Loss of genomic imprinting in mouse parthenogenetic embryonic stem cells

microRNA-670 modulates Igf2bp1 expression to regulate RNA methylation in parthenogenetic mouse embryonic development

Aberrant epigenetic modification, including N⁶-methylation of adenosine (m6A), has been frequently reported in embryos derived from parthenogenetic activation (PA). However, the role of Igf2bp1 expression pattern in m6A modification and the mechanism through which Igf2bp1 function is regulated in PA embryos remains elusive. Therefore, in this study, using si-Igf2bp1 and betaine (N,N,N-trimethylglycine, a major methyl donor), we investigated the effect of Igf2bp1 expression in m6A modification on the development of PA embryos. The results indicated that the down-regulation of Igf2bp1 reduced the cleavage and blastula rates of PA embryos. Moreover, m6A expression level was markedly down-regulated following microinjection with si-Igf2bp1. However, the treatment with betaine could significantly restore the m6A level. Further bioinformatics analysis revealed Igf2bp1 as the putative target of microRNA 670 (miR-670). Thus, to confirm this finding, mimics and inhibitor of miR-670 were microinjected into PA embryos. The results demonstrated that miR-670 inhibitor augmented the expression of Igf2bp1 and rescued cleavage and blastula rates. In addition, the miR-670 inhibitor promoted the m6A expression level. TUNEL assay revealed a loss of expression of Igf2bp1 induced cell apoptosis in PA embryos. Taken together, these results demonstrated that miR-670-3p functions as the regulator of Igf2bp1 expression and plays a crucial role in PA development through m6A modification.

The perturbed expression of m6A in parthenogenetic mouse embryos

Parthenogenetically activated oocytes cannot develop to term in mammals owing to abnormal epigenetic modifications. Methylation of the N6 position of adenosine (m6A) is a post-transcriptional epigenetic modification of RNA. To investigate the role of m6A methylation in parthenogenetic (PA) embryonic development, we analyzed METTL3, METTL14, FTO, ALKBH5, YTHDF2, IGF2BP1, and IGF2BP2 expression by quantitative real-time PCR. These genes were found dynamically expressed during the 2-cell, 4-cell, 8-cell, and blastocyst stages of the embryo. Compared to normally fertilized embryos, the expression of these genes was perturbed in PA embryos, especially at the 8-cell stage. Furthermore, immunofluorescence was used to detect m6A expression. The results demonstrated that m6A expression decreased in the 2-cell stage, whereas it increased in the 8-cell stage of PA embryos. Taken together, these results suggest that the expression of RNA methylation-related genes was perturbed, leading to abnormal m6A modification during early development in PA embryos.

Allele-specific RNA-seq expression profiling of imprinted genes in mouse isogenic pluripotent states

BACKGROUND

Genomic imprinting, resulting in parent-of-origin specific gene expression, plays a critical role in mammalian development. Here, we apply allele-specific RNA-seq on isogenic B6D2F1 mice to assay imprinted genes in tissues from early embryonic tissues between E3.5 and E7.25 and in pluripotent cell lines to evaluate maintenance of imprinted gene expression. For the cell lines, we include embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) derived from fertilized embryos and from embryos obtained after nuclear transfer (NT) or parthenogenetic activation (PGA).

RESULTS

As homozygous genomic regions of PGA-derived cells are not compatible with allele-specific RNA-seq, we developed an RNA-seq-based genotyping strategy allowing identification of informative heterozygous regions. Global analysis shows that proper imprinted gene expression as observed in embryonic tissues is largely lost in the ESC lines included in this study, which mainly consisted of female ESCs. Differentiation of ESC lines to embryoid bodies or NPCs does not restore monoallelic expression of imprinted genes, neither did reprogramming of the serum-cultured ESCs to the pluripotent ground state by the use of 2 kinase inhibitors. Fertilized EpiSC and EpiSC-NT lines largely maintain imprinted gene expression, as did EpiSC-PGA lines that show known paternally expressed genes being silent and known maternally expressed genes consistently showing doubled expression. Notably, two EpiSC-NT lines show aberrant silencing of Rian and Meg3, two critically imprinted genes in mouse iPSCs. With respect to female EpiSC, most of the lines displayed completely skewed X inactivation suggesting a (near) clonal origin.

CONCLUSIONS

Altogether, our analysis provides a comprehensive overview of imprinted gene expression in pluripotency and provides a benchmark to allow identification of cell lines that faithfully maintain imprinted gene expression and therefore retain full developmental potential.

Plausible Links Between Metabolic Networks, Stem Cells, and Longevity

Aging is an inevitable consequence of life, and all multicellular organisms undergo a decline in tissue and organ functions as they age. Several well-known risk factors, such as obesity, diabetes, and lack of physical activity that lead to the cardiovascular system, decline and impede the function of vital organs, ultimately limit overall life span. Over recent years, aging research has experienced an unparalleled growth, particularly with the discovery and recognition of genetic pathways and biochemical processes that control to some extent the rate of aging.In this chapter, we focus on several aspects of stem cell biology and aging, beginning with major cellular hallmarks of aging, endocrine regulation of aging and its impact on stem cell compartment, and mechanisms of increased longevity. We then discuss the role of epigenetic modifications associated with aging and provide an overview on a most recent search of antiaging modalities.

