclear reprogramming.

Novel retrotransposed imprinted locus identified at human 6p25

Differentially methylated regions (DMRs) are stable epigenetic features within or in proximity to imprinted genes. We used this feature to identify candidate human imprinted loci by quantitative DNA methylation analysis. We discovered a unique DMR at the 5′-end of FAM50B at 6p25.2. We determined that sense transcripts originating from the FAM50B locus are expressed from the paternal allele in all human tissues investigated except for ovary, in which expression is biallelic. Furthermore, an antisense transcript, FAM50B-AS, was identified to be monoallelically expressed from the paternal allele in a variety of tissues. Comparative phylogenetic analysis showed that FAM50B orthologs are absent in chicken and platypus, but are present and biallelically expressed in opossum and mouse. These findings indicate that FAM50B originated in Therians after divergence from Prototherians via retrotransposition of a gene on the X chromosome. Moreover, our data are consistent with acquisition of imprinting during Eutherian evolution after divergence of Glires from the Euarchonta mammals. FAM50B expression is deregulated in testicular germ cell tumors, and loss of imprinting occurs frequently in testicular seminomas, suggesting an important role for FAM50B in spermatogenesis and tumorigenesis. These results also underscore the importance of accounting for parental origin in understanding the mechanism of 6p25-related diseases.

Epiblast/germ line hypothesis of cancer development revisited: lesson from the presence of Oct-4+ cells in adult tissues

The morphology of several tumors mimics developmentally early tissues; tumors often express early developmental markers characteristic for the germ line lineage. Recently, our group identified a population of very small stem cells (SCs) in murine bone marrow (BM) and other adult organs that express several markers characteristic for epiblast/germ line-derived SCs. We named these rare cells "Very Small Embryonic/Epiblast-like Stem Cells (VSELs)." We hypothesized that these cells that express both epiblast and germ line markers are deposited during early gastrulation in developing tissues and organs and play an important role in the turnover of tissue-committed (TC) SCs. To support this, we envision that the germ line is not only the origin of SCs, but also remains as a scaffold or back-up for the SC compartment in adult life. Furthermore, we noticed that VSELs are protected from uncontrolled proliferation and teratoma formation by a unique DNA methylation pattern in some developmentally crucial imprinted genes, which show hypomethylation or erasure of imprints in paternally methylated genes and hypermethylation of imprints in the maternally methylated. In pathological situations, however, we hypothesize that VSELs could be involved in the development of several malignancies. Therefore, potential involvement of VSELs in cancerogenesis could support century-old concepts of embryonic rest- or germ line-origin hypotheses of cancer development. However, we are aware that this working hypothesis requires further direct experimental confirmation.

Characterization of the methylation status of five imprinted genes in sheep gametes

Genomic imprinting is a mammalian developmental process that uses epigenetic mechanisms to induce monoallelic and parental-specific expression of particular autosomal genes. A crucial epigenetic event consists of DNA methylation of CpG-islands, which become differentially methylated regions (DMRs) on the maternal and paternal alleles during oogenesis or spermatogenesis (germline DMRs). By contrast, somatic DMRs are acquired after fertilization. While there are several studies referring to methylation acquisition within germline DMRs in the mouse and human, a comparable methylation analysis of orthologous sequences is still lacking in sheep. To identify germline DMRs, this study analysed the methylation status of the available CpG-islands of five ovine imprinted genes (H19, IGF2R, DLK1, DIO3 and BEGAIN) in mature spermatozoa and in female gametes at different stages of their follicle growth, including in vitro matured oocytes. The 5'-end CpG-island of H19 showed a full methylation in spermatozoa and an absent methylation in growing and fully grown oocytes. The intron 2 CpG-island of IGF2R was unmethylated in male gametes, while it showed a high level of methylation in early stages of oogenesis. The promoter CpG-islands of DLK1 and DIO3 were found to be unmethylated both in spermatozoa and oocytes. Finally, the exon 9 CpG-island of BEGAIN was hypermethylated in mature male gametes, while it showed an almost complete methylation only in late stages of oocyte development. Our findings suggest that DNA methylation establishment during early stages of sheep oogenesis and subsequent in vitro maturation is gene-specific and that, of the five genes investigated, only the CpG-islands of H19 and IGF2R might represent ovine germline DMRs.

