Germinal vesicle material is essential for nucleus remodeling after nuclear transfer

Assessing the effect of interspecies oocyte nucleolar material dosage on embryonic development

Sequence differences are considered to be the basic cause of developmental failure in interspecies embryos when more distant species are combined. However, other phenomena, such as insufficient or excessive quantity of specific cellular factors, might also influence the outcome. These effects are usually not considered. One of the organelles shown to contain different amount of proteins is the oocyte nucleolus-like body. Here we show that upon interspecies transfer, a single porcine nucleolus-like body is unable to support the development of a mouse parthenogenetic embryo derived from an enucleolated oocyte. However, when the amount of the porcine nucleolar material is increased to equalize the amount of mouse nucleolar material by transferring two nucleolus-like bodies, mouse embryos are able to pass the developmental block elicited by enucleolation. These embryos progress to the blastocyst stage at rates comparable to controls. Thus, using the model of an interspecies nucleolus-like body transplantation between mouse and pig oocytes, we show that an inadequate amount of nucleolar factors, rather than the species origin, affects the development. In a wider context of interspecies nuclear transfer schemes, the observed incompatibility between more distant species might not stem simply from sequence differences but also from improper dosage of key cellular factors.

Effects of Cellular Extract on Epigenetic Reprogramming

Functional reprogramming of a differentiated cell toward pluripotent cell may have long-term applications in numerous aspects, especially in regenerative medicine. Evidences accumulating from recent studies suggest that cellular extracts from stem cells or pluripotent cells can induce epigenetic reprogramming and facilitate pluripotency in otherwise highly differentiated cell types. Epigenetic reprogramming using cellular extracts has gained increasing attention and applied to recognize the functional factors, acquire the target cell types, and explain the mechanism of reprogramming. Now, more and more researches have proved that cellular extract treatment is an important strategy of cellular reprogramming. Thus, this review mainly focused on the progresses and potential mechanisms in epigenetic reprogramming using cellular extracts.

Factors and molecules that could impact cell differentiation in the embryo generated by nuclear transfer

Somatic cell nuclear transfer is a technique to create an embryo using an enucleated oocyte and a donor nucleus. Nucleus of somatic cells must be reprogrammed in order to participate in normal development within an enucleated egg. Reprogramming refers to the erasing and remodeling of cellular epigenetic marks to a lower differentiation state. Somatic nuclei must be reprogrammed by factors in the oocyte cytoplasm to a rather totipotent state since the reconstructed embryo must initiate embryo development from the one cell stage to term. In embryos reconstructed by nuclear transfer, the donor genetic material must respond to the cytoplasmic environment of the cytoplast and recapitulate this normal developmental process. Enucleation is critically important for cloning efficiency because may affect the ultrastructure of the remaining cytoplast, thus resulting in a decline or destruction of its cellular compartments. Nonetheless, the effects of in vitro culturing are yet to be fully understood. In vitro oocyte maturation can affect the abundance of specific transcripts and are likely to deplete the developmental competence. The epigenetic modifications established during cellular differentiation are a major factor determining this low efficiency as they act as epigenetic barriers restricting reprogramming of somatic nuclei. In this review we discuss some factors that could impact cell differentiation in embryo generated by nuclear transfer.

Incidence of methylated histones H3K4 and H3K79 in cat germinal vesicles is regulated by specific nuclear factors at the acquisition of developmental competence during the folliculogenesis

PURPOSE

This study aims to characterize the regulations of histone methylations, key epigenetic markers of oocyte competence, in germinal vesicle (GV) from different follicles (preantral, early, small, or large antral stage) using the domestic cat model.

METHODS

In Experiment 1, the incidence of H3K4me3 or H3K79me2 was determined in GVs from the diverse follicle stages directly or after exposure to (1) a methyltransferase inhibitor, (2) sonication to fracture the cytoplasmic membranes and wash away the cytoplasmic content, or (3) methyltransferase inhibitor followed by sonication. In Experiment 2, the presence and maintenance of nuclear methyltransferases SMYD3 and DOT1L (regulating H3K4me3 and H3K79me2, respectively) was characterized in separate GV stages before and after sonication. Functionality of GVs from the various follicle stages (with or without transient isolation from the cytoplasm) then was assessed in Experiment 3 by transfer into recipient competent oocytes.

RESULTS

The incidence of histones H3K4me3 and H3K79me2 within the GV were influenced by the cytoplasmic environment at all stages except at the transition to the early antral stage where nuclear regulating factors appeared to be mainly involved. The methyltransferase SMYD3 and DOT1L also appeared tightly bound to the nucleus at that transition. Interestingly, oocytes reconstructed with a GV isolated from the cytoplasm for a prolonged period had the capacity to form an embryo after fertilization which proved that communication between the donor GV and the host cytoplasm (likely including the regulation of epigenetic factors) could be restored.

CONCLUSIONS

Histone methylation apparently becomes regulated by specific nuclear factors at the acquisition of competence during the folliculogenesis and does not seem to be disrupted by prolonged isolation from the surrounding cytoplasm.