Imprinting Status in Two Human Parthenogenetic Embryonic Stem Cell Lines: Analysis of 63 Imprinted Gene Expression Levels in Undifferentiated and Early Differentiated Stages

Human parthenogenetic embryonic stem cells (hPESCs) represent a source of histocompatible tissues for transplantation and carry two copies of the maternal genome, but lack the paternal genome. In this study, we selected 63 known human imprinted genes to investigate the imprinting status of hPESC. The expression level of these genes, including 27 maternally and 36 paternally imprinted were illustrated in hPESC and human embryonic stem cells (hESCs) derived from fertilized embryo lines. The expression activity changes of these genes were analyzed in undifferentiated and early differentiated hPESC lines. In addition, the methylation status of four differentially methylated regions (DMRs) of the imprinted genes was analyzed in undifferentiated and early differentiated hPESC and hESC lines. As a result, we found that all the maternally imprinted genes were expressed at similar levels in the undifferentiated hPESC lines and the hESC lines, except ZNF264 and ATP10A. Twenty-one analyzed paternal imprinted genes were expressed at the same level in two separated hPESC lines as well as compared with the hESC lines, whereas 15 other paternal imprinted genes were significantly downregulated or inactivated in hPESC lines as compared with the hESC line. During prolonged passage, the expression levels of the majority of imprinted genes remained stable in two hPESC lines. The four DMRs, including PEG3/ZIM2 (DMRs), SNURF/SNRPN DMRs, and KVDMR1 DMRs are highly methylated in the genes of two undifferentiated hPESCs and its embryonic bodies (EBs), whereas the genes of the undifferentiated hESCs and its EBs are half methylated. During the early differentiation stage, the imprinted genes showed the same expression trend and the expression levels of H19, IGF2, SLC22A2, SLC22A3/SLC22A18, and CPA4 were significantly upregulated in both hPESC lines. As conclusion, hPESCs show a substantial degree of epigenetic stability with respect to some imprinted genes.

Disruption of NNAT, NAP1L5 and MKRN3 DNA methylation and transcription in rabbit parthenogenetic fetuses

Parthenogenetically activated oocytes cannot develop to term in mammals due to lack of paternal gene expression. Disruption of imprinted gene expression and DNA methylation status in parthenogenetic fetuses has been reported in mice and pigs, but not in rabbits. In this study, the genomic imprinting status of the paternally expressed genes Neuronatin (NNAT), Nucleosome assembly protein 1-like 5 (NAP1L5), and Makorin ring finger protein 3 (MKRN3) was compared between rabbit parthenogenetic (PA) and normally fertilized fetuses (Con) using quantitative real-time PCR (qRT-PCR) and bisulfite sequencing PCR (BSP). The results revealed a significantly reduced expression of NNAT, NAP1L5, and MKRN3 in rabbit PA fetuses compared with Con fetuses (p<0.05). In addition, the BSP results demonstrated hypermethylation in the differentially methylated regions (DMRs) of NNAT, NAP1L5, and MKRN3 in rabbit PA fetuses. Taken together, these results suggest that hypermethylation of DMRs is associated with decreased NNAT, NAP1L5, and MKRN3 expression, which may be responsible for developmental failure of rabbit PA fetuses.

Non-coding RNAs: emerging regulatory factors in the derivation and differentiation of mammalian parthenogenetic embryonic stem cells

Parthenogenetic embryonic stem cells (PESCs) are ESCs derived from early parthenogenetic embryos. Haploid PESCs, containing haploid DNA, originate from a single sperm or occyte, while, diploid PESCs originate from two fused occytes. Most PESC lines used so far are diploid. PESCs exhibit representative pluripotent stem cell features, such as the capacity for self-renewal and the pariticular molecular signatures. Whereas, PESCs display distinctive properties, such as differential regulation of X-chromosome inactivation (XCI) and divergent monitor of genes involved in multiple biological processes. PESCs are considered promising in the regeneration medicine and developmental biology. Non-coding RNAs (ncRNAs), especially miRNAs and lncRNAs, have garnered increasing attention over the past 2 decades. They are now known to be involved in almost all cellular processes due to their full-range regulation of gene expression. Numerous studies have indicated that embryonic stem cells (ESCs) displayed distinct signatures of ncRNA genes, which play key roles in the pluripotency and self renewal of ESCs. However, the expression pattern of ncRNAs in PESCs and their roles in the derivation and differentiation of PESCs were rarely reported. In this paper, we reviewed recent research on the derivation and differentiation of PESCs and describe the emerging role of ncRNAs in these processes.