[Spermiogenesis: histone acetylation triggers male genome reprogramming]

During their post-meiotic maturation, male germ cells undergo an extensive reorganization of their genome, during which histones become globally hyperacetylated, are then removed and progressively replaced by transition proteins and finally by protamines. The latter are known to tightly associate with DNA in the mature sperm cell. Although this is a highly conserved and fundamental biological process, which is a necessary prerequisite for the transmission of the male genome to the next generation, its molecular basis remains mostly unknown. We have identified several key factors involved in this process, and their detailed functional study has enabled us to propose the first model describing molecular mechanisms involved in post-meiotic male genome reprogramming. One of them, Bromodomain Testis Specific (BRDT), has been the focus of particular attention since it possesses the unique ability to specifically induce a dramatic compaction of acetylated chromatin. Interestingly, a mutation was found homozygous in infertile men which, according to our structural and functional studies, disrupts the function of the protein. A combination of molecular structural and genetic approaches has led to a comprehensive understanding of new major actors involved in the male genome reprogramming and transmission.

Defining contributions of paternally methylated imprinted genes at the Igf2-H19 and Dlk1-Gtl2 domains to mouse placentation by transcriptomic analysis

Parental genome functions in ontogeny are determined by interactions among transcripts from the maternal and paternal genomes, which contain many genes whose expression is strictly dependent on their parental origin as a result of genomic imprinting. Comprehensive recognition of the interactions between parental genomes is important for understanding genomic imprinting in mammalian development. The placenta is a key organ for exploring the biological significance of genomic imprinting. To decipher the unknown roles of paternally methylated imprinted genes on chromosomes 7 and 12 in mouse placentation, we performed a transcriptomic analysis on placentae in three types of bimaternal conceptuses that contained genomes derived from both non-growing and fully grown oocytes. Furthermore, we used the Ingenuity pathway analysis software to predict key networks and identify functions specific to paternally methylated imprinted genes regulated by the Igf2-H19 imprinting control region and Dlk1-Dio3 imprinting control region. The data suggested that dynamic conversion of the gene expression profile by restoring the expression of paternally methylated imprinted genes resulted in phenotypic improvements in bimaternal placentae. These results provide a framework to further explore the role of epigenetic modifications in paternal genome during mouse placentation.

[Influence of the male genomic imprinting on the reproduction]

OBJECTIVE

Genomic imprinting is the epigenetic change that occurred differentially in the specific genes in spermatozoa and oocyte according to their paternal or maternal origin, thus allowing a monoallelic expression. This review is a critical analysis of the published information relating to the role of the male imprinting on the successful reproduction.

METHODS

We performed a literature search on some of the components that regulate the male genomic imprinting and the possible role on reproductive events such as spermatogenesis, and placental and embryo development.

RESULTS

The literature analysis allowed us to appreciate structural, genetic and epigenetic changes occurring during the formation of the male gamete that could have an impact on embryo development, mainly in the formation of extraembryonic tissues as the placenta.

CONCLUSION

Alterations in the molecular mechanisms involved in the sperm DNA methylation during the spermatogenesis, could induce alterations in the normal pattern of expression required in the fetal-placental components development.

Immunoprecipitation of methylated DNA

DNA methylation contributes to the regulation of long-term gene repression by enabling the recruitment of transcriptional repressor complexes to methylated cytosines. Several methods for detecting DNA methylation at the gene-specific and genome-wide levels have been developed. Methylated DNA immunoprecipitation, or MeDIP, consists of the selective immunoprecipitation of methylated DNA fragments using antibodies to 5-methylcytosine. The genomic site of interest can be detected by PCR, hybridization to DNA arrays, or by direct sequencing. This chapter describes the MeDIP protocol and quality control tests that should be performed throughout the procedure.

Protocol for the production of viable bimaternal mouse embryos

A reliable nuclear transfer method was first reported in 1983; it provided definite evidence that parthenogenetic embryos are lethal at early postimplantation in mammals. Subsequently, nuclear transfer has been extensively used as an important and versatile tool for investigating embryo and somatic-cell cloning and nucleo-cytoplasmic interactions. Further development of this technique has enabled the generation of bimaternal embryos containing two haploid sets of maternal genomes from female germ cells of different origins. By using a 2-d nuclear transfer system for oocyte reconstruction, viable mice can be produced solely from maternal genomes, without the participation of the paternal genome. This oocyte reconstruction system, as described in this protocol, could provide valuable guidelines for exploring the potential endowments of gametes and for conferring novel properties to them.