Cytoplasmic Determination of Meiotic Spindle Size Revealed by a Unique Inter-Species Germinal Vesicle Transfer Model

Spindle sizes are different in diverse species and cell types. In frogs, the meiotic spindle size is positively correlated with the egg cell volume. Across species, relatively small mouse oocytes (70–80 μm) have a relatively large spindle while larger pig oocytes (about 120 μm) have a considerably smaller spindle. In this study we investigated whether species-specific oocyte spindle size was determined by cytoplasmic or nuclear factors. By exchanging the germinal vesicle between mouse and pig oocytes, we obtained two kinds of reconstructed oocytes: one with mouse ooplasm and pig GV (mCy-pGV oocyte), and the other with pig ooplasm and mouse GV (pCy-mGV oocyte). We show that the MII spindle size of the mCy-pGV oocyte is similar to that of the mouse meiotic spindle and significantly larger than that of the pig meiotic spindle. The timing of oocyte maturation also followed that of the species from which the oocyte cytoplasm arose, although some impact of the origin of the GV was observed. These data suggest that spindle size and the timing of meiotic progression are governed by cytoplasmic components rather than cytoplasmic volume and GV materials.

H3.3 replacement facilitates epigenetic reprogramming of donor nuclei in somatic cell nuclear transfer embryos

Transfer of a somatic nucleus into an enucleated oocyte is the most efficient approach for somatic cell reprogramming. While this process is known to involve extensive chromatin remodeling of the donor nucleus, the maternal factors responsible and the underlying chromatin-based mechanisms remain largely unknown. Here we discuss our recent findings demonstrating that the histone variant H3.3 plays an essential role in reprogramming and is required for reactivation of key pluripotency genes in somatic cell nuclear transfer (SCNT) embryos. Maternal-derived H3.3 replaces H3 in the donor nucleus shortly after oocyte activation, with the amount of replacement directly related to the differentiation status of the donor nucleus in SCNT embryos. We provide additional evidence to suggest that de novo synthesized H3.3 replaces histone H3 carrying repressive modifications in the donor nuclei of SCNT embryos, and hypothesize that replacement may occur at specific loci that must be reprogrammed for gene reactivation.

Temporal and spatial regulation of translation in the mammalian oocyte via the mTOR-eIF4F pathway

The fully grown mammalian oocyte is transcriptionally quiescent and utilizes only transcripts synthesized and stored during early development. However, we find that an abundant RNA population is retained in the oocyte nucleus and contains specific mRNAs important for meiotic progression. Here we show that during the first meiotic division, shortly after nuclear envelope breakdown, translational hotspots develop in the chromosomal area and in a region that was previously surrounded the nucleus. These distinct translational hotspots are separated by endoplasmic reticulum and Lamin, and disappear following polar body extrusion. Chromosomal translational hotspots are controlled by the activity of the mTOR–eIF4F pathway. Here we reveal a mechanism that—following the resumption of meiosis—controls the temporal and spatial translation of a specific set of transcripts required for normal spindle assembly, chromosome alignment and segregation.

Meiotic maturation of oocytes and early development of mammalian embryos is largely dependent on the translation of mRNAs stored in the oocyte. Here the authors uncover a population of mRNA retained in the oocyte nucleus whose translation is spatially and temporally regulated by the mTOR–eIF4F pathway during meiosis.

Maternal histone variants and their chaperones promote paternal genome activation and boost somatic cell reprogramming

The mammalian egg employs a wide spectrum of epigenome modification machinery to reprogram the sperm nucleus shortly after fertilization. This event is required for transcriptional activation of the paternal/zygotic genome and progression through cleavage divisions. Reprogramming of paternal nuclei requires replacement of sperm protamines with canonical and non-canonical histones, covalent modification of histone tails, and chemical modification of DNA (notably oxidative demethylation of methylated cytosines). In this essay we highlight the role maternal histone variants play during developmental reprogramming following fertilization. We discuss how reduced maternal histone variant incorporation in somatic nuclear transfer experiments may explain the reduced viability of resulting embryos and how knowledge of repressive and activating maternal factors may be used to improve somatic cell reprogramming.

Connecting the cell cycle with cellular identity

The remarkable ability of oocytes to reinstate the totipotent state from a unipotent somatic cell, allowing the cloning of animals and the generation of human stem cells, has fascinated scientists for decades. Due to the complexity of oocytes, it has remained challenging to understand the rapid reprogramming following nuclear transfer at a molecular level. Conversely, the detailed characterization of molecular mechanisms is also often insufficient to comprehend the functional relevance of a complex molecular process, such as the dissociation of transcription factors from chromatin during cell division, the role of chromatin modifications in cellular memory, or of cell type-specific DNA replication. This review attempts to bridge the gap between nuclear transfer and molecular biology by focusing on the role of the cell cycle in reprogramming.

Identification and characterization of an oocyte factor required for porcine nuclear reprogramming

Nuclear reprogramming of somatic cells can be induced by oocyte factors. Despite numerous attempts, the factors responsible for successful nuclear reprogramming remain elusive. In the present study, we found that porcine oocytes with the first polar body collected at 42 h of in vitro maturation had a stronger ability to support early development of cloned embryos than porcine oocytes with the first polar body collected at 33 h of in vitro maturation. To explore the key reprogramming factors responsible for the difference, we compared proteome signatures of the two groups of oocytes. 18 differentially expressed proteins between these two groups of oocytes were discovered by mass spectrometry (MS). Among these proteins, we especially focused on vimentin (VIM). A certain amount of VIM protein was stored in oocytes and accumulated during oocyte maturation, and maternal VIM was specifically incorporated into transferred somatic nuclei during nuclear reprogramming. When maternal VIM function was inhibited by anti-VIM antibody, the rate of cloned embryos developing to blastocysts was significantly lower than that of IgG antibody-injected embryos and non-injected embryos (12.24 versus 22.57 and 21.10%; p < 0.05), but the development of in vitro fertilization and parthenogenetic activation embryos was not affected. Furthermore, we found that DNA double strand breaks dramatically increased and that the p53 pathway was activated in cloned embryos when VIM function was inhibited. This study demonstrates that maternal VIM, as a genomic protector, is crucial for nuclear reprogramming in porcine cloned embryos.