Regulation of CpG methylation by Dnmt and Tet in pluripotent stem cells

Vertebrate genomes are highly methylated at cytosine residues in CpG sequences. CpG methylation plays an important role in epigenetic gene silencing and genome stability. Compared with other epigenetic modifications, CpG methylation is thought to be relatively stable; however, it is sometimes affected by environmental changes, leading to epigenetic instability and disease. CpG methylation is reversible and regulated by DNA methyltransferases and demethylases including ten-eleven translocation. Here, we discuss CpG methylation instability and the regulation of CpG methylation by DNA methyltransferases and ten-eleven translocation in pluripotent stem cells.

Parthenogenesis and Human Assisted Reproduction

Parthenogenetic activation of human oocytes obtained from infertility treatments has gained new interest in recent years as an alternative approach to create embryos with no reproductive purpose for research in areas such as assisted reproduction technologies itself, somatic cell, and nuclear transfer experiments and for derivation of clinical grade pluripotent embryonic stem cells for regenerative medicine. Different activating methods have been tested on human and nonhuman oocytes, with varying degrees of success in terms of parthenote generation rates, embryo development stem cell derivation rates. Success in achieving a standardized artificial activation methodology for human oocytes and the subsequent potential therapeutic gain obtained from these embryos depends mainly on the availability of gametes donated from infertility treatments. This review will focus on the creation of parthenotes from clinically unusable oocytes for derivation and establishment of human parthenogenetic stem cell lines and their potential applications in regenerative medicine.

p53 suppresses tetraploid development in mice

Mammalian tetraploid embryos die in early development because of defects in the epiblast. Experiments with diploid/tetraploid chimeric mice, obtained via the aggregation of embryonic stem cells, clarified that while tetraploid cells are excluded from epiblast derivatives, diploid embryos with tetraploid extraembryonic tissues can develop to term. Today, this method, known as tetraploid complementation, is usually used for rescuing extraembryonic defects or for obtaining completely embryonic stem (ES) cell-derived pups. However, it is still unknown why defects occur in the epiblast during mammalian development. Here, we demonstrated that downregulation of p53, a tumour suppressor protein, rescued tetraploid development in the mammalian epiblast. Tetraploidy in differentiating epiblast cells triggered p53-dependent cell-cycle arrest and apoptosis, suggesting the activation of a tetraploidy checkpoint during early development. Finally, we found that p53 downregulation rescued tetraploid embryos later in gestation.

Epigenetic disruptions of histone signatures for the trophectoderm and inner cell mass in mouse parthenogenetic embryos

Epigenetic asymmetry has been shown to be associated with the first lineage allocation event in preimplantation development, that is, the formation of the trophectoderm (TE) and inner cell mass (ICM) lineages in the blastocyst. Since parthenogenesis causes aberrant segregation between the TE and ICM lineages, we examined several development-associated histone modifications in parthenotes, including those involved in (i) transcriptional activation [acetylated histone H3 lysine 9 (H3K9Ac) and lysine 14 (H3K14Ac), trimethylated histone H3 lysine 4 (H3K4Me3), and dimethylated histone H3 arginine 26 (H3R26Me2)] and (ii) transcriptional repression [trimethylated histone H3 lysine 9 (H3K9Me3) and lysine 27 (H3K27Me3), and mono-ubiquitinated histone H2A lysine 119 (H2AK119u1)]. Here, we report that in parthenotes, H3R26Me2 expression decreased from the morula stage, while expression patterns and levels of H3K9Ac, H3K27Me3, and H2AK119u1 were unchanged until the blastocyst stage; whereas H3K14Ac, H3K4Me3, and H3K9Me3 showed normal patterns and levels of expressions. Relative to the decrease of H3K9Ac in the ICM and increase in the TE of parthenotes, we detected reduced expression of TAT-interactive protein 60 acetyltransferase and histone deacetylase 1 deacetylase in the ICM and TE of parthenotes, respectively. Relative to the decrease of H3R26Me2, we also observed decreased expression of coactivator-associated arginine methyltransferase 1 methyltransferase and increased expression of the Wnt effector transcription factor 7L2 and miR-181c microRNA in parthenotes. Furthermore, relative to the decrease in H3K27Me3 and H2AK119u1, we found increased phosphorylation of Akt1 and enhancer of zeste homolog 2 in parthenogenetic TE. Therefore, our findings that histone signatures are impaired in parthenotes provide a mechanistic explanation for aberrant lineage segregation and TE defects.

Inducing human parthenogenetic embryonic stem cells into islet‑like clusters

In order to determine whether human parthenogenetic embryonic stem (hpES) cells have the potential to differentiate into functional cells, a modified four-step protocol was used to induce the hpES cells into islet-like clusters (ILCs) in vitro. Growth factors activin A, retinoic acid, nicotinamide, Exendin-4 and betacellulin were added sequentially to the hpES cells at each step. The terminally differentiated cells were shown to gather into ILCs. Immunohistochemistry and semi quantitative polymerase chain reaction analyses demonstrated that the ILCs expressed islet specific hormones and functional markers. Furthermore, an insulin release test indicated that the clusters had the same physiological function as islets. The ILCs derived from hpES cells shared similar characteristics with islets. These results indicate that hpES cell-derived ILCs may be used as reliable material for the treatment of type I diabetes mellitus.