Seminal discoveries in regenerative medicine: contributions of the male germ line to understanding pluripotency

Germ cells are highly specialized cells that form gametes (sperm and eggs), and they are the only cells within an organism that contribute genes to offspring. Due to the fact that the genetic information contained within germ cells is passed from generation to generation, the germ line is often thought of as immortal. Studies have revealed that germ cells are remarkably similar to pluripotent embryonic stem cells (ESCs). For example, there is a significant overlap in the gene expression profile between ESCs and primordial germ cells (PGCs), the founders of the germ cell lineage. In addition, pluripotent embryonic germ (EG) cell lines have been derived from mammalian PGCs. Secondly, a subset of testicular germ cell tumors, known as non-seminomas, often contain differentiated cells representative of all three germ layers, a definitive test of pluripotency. Lastly, recent results have demonstrated the ability of spermatogonial stem cells (SSCs) to de-differentiate into pluripotent ES-like cells, underscoring a unique relationship between the germ line and pluripotent cells that are present during the earliest stages of embryonic development. Here, we will review the factors that regulate the self-renewal and maintenance of male germline stem cells (GSCs) and discuss how these factors may allow us to manipulate the germ line to create pluripotent cells that could serve as a critical tool in cell replacement therapies and regenerative medicine.

Very small embryonic-like stem cells: characterization, developmental origin, and biological significance

Bone marrow (BM) was, for many years, primarily envisioned as the "home organ" of hematopoietic stem cells (HSC). Augmenting evidence demonstrates, however, that BM, in addition to HSC, also contains a heterogeneous population of non-HSC. Recently, our group identified in BM and other adult tissues a population of very small embryonic-like stem cells (VSELs), which express several markers characteristic for pluripotent stem cells that are characteristic for epiblast/germ line-derived stem cells. Thus, we hypothesize that VSELs are a population of epiblast-derived cells that are deposited during early gastrulation in developing tissues/organs and play an important role in turnover of tissue-specific/committed stem cells. In this context, VSELs deposited in BM can give rise to long-term repopulating HSC. VSELs could be also mobilized into peripheral blood (PB), and the number of these cells circulating in PB increases during stress and tissue/organ injuries. Finally, we envision that in pathological situations VSELs are involved in development of some malignancies (e.g., teratomas, germinal tumors).

Epigenetic reprogramming of the male genome during gametogenesis and in the zygote

During post-meiotic maturation, male germ cells undergo a formidable reorganization and condensation of their genome. During this phase most histones are globally acetylated and then replaced by sperm-specific basic proteins, named protamines, which compact the genome into a very specific structure within the sperm nucleus. Several studies suggest that this sperm-specific genome packaging structure conveys an important epigenetic message to the embryo. This paper reviews what is known about this fundamental, yet poorly understood, process, which involves not only global changes of the structure of the haploid genome, but also localized specific modifications of particular genomic regions, including pericentric heterochromatin and sex chromosomes. After fertilization, the male genome undergoes a drastic decondensation, and rapidly incorporates new histones. However, it remains different from the maternal genome, bearing specific epigenetic marks, especially in the pericentric heterochromatin region. The functional implications of male post-meiotic and post-fertilization genome reprogramming are not well known, but there is experimental evidence to show that it affects early embryonic development.

Epigenetic Basis for the Differentiation Potential of Mesenchymal and Embryonic Stem Cells

SUMMARY: Stem cells have the ability to self-renew, and give rise to one or more differentiated cell types. Embryonic stem cells can differentiate into all cell types of the body and have unlimited self-renewal capacity. Somatic stem cells are found in many adult tissues. They have an extensive but finite lifespan and can differentiate into a more restricted range of cell types. Increasing evidence indicates that the multilineage differentiation ability of stem cells is defined by the potential for expression of developmentally regulated transcription factors and of lineage specification genes. Gene expression, or as emphasized here, the potential for gene expression, is largely controlled by epigenetic modifications of DNA (DNA methylation) and chromatin (such as post-translational histone modifications) in the regulatory regions of specific genes. Epigenetic modifications can also influence the timing of DNA replication. We highlight here how mechanisms by which genes are poised for transcription in undifferentiated stem cells are being uncovered through the mapping of DNA methylation profiles on differentiation-regulated promoters and at the genome-wide level, histone modifications, and transcription factor binding. Epigenetic marks on developmentally regulated and lineage specification genes in stem cells seem to define a state of pluripotency.