Reprogramming of round spermatids by the germinal vesicle cytoplasm in mice

The birthrate following round spermatid injection (ROSI) remains low in current and evidence suggests that factors in the germinal vesicle (GV) cytoplasm and certain substances in the GV such as the nucleolus might be responsible for genomic reprogramming and embryonic development. However, little is known whether the reprogramming factors in GV oocyte cytoplasm and/or nucleolus in GV are beneficial to the reprogramming of round spermatids and development of ROSI embryos. Here, round spermatids were treated with GV cytolysates and injected this round spermatid alone or co-injected with GV oocyte nucleolus into mature metaphase II oocytes. Subsequent embryonic development was assessed morphologically and by Oct4 expression in blastocysts. There was no significant difference between experimental groups at the zygote to four-cell development stages. Blastocysts derived from oocytes which were injected with cytolysate treated-round spermatid alone or co-injected with nucleoli injection yielded 63.6% and 70.3% high quality embryos, respectively; comparable to blastocysts derived by intracytoplasmic sperm injection (ICSI), but higher than these oocytes which were co-injected with lysis buffer-treated round spermatids and nucleoli or injected with the lysis buffer-treated round spermatids alone. Furthermore, the proportion of live offspring resulting from oocytes which were co-injected with cytolysate treated-round spermatids and nucleoli or injected with cytolysate treated-round spermatids alone was higher than those were injected with lysis buffer treated-round spermaids, but comparable with the ICSI group. Our results demonstrate that factors from the GV cytoplasm improve round spermatid reprogramming, and while injection of the extra nucleolus does not obviously improve reprogramming its potential contribution, although which cannot be definitively excluded. Thus, some reprogramming factors are evidently present in GV oocyte cytoplasm and could significantly facilitate ROSI technology, while the nucleolus in GV seems also having a potential to improve reprogramming of round spermatids.

Nuclear Wave1 is required for reprogramming transcription in oocytes and for normal development

Eggs and oocytes have a remarkable ability to induce transcription of sperm after normal fertilization and in somatic nuclei after somatic cell nuclear transfer. This ability of eggs and oocytes is essential for normal development. Nuclear actin and actin-binding proteins have been shown to contribute to transcription, although their mode of action is elusive. Here, we find that Xenopus Wave1, previously characterized as a protein involved in actin cytoskeleton organization, is present in the oocyte nucleus and is required for efficient transcriptional reprogramming. Moreover, Wave1 knockdown in embryos results in abnormal development and defective hox gene activation. Nuclear Wave1 binds by its WHD domain to active transcription components, and this binding contributes to the action of RNA polymerase II. We identify Wave1 as a maternal reprogramming factor that also has a necessary role in gene activation in development.

Nuclear reprogramming

There is currently particular interest in the field of nuclear reprogramming, a process by which the identity of specialised cells may be changed, typically to an embryonic-like state. Reprogramming procedures provide insight into many mechanisms of fundamental cell biology and have several promising applications, most notably in healthcare through the development of human disease models and patient-specific tissue-replacement therapies. Here, we introduce the field of nuclear reprogramming and briefly discuss six of the procedures by which reprogramming may be experimentally performed: nuclear transfer to eggs or oocytes, cell fusion, extract treatment, direct reprogramming to pluripotency and transdifferentiation.

Pre-stem cell formation by non-platelet RNA-containing particle fusion

We found a group of non-platelet RNA-containing particles (NPRCP) in human umbilical cord blood. To understand the origin, characterization and differentiation of NPRCP, we examined cord blood-isolated NPRCP in vitro. The NPRCP range in size from < 1 to 5 μm, have a thin bilayer membrane and various morphological features, contain short RNA and microRNA and express octamer-binding transcription factor 4 (OCT4), sex-determining region Y 2 (SOX2) and DEAD box polypeptide 4 (DDX4). On coculture with nucleated cells from umbilical cord blood, NPRCP fuse to small, active, non-nucleated cells called 'particle fusion-derived non-nucleated cells' (PFDNC). The PFDNC are approximately 8 μm in diameter and are characterized by their twisting movement in culture plates. They can easily move into and out of nucleated cells and finally differentiate into mesenchymal-like cells. In addition, the larger non-nucleated cellular structures that are derived from the aggregation and fusion of multiple NPRCP can further differentiate into large stem cells that also release OCT4- and SOX2-positive non-nucleated small cells. Our data provide strong evidence that NPRCP can fuse into PFDNC, which further differentiate into mesenchymal-like cells. Multiple NPRCP also fuse into other types of large stem cells. We believe that stem cells are derived from NPRCP fusion. There is considerable potential for the use of NPRCP in clinical therapy.

Xenopus egg extract treatment reduced global DNA methylation of donor cells and enhanced somatic cell nuclear transfer embryo development in pigs