Concise review: parthenote stem cells for regenerative medicine: genetic, epigenetic, and developmental features

Embryonic stem cells (ESCs) have the potential to provide unlimited cells and tissues for regenerative medicine. ESCs derived from fertilized embryos, however, will most likely be rejected by a patient's immune system unless appropriately immunomatched. Pluripotent stem cells (PSCs) genetically identical to a patient can now be established by reprogramming of somatic cells. However, practical applications of PSCs for personalized therapies are projected to be unfeasible because of the enormous cost and time required to produce clinical-grade cells for each patient. ESCs derived from parthenogenetic embryos (pESCs) that are homozygous for human leukocyte antigens may serve as an attractive alternative for immunomatched therapies for a large population of patients. In this study, we describe the biology and genetic nature of mammalian parthenogenesis and review potential advantages and limitations of pESCs for cell-based therapies.

Genome engineering of mammalian haploid embryonic stem cells using the Cas9/RNA system

Haploid embryonic stem cells (ESCs) are useful for studying mammalian genes because disruption of only one allele can cause loss-of-function phenotypes. Here, we report the use of haploid ESCs and the CRISPR RNA-guided Cas9 nuclease gene-targeting system to manipulate mammalian genes. Co-transfection of haploid ESCs with vectors expressing Cas9 nuclease and single-guide RNAs (sgRNAs) targeting Tet1, Tet2, and Tet3 resulted in the complete disruption of all three genes and caused a loss-of-function phenotype with high efficiency (50%). Co-transfection of cells with vectors expressing Cas9 and sgRNAs targeting two loci on the same chromosome resulted in the creation of a large chromosomal deletion and a large inversion. Thus, the use of the CRISPR system in combination with haploid ESCs provides a powerful platform to manipulate the mammalian genome.

Derivation of embryonic stem cell lines from parthenogenetically developing rat blastocysts

This study was undertaken to establish rat embryonic stem (ES) cells from parthenogenetically developing blastocysts. Ten blastocysts were prepared by treatment of ovulated rat oocytes with ionomycin and cycloheximide, and three alkaline phosphatase-positive ES cell lines were established using the N2B27 medium supplemented with mitogen activated protein kinase kinase inhibitor PD0325901, glycogen synthase kinase 3 inhibitor CHIR99021, rat leukemia inhibitory factor, and forskolin. Expression of stem cell marker genes (Oct-4, rNanog, Fgf-4, and Rex-1) was confirmed in all three ES cell lines by reverse transcriptase-polymerase chain reaction (RT-PCR). Combined bisulfite restriction analysis showed that the differentially methylated region locus of five imprinted genes (H19, Meg3IG, Igf2r, Peg5, and Peg10) in these ES cells remained to be demethylated or was hypomethylated, which was similar to that in control ES cells established from normal blastocysts. Characteristics of the parthenogenetic blastocyst-derived ES cells were successfully transmitted to the next generation through a chimeric rat for one of the three ES cell lines. This is the first report on germline-competent (genuine) ES cells derived from parthenogenetically developing rat blastocysts.

Conversion of genomic imprinting by reprogramming and redifferentiation

Induced pluripotent stem cells (iPSCs), generated from somatic cells by overexpression of transcription factors, Oct4, Sox2, Klf4, and c-Myc, have the same characteristics as pluripotent embryonic stem cells (ESCs). iPSCs reprogrammed from differentiated cells undergo epigenetic modification during reprogramming, and ultimately acquire a similar epigenetic state to that of ESCs. In this study, these epigenetic changes were observed in reprogramming of uniparental parthenogenetic somatic cells. The parthenogenetic pattern of imprinted genes changes during the generation of parthenogenetic maternal iPSCs (miPSCs), a process referred to as pluripotent reprogramming. Here, we determined whether altered imprinted genes are maintained or reverted to the parthenogenetic state when the reprogrammed cells are redifferentiated into specialized cell types. To address this question, we redifferentiated miPSCs into neural stem cells (miPS-NSCs) and compared them with biparental female NSCs (fNSCs) and parthenogenetic NSCs (pNSCs). We found that pluripotent reprogramming of parthenogenetic somatic cells could reset parthenogenetic DNA methylation patterns in imprinted genes, and that alterations in DNA methylation were maintained even after miPSCs were redifferentiated into miPS-NSCs. Notably, maternally methylated imprinted genes (Peg1, Peg3, Igf2r, Snrpn, and Ndn) whose differentially methylated regions (DMRs) were fully methylated in pNSCs, were demethylated, and their expression levels were found to be close to the levels in normal biparental fNSCs after reprogramming and redifferentiation. Our findings suggest that pluripotent reprogramming of parthenogenetic somatic cells followed by redifferentiation leads to changes in DNA methylation of imprinted genes and the reestablishment of gene expression levels to those of normal biparental cells.