Very small embryonic-like stem cells in adult tissues-potential implications for aging

Recently our group identified in murine bone marrow (BM) and human cord blood (CB), a rare population of very small embryonic-like (VSEL) stem cells. We hypothesize that these cells are deposited during embryonic development in BM as a mobile pool of circulating pluripotent stem cells (PSC) that play a pivotal role in postnatal tissue turnover both of non-hematopoietic and hematopoietic tissues. During in vitro co-cultures with murine myoblastic C2C12 cells, VSELs form spheres that contain primitive stem cells. Cells isolated from these spheres may give rise to cells from all three germ layers when plated in tissue specific media. The number of murine VSELs and their ability to form spheres decreases with the age and is reduced in short-living murine strains. Thus, developmental deposition of VSELs in adult tissues may potentially play an underappreciated role in regulating the rejuvenation of senescent organs. We envision that the regenerative potential of these cells could be harnessed to decelerate aging processes.

Hunt for pluripotent stem cell -- regenerative medicine search for almighty cell

Regenerative medicine and tissue engineering are searching for a novel stem cell based therapeutic strategy that will allow for efficient treatment or even potential replacement of damaged organs. The pluripotent stem cell (PSC), which gives rise to cells from all three germ lineages, seems to be the most ideal candidate for such therapies. PSC could be extracted from developing embryos. However, since this source of stem cells for potential therapeutic purposes remains controversial, stem cell researchers look for PSC that could be isolated from the adult tissues or generated from already differentiated cells. True PSC should possess both potential for multilineage differentiation in vitro and, more importantly, also be able to complement in vivo blastocyst development. This review will summarize current approaches and limitations to isolate PSC from adult tissues or, alternatively, to generate it by nuclear reprogramming from already differentiated somatic cells.

Programming the genome in embryonic and somatic stem cells

In opposition to terminally differentiated cells, stem cells can self-renew and give rise to multiple cell types. Embryonic stem cells retain the ability of the inner cell mass of blastocysts to differentiate into all cell types of the body and have acquired in culture unlimited self-renewal capacity. Somatic stem cells are found in many adult tissues, have an extensive but finite lifespan and can differentiate into a more restricted array of cell types. A growing body of evidence indicates that multi-lineage differentiation ability of stem cells can be defined by the potential for expression of lineage-specification genes. Gene expression, or as emphasized here, potential for gene expression, is largely controlled by epigenetic modifications of DNA and chromatin on genomic regulatory and coding regions. These modifications modulate chromatin organization not only on specific genes but also at the level of the whole nucleus; they can also affect timing of DNA replication. This review highlights how mechanisms by which genes are poised for transcription in undifferentiated stem cells are being uncovered through primarily the mapping of DNA methylation, histone modifications and transcription factor binding throughout the genome. The combinatorial association of epigenetic marks on developmentally regulated and lineage-specifying genes in undifferentiated cells seems to define a pluripotent state.

Adult marrow-derived very small embryonic-like stem cells and tissue engineering

A population of CXCR4(+) lin(-) CD45(-) cells that express SSEA, Oct-4 and Nanog has been identified in adult bone marrow. These cells are very small and display several features typical for primary embryonic stem cells such as: i) a large nuclei surrounded by a narrow rim of cytoplasm; ii) open-type chromatin (euchromatin); and iii) high telomerase activity. These cells were named very small embryonic-like stem cells (VSEL-SC). The authors hypothesized that they are direct descendants of the germ lineage. Germ lineage, in order to pass genes on to the next generation, has to create soma and thus becomes a 'mother lineage' for all somatic cell lineages present in the adult body. Germ potential is established after conception in a totipotent zygote and retained subsequently during development in blastomers of morula, cells form the inner cell mass of blastocyst, epiblast and population of primordial germ cells. The authors envision that VSEL-SC are epiblast-derived pluripotent stem cells and could potentially become a less-controversial source of stem cells for regeneration.