The efficiency to produce offspring by somatic cell nuclear transfer (SCNT) is low. It has been showed that treatment of donor cells with Xenopus oocyte extract increased live births in ovine and handmade cloned embryo development in pigs. Scriptaid treatment after oocyte activation is another approach to improve SCNT efficiency. The present study was carried out to investigate (a) the effects of treatment of donor cells with Xenopus egg extract on donor cell DNA methylation at days 0 and 4 with two digitonin permeabilization concentrations (10 and 15 μg/mL), (b) the effects of treatment of donor cells with Xenopus egg extract on early development of cloned embryos, and (c) the effects of combined treatments, treating donor cells with extract before nuclear transfer and treatment of cloned embryos with scriptaid after oocyte activation, on embryo development. Compared to the control, a decrease of DNA methylation in donor cells was observed at 2.5 h after extract treatment. However, this effect was not observed after the cells were cultured for four more days. More embryos developed into blastocysts in the Xenopus egg extract-treated group than in the control (13.4±1.9% vs. 9.1±1.9%, p=0.01). Furthermore, scriptaid treatment of cloned embryos further increased the frequency of development to blastocyst, compared to the control reconstructed with the same extract-treated cells (22.5±0.9% vs. 15.3±0.9%, p<0.01). In addition, egg extract treatments increased the cell number in the blastocysts. This study demonstrated that Xenopus egg extract treatment reduced donor cell DNA methylation and enhanced the SCNT embryo development. Moreover, the combined treatments of donor cells with egg extract before nuclear transfer and of cloned embryos with scriptaid could improve cloned embryo development additively.

Follicular oocytes better support development in rabbit cloning than oviductal oocytes

This study was conducted to determine the effect of rabbit oocytes collected from ovaries or oviducts on the developmental potential of nuclear transplant embryos. Donor nuclei were obtained from adult skin fibroblasts, cumulus cells, and embryonic blastomeres. Rabbit oocytes were flushed from the oviducts (oviductal oocytes) or aspirated from the ovaries (follicular oocytes) of superovulated does at 10, 11, or 12 h post-hCG injection. The majority of collected oocytes were still attached to the sites of ovulation on the ovaries. We found that follicular oocytes had a significantly higher rate of fusion with nuclear donor cells than oviductal oocytes. There was no difference in the cleavage rate between follicular and oviductal groups, but morula and blastocyst development was significantly higher in the follicular group than in the oviductal group. Two live clones were produced in follicular group using blastomere and cumulus nuclear donors, whereas one live clone was produced in the oviductal group using a cumulus nuclear donor. These results demonstrate that cloned rabbit embryos derived from follicular oocytes have better developmental competence than those derived from oviductal oocytes.

The competence of germinal vesicle oocytes is unrelated to nuclear chromatin configuration and strictly depends on cytoplasmic quantity and quality in the cat model

BACKGROUND

Chromatin configuration of the germinal vesicle (GV) and quality of the cytoplasm are critical factors in achieving oocyte meiotic and developmental capacity during folliculogenesis. Besides gaining new insights into the timing and cellular mechanisms associated with the acquisition and regulation of GV oocyte competence, the domestic cat model was used to examine (i) the relation between GV chromatin configuration and oocyte functionality during folliculogenesis and (ii) the role of the cytoplasmic environment on the GV competence and stability.

METHODS

Structural and functional properties of GV oocytes were characterized after isolation from different follicle stages of non-stimulated cat ovaries. GV transfers, artificial chromatin compaction and oocyte vitrification were used to demonstrate the respective roles of GV and cytoplasm on the oocyte functionality.

RESULTS

GVs acquired the intrinsic capability to resume meiosis during the pre-antral follicle stage, whereas the capacity to support embryo development occurred while the antrum started to form. Chromatin configuration of the GV did not undergo extensive modification during the acquisition of competence or during the arrest of transcriptional activity at the large antral follicle stage. However, the quality and quantity of the cytoplasm regulated and enhanced GV functionality. This finding also held for GVs transferred from incompetent or subpar oocytes into the cytoplasm of good quality oocytes or when chromatin was artificially modified or vitrified.

CONCLUSIONS

The cat model provides a new insight into GV oocyte structure and function during folliculogenesis while challenging current concepts about oocyte quality criteria based on the GV morphology. This suggests alternative evaluative approaches for oocytes from other species too, including humans. Cat GVs also appear competent at an early follicle stage and are resilient to perturbations which designate this organelle as an attractive target for developing novel fertility preservation tactics.

Nuclear transfer to eggs and oocytes

We review experiments in which somatic cell nuclei are transplanted singly to enucleated eggs (metaphase II) in amphibia and mammals and as multiple nuclei to the germinal vesicle of amphibian oocytes (prophase I). These experiments have shown the totipotency of some somatic cell nuclei, as well as switches in cell type and changes in gene expression. Abnormalities of nuclear transplant embryo development increase greatly as nuclei are taken from progressively more differentiated donor cells. The molecular changes that accompany the reprogramming of transplanted nuclei help to indicate the mechanisms used by eggs and oocytes to reprogram gene expression. We discuss the importance of chromosomal protein exchange, of transcription factor supply, and of chromatin access in reprogramming.

Reprogramming factors involved in hybrids and cybrids of human embryonic stem cells fused with hepatocytes

Embryonic stem cells (ESCs) have the potential to reprogram somatic cells into ESC-like cells through cell fusion. In the present study, the potential of human (h)ESC cytoplasts and karyoplasts to reprogram human hepatocytes was evaluated. Green fluorescent protein (GFP) transfected hESCs (ENVY cells) were fused with SNARF-1 (CellTracker)-labeled human hepatocytes using polyethylene glycol (PEG) and fluorescence-activated cell sorting (FACS) to produce hESC-hepatocyte hybrids. Immunocytochemical analysis of ESC markers showed that the hybrids expressed OCT4, TRA-1-60, TRA-1-81, SSEA-4, and GCTM-2. However, SSEA-1, which is typically low or absent on hESCs, was detected on hESC–hepatocyte hybrids. Moreover, reverse transcriptase polymerase chain reaction (RT-PCR) showed that alpha-fetoprotein, which is highly expressed in hepatocytes, was erased in the hybrids. These results indicated that hESCs have the potential to reprogram hepatocyte phenotype to a relatively undifferentiated state, but such hybrid cells are not identical to hESCs. Although hESC–hepatocyte hybrids were aneuploid, they were able to differentiate into embryoid bodies and some types of somatic cells. Furthermore, cybrids of enucleated hESCs and hepatocytes were produced by cell fusion, but the cybrids were unable to self-renew in the same way as hESCs. Presumably, the reprogramming factors are associated with the karyoplast and not the cytoplast of hESCs.