Ectopic expression of Fgf3 leads to aberrant lineage segregation in the mouse parthenote preimplantation embryos

BACKGROUND

Parthenogenetic mammalian embryos were reported to die in utero no later than the 25-somite stage due to abnormal development of both embryonic and extraembryonic lineages. Interestingly, it has been shown that parthenogenetic ICM cells tend to differentiate more into primitive endoderm cells and less into epiblast and ES cells. Hence we are interested in studying the molecular mechanisms underlying lineage defects of parthenotes.

RESULTS

We found that parthenote inner cell masses (ICMs) contained decreased numbers of Sox2(+) /Nanog(+) epiblast cells but increased numbers of Gata4(+) primitive endoderm cells, indicating an unusual lineage segregation. We demonstrate for the first time that the increased Gata4 level in parthenotes may be explained by the strong up-regulation of Fgf3 and Fgfr2 phosphorylation. Inhibition of Fgfr2 activation by SU5402 in parthenotes restored normal Nanog and Gata4 levels without affecting Fgf3, indicating that Fgf3 is upstream of Fgfr2 activation. In parthenote trophectoderm, we detected normal Cdx2 but ectopic Gata4 expression and reduced Elf5 and Tbr2(Eomes) levels.

CONCLUSIONS

Taken together, our work provides for the first time the insight into the molecular mechanisms of the developmental defects of parthenogenetic embryos in both the trophectoderm and ICM.

Aggregation of pre-implantation embryos improves establishment of parthenogenetic stem cells and expression of imprinted genes

Parthenogenetic embryonic stem cells (PgES) might advance cell replacement therapies and provide a valuable in vitro model system to study the genomic imprinting. However, the differential potential of PgES cells was limited. It could result from relative low heterology of PgES cells compared with ES cells from fertilization (fES), which produce different expression of most imprinted genes. Here, we described the establishment of PgES cells by aggregating parthenogenetic embryos at the 8-cell stage (aPgES cells), which may increase heterozygy. We found that derivation of aPgES cells in association with an increased number of inner cell mass cells by aggregating was more efficient than that of PgES cells from a single parthenogenetic blastocyst. The aPgES cells have normal karyotype, stain positive for alkaline phosphatase, express high levels of ES cell markers and can differentiate into teratomas composed of the three germ layers. Moreover, compared with PgES cells, the more highly upregulated paternally expressed imprinted genes were observed in aPgES cells, the same change was not shown in aPg blastocysts. This suggested that the aggregation induced effect could modify the expression of paternally expressed imprinted genes. Our studies showed that aPgES cells, the expression of imprinted genes in which more closely resemble fES cells than PgES cells, would contribute to all organs and avoiding immuno-rejection, which may provide invaluable material for regeneration medicine.

Derivation of induced pluripotent stem cells by retroviral gene transduction in mammalian species

Pluripotent stem cells can provide us with an enormous cell source for in vitro model systems for development. In 2006, new methodology was designed to generate pluripotent stem cells directly from somatic cells, and these cells were named induced pluripotent stem cells (iPSCs). This method consists of technically simple procedures: donor cell preparation, gene transduction, and isolation of embryonic stem cell-like colonies. The iPSC technology enables cell biologists not only to obtain pluripotent stem cells easily but also to study the reprogramming events themselves. Here, we describe the protocols to generate iPSCs from somatic origins by using conventional viral vectors. Specifically, we state the usage of three mammalian species: mouse, common marmoset, and human. As mouse iPSC donors, fibroblasts are easily prepared, while mesenchymal stem cells are expected to give rise to highly reprogrammed iPSCs efficiently. Common marmoset (Callithrix jacchus), a nonhuman primate, represents an alternative model to the usual laboratory animals. Finally, patient-specific human iPSCs give us an opportunity to examine the pathology and mechanisms of dysregulated genomic imprinting. The iPSC technology will serve as a valuable method for studying genomic imprinting, and conversely, the insights from these studies will offer valuable criteria to assess the potential of iPSCs.

The Dnmt3b splice variant is specifically expressed in in vitro-manipulated blastocysts and their derivative ES cells

Manipulation of preimplantation embryos in vitro, such as in vitro fertilization (IVF), in vitro culture (IVC), intracytoplasmic sperm injection (ICSI), somatic cell nuclear transfer (SCNT) and other assisted reproduction technologies (ART), has contributed to the development of infertility treatment and new animal reproduction methods. However, such embryos often exhibit abnormal DNA methylation patterns in imprinted genes and centromeric satellite repeats. These DNA methylation patterns are established and maintained by three DNA methyltransferases: Dnmt1, Dnmt3a and Dnmt3b. Dnmt3b is responsible for the creation of methylation patterns during the early stage of embryogenesis and consists of many alternative splice variants that affect methylation activity; nevertheless, the roles of these variants have not yet been identified. In this study, we found an alternatively spliced variant of Dnmt3b lacking exon 6 (Dnmt3bΔ6) that is specific to mouse IVC embryos. Dnmt3bΔ6 also showed prominent expression in embryonic stem (ES) cells derived from in vitro manipulated embryos. Interestingly, IVC blastocysts were hypomethylated in centromeric satellite repeat regions that could be susceptible to methylation by Dnmt3b. In vitro methylation activity assays showed that Dnmt3bΔ6 had lower activity than normal Dnmt3b. Our findings suggest that Dnmt3bΔ6 could induce a hypomethylation status especially in in vitro manipulated embryos.