Identification of very small embryonic like (VSEL) stem cells in bone marrow

Bone marrow (BM) develops in mammals by the end of the second/beginning of the third trimester of gestation and becomes a major hematopoietic organ in postnatal life. The alpha-chemokine stromal derived factor-1 (SDF-1) to CXCR4 (G ai-protein-coupled seven transmembrane-spanning chemokine receptor) axis plays a major role in BM colonization by stem cells. By the end of the second trimester of gestation, BM becomes colonized by hematopoietic stem cells (HSC), which are chemoattracted from the fetal liver in a CXCR4-SDF-1-dependent manner. Whereas CXCR4 is expressed on HSC, SDF-1 is secreted by BM stroma and osteoblasts that line BM cavities. Mounting evidence indicates that BM also contains rare CXCR4(+) pluripotent stem cells (PSC). Recently, our group has identified a population of CXCR4(+) very small embryonic like stem cells in murine BM and human cord blood. We hypothesize that these cells are deposited during development in BM as a mobile pool of circulating PSC that play a pivotal role in postnatal tissue turnover, both of non-hematopoietic and hematopoietic tissues.

A hypothesis for an embryonic origin of pluripotent Oct-4(+) stem cells in adult bone marrow and other tissues

Accumulating evidence demonstrates that adult tissues contain a population of stem cells that express early developmental markers such as stage-specific embryonic antigen and transcription factors Oct-4 and Nanog. These are the markers characteristic for embryonic stem cells, epiblast stem cells and primordial germ cells. The presence of these stem cells in adult tissues including bone marrow, epidermis, bronchial epithelium, myocardium, pancreas and testes supports the concept that adult tissues contain some population of pluripotent stem cells that is deposited in embryogenesis during early gastrulation. In this review we will discuss these data and present a hypothesis that these cells could be direct descendants of the germ lineage. The germ lineage in order to pass genes on to the next generations creates soma and thus becomes a 'mother lineage' for all somatic cell lineages present in the adult body.

Stem cells adaptive network: mechanism and implications for evolution and disease development

During development, different cells and tissues acquire different programmes of gene expression, so that cells are related to each other through a somatic cells tree or cluster and adult pluripotential stem cells (PSC) may be defined as progenitors that we distinguish in four types according to their biological behaviour. This clustering may segregate specific pathways establishing spatial patterns of cell-cell communications. Thus, we suggest that normal somatic cells renewal is tributary of multipotential stem cells (MSC), while renewal of cells undergoing stress or abnormal death is tributary of PSC through specific pathway(s) from cluster, thus, defining the cell repertoire that will be produced. We also assume that PSC play a pivotal role in evolutionary and propose the theory of "internal clusters competition". According to the functional duality of stem cells (SC) we define a stem cells adaptive network (SCAN) which we believe is linked to the central clock and display two pathways. The diurnal pathway includes SC-somatic cells communications, while the nocturnal pathway includes inter-SC network. These alternate pathways could be activated or repressed as a consequence of change in the biological chirality. This new approach of SC may contribute to our understanding on how some diseases may develop including cancer which could be linked to "cluster illness", while demyelinating and systemic diseases could be related to "PSC locus illness" or "focalised SCAN disturbances" and it explains how any environment stress may act on organism evolution.

Impact of folate and homocysteine metabolism on human reproductive health

Folates belong to the vitamin B group and are involved in a large number of biochemical processes, particularly in the metabolism of homocysteine. Dietary or genetically determined folate deficiency leads to mild hyperhomocysteinemia, which has been associated with various pathologies. Molecular mechanisms of homocysteine-induced cellular dysfunction include increased inflammatory cytokine expression, altered nitric oxide bioavailability, induction of oxidative stress, activation of apoptosis and defective methylation. Whereas the involvement of folate metabolism and homocysteine in ageing-related diseases, in several developmental abnormalities and in pregnancy complications has given rise to a large amount of scientific work, the role of these biochemical factors in the earlier stages of mammalian reproduction and the possible preventive effects of folate supplementation on fertility have, until recently, been much less investigated. In the present article, the possible roles of folates and homocysteine in male and female subfertility and related diseases are systematically reviewed, with regard to the epidemiological, pathological, pharmacological and experimental data of the literature from the last 25 years.