Germline competency of parthenogenetic embryonic stem cells from immature oocytes of adult mouse ovary

Parthenogenetic embryonic stem cells (pESCs) have been generated in several mammalian species from parthenogenetic embryos that would otherwise die around mid-gestation. However, previous reports suggest that pESCs derived from in vivo ovulated (IVO) mature oocytes show limited pluripotency, as evidenced by low chimera production, high tissue preference and especially deficiency in germline competence, a critical test for genetic integrity and pluripotency of ESCs. Here, we report efficient generation of germline-competent pESC lines (named as IVM pESCs) from parthenogenetic embryos developed from immature oocytes of adult mouse ovaries following in vitro maturation (IVM) and artificial activation. In contrast, pESCs derived from IVO oocytes show defective germline competence, consistent with previous reports. Further, IVM pESCs resemble more ESCs from fertilized embryos (fESCs) than do IVO pESCs on genome-wide DNA methylation and global protein profiles. In addition, IVM pESCs express higher levels of Blimp1, Lin28 and Stella, relative to fESCs, and in their embryoid bodies following differentiation. This may indicate differences in differentiation potentially to the germline. The mechanisms for acquisition of pluripotency and germline competency of IVM pESCs from immature oocytes remain to be determined.

Efficiencies and mechanisms of nuclear reprogramming

The differentiated state of somatic cells is highly stable, but it can be experimentally reversed. The resulting cells can then be redirected into many different pathways. Nuclear reprogramming has been achieved by nuclear transfer to eggs, cell fusion, and overexpression of transcription factors. The mechanisms of nuclear reprogramming are not understood, but some insight into them is provided by comparing the efficiencies of different reprogramming strategies. Here, we compare these efficiencies by describing the frequency and rapidity with which reprogramming is induced and by the proportion of cells and level of expression in which reprogramming is achieved. We comment on the mechanisms that lead to successful somatic-cell reprogramming and on those that resist in helping to maintain the differentiated state of somatic cells.

Proteome of mouse oocytes at different developmental stages

The mammalian oocyte possesses powerful reprogramming factors, which can reprogram terminally differentiated germ cells (sperm) or somatic cells within a few cell cycles. Although it has been suggested that use of oocyte-derived transcripts may enhance the generation of induced pluripotent stem cells, the reprogramming factors in oocytes are undetermined, and even the identified proteins composition of oocytes is very limited. In the present study, 7,000 mouse oocytes at different developmental stages, including the germinal vesicle stage, the metaphase II (MII) stage, and the fertilized oocytes (zygotes), were collected. We successfully identified 2,781 proteins present in germinal vesicle oocytes, 2,973 proteins in MII oocytes, and 2,082 proteins in zygotes through semiquantitative MS analysis. Furthermore, the results of the bioinformatics analysis indicated that different protein compositions are correlated with oocyte characteristics at different developmental stages. For example, specific transcription factors and chromatin remodeling factors are more abundant in MII oocytes, which may be crucial for the epigenetic reprogramming of sperm or somatic nuclei. These results provided important knowledge to better understand the molecular mechanisms in early development and may improve the generation of induced pluripotent stem cells.

Recipient cell nuclear factors are required for reprogramming by nuclear transfer

Nuclear transfer allows the reprogramming of somatic cells to totipotency. The cell cycle state of the donor and recipient cells, as well as their extent of differentiation, have each been cited as important determinants of reprogramming success. Here, we have used donor and recipient cells at various cell cycle and developmental stages to investigate the importance of these parameters. We found that many stages of the cell cycle were compatible with reprogramming as long as a sufficient supply of essential nuclear factors, such as Brg1, were retained in the recipient cell following enucleation. Consistent with this conclusion, the increased efficiency of reprogramming when using donor nuclei from embryonic cells could be explained, at least in part, by reintroduction of embryonic nuclear factors along with the donor nucleus. By contrast, cell cycle synchrony between the donor nucleus and the recipient cell was not required at the time of transfer, as long as synchrony was reached by the first mitosis. Our findings demonstrate the remarkable flexibility of the reprogramming process and support the importance of nuclear transcriptional regulators in mediating reprogramming.

Reprogramming after chromosome transfer into mouse blastomeres

It is well known that oocytes can reprogram differentiated cells, allowing animal cloning by nuclear transfer. We have recently shown that fertilized zygotes retain reprogramming activities, suggesting that such activities might also persist in cleavage-stage embryos. Here, we used chromosome transplantation techniques to investigate whether the blastomeres of two-cell-stage mouse embryos can reprogram more differentiated cells. When chromosomes from one of the two blastomeres were replaced with the chromosomes of an embryonic or CD4(+) T lymphocyte donor cell, we observed nuclear reprogramming and efficient contribution of the manipulated cell to the developing blastocyst. Embryos produced by this method could be used to derive stem cell lines and also developed to term, generating mosaic "cloned" animals. These results demonstrate that blastomeres retain reprogramming activities and support the notion that discarded human preimplantation embryos may be useful recipients for the production of genetically tailored human embryonic stem cell lines.