Downregulation of H19 improves the differentiation potential of mouse parthenogenetic embryonic stem cells

Parthenogenetic embryonic stem cells (P-ESCs) offer an alternative source of pluripotent cells, which hold great promise for autologous transplantation and regenerative medicine. P-ESCs have been successfully derived from blastocysts of several mammalian species. However, compared with biparental embryonic stem cells (B-ESCs), P-ESCs are limited in their ability to fully differentiate into all 3 germ layers. For example, it has been observed that there is a differentiation bias toward ectoderm derivatives at the expense of endoderm and mesoderm derivatives-muscle in particular-in chimeric embryos, teratomas, and embryoid bodies. In the present study we found that H19 expression was highly upregulated in P-ESCs with more than 6-fold overexpression compared with B-ESCs. Thus, we hypothesized that manipulation of the H19 gene in P-ESCs would alleviate their limitations and allow them to function like B-ESCs. To test this hypothesis we employed a small hairpin RNA approach to reduce the amount of H19 transcripts in mouse P-ESCs. We found that downregulation of H19 led to an increase of mesoderm-derived muscle and endoderm in P-ESCs teratomas similar to that observed in B-ESCs teratomas. This phenomenon coincided with upregulation of mesoderm-specific genes such as Myf5, Myf6, and MyoD. Moreover, H19 downregulated P-ESCs differentiated into a higher percentage of beating cardiomyocytes compared with control P-ESCs. Collectively, these results suggest that P-ESCs are amenable to molecular modifications that bring them functionally closer to true ESCs.

Quantitative proteomics analysis of parthenogenetically induced pluripotent stem cells

Parthenogenetic embryonic stem (pES) cells isolated from parthenogenetic activation of oocytes and embryos, also called parthenogenetically induced pluripotent stem cells, exhibit pluripotency evidenced by both in vitro and in vivo differentiation potential. Differential proteomic analysis was performed using differential in-gel electrophoresis and isotope-coded affinity tag-based quantitative proteomics to investigate the molecular mechanisms underlying the developmental pluripotency of pES cells and to compare the protein expression of pES cells generated from either the in vivo-matured ovulated (IVO) oocytes or from the in vitro-matured (IVM) oocytes with that of fertilized embryonic stem (fES) cells derived from fertilized embryos. A total of 76 proteins were upregulated and 16 proteins were downregulated in the IVM pES cells, whereas 91 proteins were upregulated and 9 were downregulated in the IVO pES cells based on a minimal 1.5-fold change as the cutoff value. No distinct pathways were found in the differentially expressed proteins except for those involved in metabolism and physiological processes. Notably, no differences were found in the protein expression of imprinted genes between the pES and fES cells, suggesting that genomic imprinting can be corrected in the pES cells at least at the early passages. The germline competent IVM pES cells may be applicable for germ cell renewal in aging ovaries if oocytes are retrieved at a younger age.

Epigenetic differences between embryonic stem cells generated from blastocysts developed in vitro and in vivo

Embryonic stem (ES) cells constitute a very important tool for regenerative medicine today. These ES cells, and human ES cells in particular, are almost all derived from embryos obtained by in vitro fertilization (IVF) and from in vitro culture (IVC); however, such in vitro manipulated embryos often show abnormal genomic imprinting, which can lead to the development of various diseases. Nevertheless, several reports have evaluated ES cells derived from in vitro manipulated embryos. In this study, we established ES cells derived from both in vivo and in vitro developed blastocysts (Vivo ES cells and Vitro ES cells, respectively) to compare the methylation status of imprinted genes and gene expression patterns. At very early passages, Vitro ES cells showed an increase in abnormal genomic imprinting compared to Vivo ES cells. In addition, we found that the gene expression patterns of several methylation related-genes frequently shifted to promote demethylation and to inhibit methylation in early-passage Vitro ES cells. In contrast, at later passages, we found no significant differences between Vivo and Vitro ES cells. In conclusion, it is advisable to use early passage Vivo ES cells whenever feasible, or to select ES cell lines with a normal epigenotype.

RFD Award Lecture 2009. In vitro maturation of farm animal oocytes: a useful tool for investigating the mechanisms leading to full-term development

Due to logistical and economic reasons, assisted reproduction of domestic animals has been based mostly on the use of oocytes isolated from ovaries collected at the slaughterhouse. In order to propagate valuable or rare genetic material, perform somatic cell nuclear transfer or generate genetically modified animals, it is essential to obtain fully competent oocytes that will allow full-term development of the in vitro-produced embryos. Such a need makes clear the crucial role played by oocyte quality. In fact, it is easy to compromise the oocyte's developmental potential but it is impossible to restore once it has been lost. Almost three decades after the first cow, sheep, goat, horse and pig in vitro-generated offspring were born, a large body of information has accumulated on the mechanisms regulating oocyte competence and on how the latter may be preserved during all the required manipulations. The amount of knowledge is far from complete and many laboratories are actively working to further expand it. In this review we will highlight the aspects of the ongoing research in which we have been actively involved.