Paternal dual barrier by Ifg2-H19 and Dlk1-Gtl2 to parthenogenesis in mice

The functional difference between the maternal and paternal genome, which is characterized by epigenetic modifications during gametogenesis, that is genomic imprinting, prevents mammalian embryos from parthenogenesis. Genomic imprinting leads to nonequivalent expression of imprinted genes from the maternal and paternal alleles. However, our research showed that alteration of maternal imprinting by oocyte reconstruction using nongrowing oocytes together with deletion of the H19 gene, provides appropriate expression of maternally imprinted genes. Here we discuss that further alteration of paternally imprinted gene expressions at chromosomes 7 and 12 allows the ng/fg parthenogenetic embryos to develop to term, suggesting that the paternal contribution is obligatory for the descendant.

Bone marrow-derived very small embryonic-like stem cells: Their developmental origin and biological significance

Data from our and other laboratories provide evidence that bone marrow (BM) contains a population of stem cells that expresses early developmental markers such as (1) stage-specific embryonic antigen (SSEA) and (2) transcription factors Oct-4 and Nanog. These are the markers characteristic for embryonic stem cells, epiblast stem cells, and primordial germ cells (PGC). The presence of these stem cells in adult BM supports the concept that this organ contains some population of pluripotent stem cells that is deposited in embryogenesis during early gastrulation. We hypothesize that these cells could be direct descendants of the germ lineage that, to pass genes on to the next generations, has to create soma and, thus, becomes a "mother lineage" for all somatic cell lineages present in the adult body. Germ potential is established after conception in totipotent zygotes and retained in blastomeres of morula, cells from the inner cell mass of blastocyst, epiblast, and population of PGC. We will present a concept that SSEA(+) Oct-4(+) Nanog(+) cells identified in BM could be descendants of epiblast cells as well as some rare migrating astray PGC.

[Epigenetics of the sperm cell]

In addition to genetic information, the spermatozoon carries another type of information, named epigenetic, which is not associated with variations of the DNA sequence. In somatic cells, it is now generally admitted that epigenetic information is not only regulated by DNA methylation but also involves modifications of the genome structure, or epigenome. During male germ cell maturation, the epigenome is globally re-organized, since most histones, which are associated to DNA in somatic cells, are removed and replaced by sperm specific nuclear proteins, the protamines, responsible for the tight compaction of the sperm DNA. However, a small proportion of histones, and probably other proteins, are retained within the sperm nucleus, and the structure of the sperm genome is actually heterogeneous. This heterogeneity of the sperm epigenome could support an epigenetic information, transmitted to the embryo, which could be crucial for its development. Although it is nowadays possible to appreciate the global structure of the sperm genome, the precise constitution of the sperm epigenome remains unknown. In particular, very recent data suggest that specific regions of the genome could be associated with particular proteins and define specific structures. This structural partitioning of the sperm genome could convey important epigenetic information, crucial for the embryo development.

Genomic imprinting is a barrier to parthenogenesis in mammals

Only mammals have relinquished parthenogenesis as a means of producing descendants. Bi-parental reproduction is necessary due to parent-specific epigenetic modification of the genome during gametogenesis, which leads to non-equivalent expression of imprinted genes from the maternal and paternal alleles. However, a series of our work showed that alteration of maternal imprinting by oocyte reconstruction using non-growing oocytes, together with deletion of the H19 gene provide appropriate expression of imprinted genes from the maternal genome. The resulting ng (non-growing)/fg (fully-grown) parthenogenic embryos were developed to term. Here, we discuss how the parthenogenetic embryos survived as normal individuals.