Haploid male germ cell- and oocyte-specific Mbd3l1 and Mbd3l2 genes are dispensable for early development, fertility, and zygotic DNA demethylation in the mouse

Genome-wide erasure of CpG methylation occurs along the paternal pronucleus in fertilized oocytes. This process involves an active, replication-independent enzymatic step, which has remained enigmatic. MBD3L1 and MBD3L2 are two mammalian homologues of the methyl-CpG-binding protein genes MBD2 and MBD3 that arose from recent gene duplication events. Expression of Mbd3l1 occurs specifically in haploid male germ cells. Mbd3l2 expression is restricted to metaphase II oocytes and zygotes making both proteins candidates for the zygotic demethylation process. Neither of these genes was able to promote reactivation of a methylation-silenced reporter gene. We created Mbd3l1 and Mbd3l2 knockout mice, which were viable and fertile. We show that demethylation of the paternal pronucleus in Mbd3l1-/- and Mbd3l2-/- mice is identical to that in wild-type controls. These data suggest that Mbd3l1 and Mbd3l2 are not involved in genome-wide demethylation of paternal genomes in mouse zygotes and are dispensable for normal development.

Cell-free extracts from mammalian oocytes partially induce nuclear reprogramming in somatic cells

Nuclear transfer has been regarded as the only reliable tool for studying nuclear reprogramming of mammalian somatic cells by oocytes. However, nuclear transfer is not well suited for biochemical analyses of the molecular mechanisms of reprogramming. A cell-free system from oocytes is an attractive alternative way to mimic reprogramming in vitro, since a large number of cells can be treated and analyzed. Nevertheless, a cell-free system using oocytes has not been developed in mammals. Here, cell extracts from porcine oocytes were prepared and their ability to induce nuclear reprogramming was evaluated. Extracts from metaphase II (MII) oocytes erased the machinery for regulating gene expression in reversibly permeabilized somatic cells. For example, the extracts caused histone deacetylation and the disappearance of TATA box-binding protein from the nuclei. However, MII-extract-treated cells did not show any obvious changes after cell culture. In contrast, extracts from germinal vesicle (GV) oocytes activated pluripotent marker genes, especially NANOG, and induced partial dedifferentiation after cell culture. The activation of pluripotent marker genes by GV extracts was associated with histone acetylation that was induced during extract treatment. These results indicate that GV- and MII-oocyte extracts have different roles on nuclear reprogramming. Furthermore, both oocyte extracts induced site-specific demethylation in the upstream region of NANOG. These results indicate that cell-free extracts derived from GV- and MII-oocytes could be useful for studying the mechanisms involved in nuclear reprogramming.

The cytoplasm of mouse germinal vesicle stage oocytes can enhance somatic cell nuclear reprogramming

In mammalian cloning, evidence suggests that genomic reprogramming factors are located in the nucleus rather than the cytoplasm of oocytes or zygotes. However, little is known about the mechanisms of reprogramming, and new methods using nuclear factors have not succeeded in producing cloned mice from differentiated somatic cell nuclei. We aimed to determine whether there are functional reprogramming factors present in the cytoplasm of germinal vesicle stage (GV) oocytes. We found that the GV oocyte cytoplasm could remodel somatic cell nuclei, completely demethylate histone H3 at lysine 9 and partially deacetylate histone H3 at lysines 9 and 14. Moreover, cytoplasmic lysates of GV oocytes promoted somatic cell reprogramming and cloned embryo development, when assessed by measuring histone H3-K9 hypomethylation, Oct4 and Cdx2 expression in blastocysts, and the production of cloned offspring. Thus, genomic reprogramming factors are present in the cytoplasm of the GV oocyte and could facilitate cloning technology. This finding is also useful for research on the mechanisms involved in histone deacetylation and demethylation, even though histone methylation is thought to be epigenetically stable.

Mediators of reprogramming: transcription factors and transitions through mitosis

It is thought that most cell types of the human body share the same genetic information as that contained in the zygote from which they originate. Consistent with this view, animal cloning studies demonstrated that the intact genome of a differentiated cell can be reprogrammed to support the development of an entire organism and allow the production of pluripotent stem cells. Recent progress in reprogramming research now points to an important role for transcription factors in the establishment and the maintenance of cellular phenotypes, and to cell division as a mediator of transitions between different states of gene expression.

The maternal nucleolus is essential for early embryonic development in mammals

With fertilization, the paternal and maternal contributions to the zygote are not equal. The oocyte and spermatozoon are equipped with complementary arsenals of cellular structures and molecules necessary for the creation of a developmentally competent embryo. We show that the nucleolus is exclusively of maternal origin. The maternal nucleolus is not necessary for oocyte maturation; however, it is necessary for the formation of pronuclear nucleoli after fertilization or parthenogenetic activation and is essential for further embryonic development. In addition, the nucleolus in the embryo produced by somatic cell nuclear transfer originates from the oocyte, demonstrating that the maternal nucleolus supports successful embryonic development.