Embryonic stem cell-like cells established by culture of adult ovarian cells in mice

OBJECTIVE

To suggest an alternative strategy for deriving histocompatible stems cells without undertaking genetic manipulation.

DESIGN

Prospective approach using an animal model.

SETTING

Stem cell and bioevaluation laboratory, Seoul National University.

ANIMAL(S)

F1 (C57BL6 X DBA2) and outbred (ICR) mice.

INTERVENTION(S)

Ovarian stroma cells of less than 40 mum in diameter were subcultured with fibroblast monolayer, and colony-forming cells were characterized.

MAIN OUTCOME MEASURE(S)

Stemness, genotype, and imprinted gene methylation.

RESULT(S)

Two-lines of colony-forming cells were established, which expressed markers specific for embryonic stem cells (ESC) and formed embryoid bodies and teratomas. Complete matching of microsatellite markers with the cell donor strain confirmed their establishment from ovarian tissue, and identification of both homozygotic and heterozygotic chromosomes raised the possibility of their derivation from parthenogenetic oocytes. However, the use of cells smaller than mature oocytes for primary culture, the difference in imprinted gene methylation compared with parthenogenetic ESCs, and failure to establish the ESC-like cells by primary follicle culture collectively suggested the irrelevancy to gametes.

CONCLUSION(S)

Coculture of adult ovarian cells with somatic fibroblasts can yield colony-forming cells having ESC-like activity, which may provide an alternative for establishing autologous stem cells from adults that can be obtained without genetic manipulation.

Generation of histocompatible androgenetic embryonic stem cells using spermatogenic cells

Androgenetic embryonic stem (aES) cells, produced by pronuclear transplantation, offer an important autologous pluripotent stem cell source. However, the isolation of aES cells, particularly individual-specific aES cells, with the use of fertilized embryos has limited the practical applications of this technology in humans. In this study, we applied a new approach, essentially described as somatic cell nuclear transfer, and generated three aES cell line types with the use of spermatogenic cells including primary spermatocytes, round spermatids, and mature spermatozoa as donor cells, omitting the need to use fertilized embryos. Although abnormality of chimeras and absent germline competency indicated that all three types of aES cells exhibited limited pluripotency, the epigenetic status of the aES cell lines tended to resemble normal ES cells during long-term culture, and some parental-specific imprinted genes were expressed at levels comparable to those of normal ES cells. Furthermore, the histocompatibility of the aES cells was investigated by transplanting the differentiation progenies of the aES cells into major histocompatibility (MHC)-matched and -mismatched recipient mice. The results indicated that these aES cells were histocompatible with MHC-matched mice after transplantation. Our study provides evidence that MHC-competent autologous aES cells could be generated from different spermatogenic cells using nuclear transfer into oocytes, a process that could avoid the use of fertilized embryos.

Generation of pluripotent stem cells from eggs of aging mice

Oocytes can reprogram genomes to form embryonic stem (ES) cells. Although ES cells largely escape senescence, oocytes themselves do senesce in the ovaries of most mammals. It remains to be determined whether ES cells can be established using eggs from old females, which exhibit reproductive senescence. We attempted to produce pluripotent stem cell lines from artificial activation of eggs (also called pES) from reproductive aged mice, to determine whether maternal aging affects pES cell production and pluripotency. We show that pES cell lines were generated with high efficiency from reproductive aged (old) mice, although parthenogenetic embryos from these mice produced fewer ES clones by initial two passages. Further, pES cell lines generated from old mice showed telomere length, expression of pluripotency molecular markers (Oct4, Nanog, SSEA1), alkaline phosphatase activity, teratoma formation and chimera production similar to young mice. Notably, DNA damage was reduced in pES cells from old mice compared to their progenitor parthenogenetic blastocysts, and did not differ from that of pES cells from young mice. Also, global gene expression differed only minimally between pES cells from young and old mice, in contrast to marked differences in gene expression in eggs from young and old mice. These data demonstrate that eggs from old mice can generate pluripotent stem cells, and suggest that the isolation and in vitro culture of ES cells must select cells with high levels of DNA and telomere integrity, and/or with capacity to repair DNA and telomeres.

Generation of parthenogenetic induced pluripotent stem cells from parthenogenetic neural stem cells

Somatic cells can achieve a pluripotent cell state in a process called pluripotential reprogramming. Multipotent stem cells can differentiate into cells of only one lineage, but pluripotent stem cells can give rise to cells of all three germ layers of an organism. In this study, we generated induced pluripotent stem (iPS) cells from bimaternal (uniparental) parthenogenetic neural stem cells (pNSCs) by transduction with either four (4F: Oct4, Klf4, Sox2, and c-Myc) or two (2F: Oct4 and Klf4) transcription factors. The resultant maternal iPS cells, which were reprogrammed directly from pNSCs, were capable of generating germ line-competent chimeras. Interestingly, analysis of global gene expression and imprinting status revealed that parthenogenetic iPS cells clustered closer to parthenogenetic ESCs than to female ESCs, with patterns that were clearly distinct from those of pNSCs.