Consequences of paternal cocaine exposure in mice

The present study examined the potential neuroteratological effects of paternal cocaine (COC) exposure using the novel mouse model of inhalational drug administration. In this model, mice were trained to self-administer COC in multi-hour daily inhalation sessions reminiscent of crack binges. The controls included males pair-fed with COC-inhaling animals as well as ad-lib-fed males. All males were bred with drug-naive females. The newborn pups sired by COC-inhaling males had a reduced biparietal head diameter, suggesting a decreased cerebral volume. When the pups reached adulthood, their sustained visuo-spatial attention and spatial working memory were tested using a 5-arm maze paradigm. During the attention tests, the percentage of correct trials at the shortest stimulus duration employed in the study (0.5 s) was significantly lower for the male offspring of COC-inhaling fathers as compared to the offspring of both pair-fed and ad-lib-fed controls. For the females sired by COC-inhaling fathers, the deficit was observed at light stimulus durations of 0.5 and 0.75 s. Also, during the working memory tests, the male offspring of COC-inhaling fathers required more sessions than the offspring of either pair-fed or ad-lib-fed fathers to reach the selected criterion at retention intervals of 16 min and longer. The impairment of working memory in female offspring of COC-inhaling fathers was even stronger, as the offspring needed more sessions to reach the criterion as compared to their control counterparts, even at retention intervals as short as 4 min. These findings suggest that paternal COC abuse prior to coitus may impact the development of the offspring, particularly if they are females. We further showed that chronic COC exposure in male mice does not result in substantial breakage of spermatozoal DNA, but significantly alters expression of DNA methyltransferases 1 and 3a in the germ cell-rich seminiferous tubules of the testis. Since these enzymes are essential for generating and maintaining parental gene imprinting in germ cells, our observations point to an intriguing possibility that COC may cause paternally induced neuroteratological effects by interfering with gene-imprinting patterns in male gametes.

History and perspective of stem cell research

Several types of stem cell have been discovered from germ cells, the embryo, fetus and adult. Each of these has promised to revolutionize the future of regenerative medicine through the provision of cell-replacement therapies to treat a variety of debilitating diseases. Stem cell research is politically charged, receives considerable media coverage, raises many ethical and religious debates and generates a great deal of public interest. The tremendous versatility of embryonic stem cells versus the unprecedented reports describing adult stem cell plasticity have ignited debates as to the choice of one cell type over another for future application. However, the biology of these mysterious cells have yet to be understood and a lot more basic research is needed before new therapies using stem-cell-differentiated derivatives can be applied. Stem cell research opens-up the new field of 'cell-based therapies' and, as such, several safety measures have also to be evaluated.

Establishment of male-specific epigenetic information

The setting of male-specific epigenetic information is a complex process, which involves a major global re-organisation, as well as localized changes of the nucleus structure during the pre-meiotic, meiotic and post-meiotic stages of the male germ cell differentiation. Although it has long been known that DNA methylation in targeted regions of the genome is associated with male-specific genomic imprinting, or that most core histones are hyperacetylated and then replaced by sperm-specific proteins during the post-meiotic condensation of the nucleus, many questions remain unanswered. How these changes interact, how they affect the epigenetic information and how the paternal epigenetic marks contribute to the future genome are indeed major issues remaining to be explored.

Intracytoplasmic injection of spermatozoa and spermatogenic cells: its biology and applications in humans and animals

Intracytoplasmic sperm injection (ICSI) has become the method of choice to overcome male infertility when all other forms of assisted fertilization have failed. Animals in which ICSI has produced normal offspring include many species. Success rate with normal spermatozoa is well above 50% in the mouse but ICSI success rates in other animals have been low, ranging from 0.3 to 16.5%. Mouse ICSI revealed that spermatozoa that cannot participate in normal fertilization can produce normal offspring by ICSI, provided their nuclei are genomically intact. Human ICSI using infertile spermatozoa has been highly successful perhaps because of the intrinsic instability of human sperm plasma membrane. The health of children born after ICSI and other assisted fertilization techniques is of major concern. Careful analyses suggest that higher incidences of congenital malformations and/or low birth weights after assisted fertilization are largely attributable to parental genetic background and increased incidence of multiple births, rather than to the techniques of assisted fertilization. Since the physiological and nutritional environments of developing embryos may cause persisting alteration in DNA methylation, extreme caution must be exercised in handling gametes and embryos in vitro. In the mouse, round spermatid injection (ROSI) has been routinely successful but its use in humans is controversial. Whether human ROSI and assisted fertilization involving younger spermatogenic cells are medically safe must be the subject of further investigations.