Evaluation in mammalian oocytes of gene transcripts linked to epigenetic reprogramming

The mature mammalian metaphase II (MII) oocyte has a unique ability to reprogram sperm chromatin and support early embryonic development. This feature even extends to the epigenetic reprogramming of a terminally differentiated cell nucleus as observed in connection with somatic cell nuclear transfer. Epigenetic nuclear reprogramming is highly linked to chromatin structure and includes covalent modifications of DNA and core histone proteins as well as reorganization of higher-order chromatin structure. A group of conserved enzymes mediating DNA methylation, methyl-CpG-binding protein (MeCP), histone acetylation and methylation, and chromatin remodeling are extensively involved in epigenetic reprogramming in mammalian cells. Using the oligonucleotide microarray technique, the present study compared the expression levels of 86 genes associated with epigenetic reprogramming in murinein vivomatured MII oocytes with that of germinal vesicle oocytes. Correlation between biological replicates was high. A total of 57 genes with potential reprogramming effect were detected. In MII oocytes, four genes were significant up-regulated, whereas 18 were down-regulated and 35 unchanged. The significantly regulated genes were validated by real-time quantitative RT-PCR. For example, MII oocytes showed a significant down-regulation of oocyte-specific maintenance DNA methyltransferase, Dnmt1o, and up-regulation of MeCP transcript, methyl-CpG binding domain protein 2. Furthermore, histone acetyltransferases were proportionally overrepresented when compared with histone deacetylases. These data elucidate for the first time some of the mechanisms that the oocyte may employ to reprogram a foreign genome either in form of a spermatozoa or a somatic nucleus and may thus be of importance for advancing the fields of stem cell research and regenerative medicine.

Developmental reprogramming after chromosome transfer into mitotic mouse zygotes

Until now, animal cloning and the production of embryonic stem cell lines by somatic cell nuclear transfer have relied on introducing nuclei into meiotic oocytes. In contrast, attempts at somatic cell nuclear transfer into fertilized interphase zygotes have failed. As a result, it has generally been assumed that unfertilized human oocytes will be required for the generation of tailored human embryonic stem cell lines from patients by somatic cell nuclear transfer. Here we report, however, that, unlike interphase zygotes, mouse zygotes temporarily arrested in mitosis can support somatic cell reprogramming, the production of embryonic stem cell lines and the full-term development of cloned animals. Thus, human zygotes and perhaps human embryonic blastomeres may be useful supplements to human oocytes for the creation of patient-derived human embryonic stem cells.

Mouse zygotes as recipients in embryo cloning

Zygotes have not been recognized as nuclear recipients since enucleated zygotes receiving nuclei from beyond two-cell stage embryos are not able to form blastocysts. In the present study, a new technique of zygote enucleation is presented, which consists in selectively removing the nuclear membrane with genetic material of pronuclei, but leaving other pronuclear components in the cytoplasm. With selective enucleation it is possible - after transfer of eight-cell stage nuclei - to obtain 70.5 and 7.8% of preimplantation and full-term development respectively. Origin of cloned mice from introduced nuclei was confirmed by the coat colour and glucose phosphate isomerase (GPI) isozyme of the donor. We suggest that some pronuclear factors - taken away from the zygotes in the karyoplasts upon classical enucleation - are needed to reprogram the introduced nuclei.

Dynamic changes in microtubules and early development of reconstructed embryos after somatic cell nuclear transfer in a non-human primate

In order to improve somatic cell nuclear transfer (SCNT) efficiency and to understand cellular changes in SCNT, the dynamic changes in microtubules/DNA and early development of SCNT embryos with single or multiple pronuclei were investigated, along with activation timing on efficiency of SCNT, were studied in the Cynomolgus monkey. The confocal images showed that microtubules assembled around condensed DNA at 1h after cell injection; normal or abnormal reconstructed spindle formed at 2 h after cell injection; and reconstructed spindle separated at 2 h after activation. The results of nuclear formation showed that 61.3% of the reconstructed embryos did not form pronuclei; 19.3% formed a single nucleus, and 11.9% and 7.5% formed two and more than two reconstructed pronuclei, respectively. The cleavage and 8-cell development rates of SCNT embryos with pronuclei were significantly higher than those without pronuclei, but there was no difference in development rates among NT embryos with single, two and more then two pronuclei. Activation at 2 h after cell injection yielded more embryos with pronuclei and yielded 8-cell NT embryos more reliably than did activation at 3-4 h. In conclusion, microtubules assembled around condensed DNA at 1-2 h after cell injection, and formed a spindle at 2 h after SCNT, which separated at 2 h after activation; early development was affected by activation time, but no different between single and multiple pronuclei.

Cytoskeletal and nuclear organization in mouse embryos derived from nuclear transfer and ICSI: A comparison of agamogony and syngamy before and during the first cell cycle

In this study, somatic cell nuclear transfer (SCNT) and intracytoplasmic sperm injection (ICSI) are used as models of agamogony and syngamy, respectively. In order to elucidate the reasons of low efficiency of somatic cell cloning, cytoskeletal and nuclear organization in cloned mouse embryos was monitored before and during the first cell cycle, and compared with the pattern of ICSI zygote. A metaphase-like spindle with alignment of condensed donor chromosomes was assembled within 3 hr after NT, followed by formation of pronuclear-like structures at 3-6 hr after activation, indicating that somatic nuclear remodeling depends on microtubular network organization. The percentage of two (pseudo-) pronuclei in cloned embryos derived from delayed activation was greater than that in immediate activation group (68.5% vs. 30.8%, P<0.01), but similar to that of ICSI group (68.5% vs. 65.5%, P>0.05). The 2-cell rate in NT embryos was significantly lower than that in zygotes produced by ICSI (64.8% vs. 82.5%, P<0.01). Further studies testified that the cloned embryos reached the metaphase of the first mitosis 10 hr after activation, whereas this occurred at 18 hr in the ICSI zygotes. Comparision of the pattern of microfilament assembly in early NT embryos with that in syngamic zygotes suggested that abnormal microfilamental pattern in cloned embryos may threaten subsequent embryonic development. In conclusion, agamogony, in contrast to syngamy, displays some unique features in respect of cytoskeletal organization, the most remarkable of which is that the first cell cycle is initiated ahead distinctly, which probably leads to incomplete organization of the first mitotic spindle, and contributes to low efficiency of cloning.