Novel epigenetic mechanisms that control pluripotency and quiescence of adult bone marrow-derived Oct4(+) very small embryonic-like stem cells

Recently, we identified in adult tissues a population of Oct4(+)SSEA-1(+)Sca-1(+)lin(-)CD45(-) very small embryonic-like stem cells (VSELs). First, to address recent controversies on Oct4 expression in cells isolated from adult organs, we show here evidence that Oct4 promoter in bone marrow (BM)-derived VSELs has an open chromatin structure and is actively transcribed. Next, to explain VSELs quiescence and lack of teratoma formation, we demonstrate a unique DNA methylation pattern at some developmentally crucial imprinted genes, showing hypomethylation/erasure of imprints in paternally methylated and hypermethylation of imprints in maternally methylated ones. These epigenetic characteristics leading to upregulation in VSELs of H19 and p57(KIP2) (also known as Cdkn1c) and repression of Igf2 and Rasgrf1 explain VSEL's quiescent status. Interestingly, this unique pattern in imprinted gene methylation is reverted in cocultures with a C2C12 supportive cell-line when VSELs are induced to form VSEL-derived spheres (VSEL-DSs) enriched for stem cells able to differentiate into all three germ layers. Therefore, we suggest that the proliferative/developmental potential of Oct4(+) VSELs is epigenetically regulated by expression of Oct4 and some imprinted genes, and postulate that restoring the proper methylation pattern of imprinted genes will be a crucial step for using these cells in regenerative medicine.

H19 gene is epigenetically stable in mouse multipotent germline stem cells

Testis-derived germline stem (GS) cells can undergo re-programming to acquire multipotency when cultured under appropriate culture conditions. These multipotent GS (mGS) cells have been known to differ from GS cells in their DNA methylation pattern. In this study, we examined the DNA methylation status of the H19 imprinting control region (ICR) in multipotent adult germline stem (maGS) cells to elucidate how epigenetic imprints are altered by culture conditions. DNA methylation was analyzed by bisulfite sequencing PCR of established maGS cells cultured in the presence of glial cell line-derived neurotrophic factor (GDNF) alone or both GDNF and leukemia inhibitory factor (LIF). The results showed that the H19 ICR in maGS cells of both groups was hypermethylated and had an androgenetic pattern similar to that of GS cells. In line with these data, the relative abundance of the Igf2 mRNA transcript was two-fold higher and that of H19 was three fold lower than in control embryonic stem cells. The androgenetic DNA methylation pattern of the H19 ICR was maintained even after 54 passages. Furthermore, differentiating maGS cells from retinoic acid-treated embryoid bodies maintained the androgenetic imprinting pattern of the H19 ICR. Taken together these data suggest that our maGS cells are epigenetically stable for the H19 gene during in vitro modifications. Further studies on the epigenetic regulation and chromatin structure of maGS cells are therefore necessary before their full potential can be utilized in regenerative medicine.

Correlation of expression and methylation of imprinted genes with pluripotency of parthenogenetic embryonic stem cells

Mammalian parthenogenetic embryos (pE) are not viable due to placental deficiency, presumably resulting from lack of paternally expressed imprinted genes. Pluripotent parthenogenetic embryonic stem (pES) cells derived from pE could advance regenerative medicine by avoiding immuno-rejection and ethical roadblocks. We attempted to explore the epigenetic status of imprinted genes in the generation of pES cells from parthenogenetic blastocysts, and its relationship to pluripotency of pES cells. Pluripotency was evaluated for developmental and differentiation potential in vivo, based on contributions of pES cells to chimeras and development to day 9.5 of pES fetuses complemented by tetraploid embryos (TEC). Consistently, pE and fetuses failed to express paternally expressed imprinted genes, but pES cells expressed those genes in a pattern resembling that of fertilized embryos (fE) and fertilized embryonic stem (fES) cells derived from fE. Like fE and fES cells, but unlike pE or fetuses, pES cells and pES cell-fetuses complemented by TEC exhibited balanced methylation of Snrpn, Peg1 and U2af1-rs1. Coincidently, global methylation increased in pE but decreased in pES cells, further suggesting dramatic epigenetic reprogramming occurred during isolation and culture of pES cells. Moreover, we identified decreased methylation of Igf2r, Snrpn, and especially U2af1-rs1, in association with increased contributions of pES cells to chimeras. Our data show that in vitro culture changes epigenetic status of imprinted genes during isolation of pES cells from their progenitor embryos and that increased expression of U2af1-rs1 and Snrpn and decreased expression of Igf2r correlate with pluripotency of pES cells.