Birth of parthenogenetic mice that can develop to adulthood

Only mammals have relinquished parthenogenesis, a means of producing descendants solely from maternal germ cells. Mouse parthenogenetic embryos die by day 10 of gestation. Bi-parental reproduction is necessary because of parent-specific epigenetic modification of the genome during gametogenesis. This leads to unequal expression of imprinted genes from the maternal and paternal alleles. However, there is no direct evidence that genomic imprinting is the only barrier to parthenogenetic development. Here we show the development of a viable parthenogenetic mouse individual from a reconstructed oocyte containing two haploid sets of maternal genome, derived from non-growing and fully grown oocytes. This development was made possible by the appropriate expression of the Igf2 and H19 genes with other imprinted genes, using mutant mice with a 13-kilobase deletion in the H19 gene as non-growing oocytes donors. This full-term development is associated with a marked reduction in aberrantly expressed genes. The parthenote developed to adulthood with the ability to reproduce offspring. These results suggest that paternal imprinting prevents parthenogenesis, ensuring that the paternal contribution is obligatory for the descendant.

Natural Selection and the Evolution of Genome Imprinting

Sexual reproduction results from the fusion of gametes in which the chromatin configuration of maternal and paternal chromosomes is distinct at fertilization. Although many of the differences are erased during successive cellular divisions and chromatin modifications, some are retained in both somatic and germline cells. These epigenetic modifications can confer different characteristics on maternal and paternal chromosomes and such differences can be selected during any process that has the ability to distinguish between homologues. The end result of these selective forces are parental origin effects, writ large. The range of effects observed, including transcriptional imprinting and effects on chromosome segregation and heterochromatization, reflects the diversity of selective forces in operation. However, a closer look at these effects suggests that parental origin-dependent differences in chromatin structure might be subject to some common forces and that these forces may explain many of the "nontranscriptional" parental origin effects observed in mammals.

Germ cell tumours in neonates and infants: a distinct subgroup?

Human germ cell tumours (GCTs) constitute a heterogeneous group of tumours that can be classified into four major subgroups. One of these subgroups encompasses (immature) teratomas and yolk sac tumours of patients under the age of 5 years. In this paper we review the various clinical, histological and cytogenetical aspects of these infantile GCTs. The primordial germ cell (PGC) has been suggested to be the cell of origin for GCTs. Infantile GCTs, however, have been suggested to originate from PGCs at a different stage of maturation than adult GCTs. The cytogenetic constitution of infantile GCTs also appears to differ from the adult GCTs and includes recurrent losses of lp and 6q. Recently, two cases of infantile GCT were detected with constitutional 12q13 translocations. These exceptional cases may be instrumental in the search for candidate genes related to infantile and/or adult GCT development.

The novel BORIS + CTCF gene family is uniquely involved in the epigenetics of normal biology and cancer

CTCF is a ubiquitous 11 zinc finger (ZF) protein with highly versatile functions: in addition to transcriptional silencing or activating in a context-dependent fashion, it organizes epigenetically controlled chromatin insulators that regulate imprinted genes in soma. Recently, we have identified a CTCF paralogue, termed BORIS for Brother of the Regulator of Imprinted Sites, that is expressed only in the testis. BORIS has the same exons encoding the 11 ZF domain as mammalian CTCF genes, and hence interacts with similar cis elements, but encodes amino and carboxy termini distinct from those in CTCF. Normally, CTCF and BORIS are expressed in a mutually exclusive pattern that correlates with re-setting of methylation marks during male germ cell differentiation. The antagonistic features of these two gene siblings are underscored by showing that while CTCF overexpression blocks cell proliferation, expression of BORIS in normally BORIS-negative cells promotes cell growth which can lead to transformation. The suggestion that BORIS directs epigenetic reprogramming at CTCF target sites impinges on the observations that human BORIS is not only abnormally activated in a wide range of human cancers, but also maps to the cancer-associated amplification region at 20q13. The sibling rivalry occasioned by aberrant expression of BORIS in cancer may interfere with normal functions of CTCF including growth suppression, and contribute to epigenetic dysregulation which is a common feature in human cancer.

Reprogramming of genome function through epigenetic inheritance

Most cells contain the same set of genes and yet they are extremely diverse in appearance and functions. It is the selective expression and repression of genes that determines the specific properties of individual cells. Nevertheless, even when fully differentiated, any cell can potentially be reprogrammed back to totipotency, which in turn results in re-differentiation of the full repertoire of adult cells from a single original cell of any kind. Mechanisms that regulate this exceptional genomic plasticity and the state of totipotency are being unravelled, and will enhance our ability to manipulate stem cells for therapeutic purposes.