Artificial gametes

In vitro fertilization (IVF) has been an efficient medical treatment for infertility in the past decades. However, conventional IVF approaches may be insufficient when gametes are lacking or non-viable thus precluding a significant number of patients from treatment. Ultimately, creation of artificial gametes may provide an universal solution for all indications. Somatic cell nuclear transfer (SCNT) has provided successful cloning in different animal species indicating that a derived technology may be applicable in infertility treatment procedures. Attempts to produce functional male or female gamete through nuclear transfer have been described through the process called haploidization. Initial successes have been observed, however, significant alterations at spindle construction and chromosomal segregation were also described. Stem cell technology may provide an alternative route to obtain fully functional gametes. Both sperm cells and oocytes were obtained using specific culture conditions for embryo originated stem cell. These two mainstream approaches are presented in the current review. Both of these techniques are involving sophisticated methods and consequently both of them demonstrate technical and ethical challenges. Related questions on (mitotic/meiotic) cell division, genetic/epigenetic alterations and cell renewal are needed to be addressed before clinical application.

Somatic Cell Nuclear Transfer (SCNT) in Mammals

It is now more than nine years since Dolly, the world's first somatic cell cloned mammal was born, and the success of somatic cell nuclear transfer (SCNT) is still disappointingly low. Only about 3-5% of reconstructed embryos develop to term, and it is also evident that even if some clones are born, they are not necessarily fully developed and healthy. Embryonic and neonatal abnormalities of cloned offspring are probably a result of incorrect or incomplete reprogramming of the transferred donor cell nuclei. Such an incomplete reprogramming reflects the extremely low efficiency of SCNT. The key role in the process of reprogramming has been attributed to the enucleated oocyte-cytoplast into which the somatic cell nucleus is transferred. In our chapter, we will discuss the methodological approaches used for the preparation of cytoplasts and their possible reprogramming activities.

Reprogramming somatic gene activity by fusion with pluripotent cells

Fertilized eggs and early blastomeres, that have the potential to develop to fetuses when placed into a uterus, are totipotent. Those cells in the embryo, that can give rise to all cell types of an organism, but not to an organism itself, are pluripotent. Embryonic stem (ES), embryonic carcinoma (EC), and embryonic germ (EG) cells are powerful in vitro artifacts derived from different embryonic stages and are pluripotent. Totipotent and pluripotent cells have the potential to greatly benefit biological research and medicine. One powerful feature is that the genetic program of somatic cells can be converted into that of totipotent or pluripotent cells, as shown by nuclear transfer or cell fusion experiments. During reprogramming by cell fusion various features of pluripotent cells are acquired. These include the typical morphology of the respective pluripotent fusion partner, a specific epigenetic state, a specific gene profile, inactivation of tissue-specific genes expressed in the somatic fusion partner, and the developmental as well as differentiation potential of pluripotent cells. In this review, we will discuss what is known about the reprogramming process mediated by cell fusion and the potential use of fusion-induced reprogramming for therapeutic applications.

Cell-cell fusion as a means to establish pluripotency

Embryonic stem cells (ESCs), embryonic germ cells (EGCs), and embryonic carcinoma cells (ECCs) are three types of pluripotent cells derived from mammalian embryos. The three cell types are capable not only of self-renewal, but also of having the potential to give rise to cells of all tissue types in the fetal and adult body. In several reports, ESCs, ECCs, and EGCs have been described to reprogram somatic cells in vitro. After reprogramming caused by fusion, somatic cells exhibit various features of pluripotent cells: expression of pluripotency markers (e.g., Oct4, nanog, and Rex-1), absence of tissue-specific gene expression, reactivation of inactive X chromosome of female somatic cells, demethylation, as well as histone modification. An activity in pluripotent stem cells appears to be capable of inducing the global changes inherent in the reprogramming of somatic cells. Investigations involving pluripotent stem cells will yield substantial insight into various fundamental biological processes, such as cellular differentiation and de-differentiation. Most importantly for the public, however, is that such studies might lead into cell-based therapies and as such have the potential to change regenerative medicine.

Oocyte nucleus controls progression through meiotic maturation

We analyzed progression through the meiotic maturation in oocytes manipulated to replace the prophase oocyte nucleus with the nucleus from a cumulus cell, a pachytene spermatocyte or the pronucleus from a fertilized egg. Removal of the oocyte nucleus led to a significant reduction in histone H1 kinase activity. Replacement of the oocyte nucleus by a pronucleus followed by culture resulted in premature pseudomeiotic division and occasional abnormal cytokinesis; however, histone H1 kinase activity was rescued, microtubules formed a bipolar spindle, and chromosomes were condensed. In addition to the anomalies observed after pronuclear transfer, those after transfer of the nucleus from a cumulus cell or spermatocyte included a dramatically impaired ability to form the bipolar spindle or to condense chromosomes, and histone H1 kinase activity was not rescued. Expression of a cyclin B-YFP in enucleated oocytes receiving the cumulus cell nucleus rescued histone H1 kinase activity, but spindle formation and chromosome condensation remained impaired, indicating a pleiotropic effect of oocyte nucleus removal. However, when the cumulus cell nucleus was first transformed into pronuclei (transfer into a metaphase II oocyte followed by activation), such pronuclei supported maturation after transfer into the oocyte in a manner similar to that of normal pronuclei. These results show that the oocyte nucleus contains specific components required for the control of progression through the meiotic maturation and that some of these components are also present in pronuclei.

Small GTPase RhoA is required for ooplasmic segregation and spindle rotation, but not for spindle organization and chromosome separation during mouse oocyte maturation, fertilization, and early cleavage

RhoA, a small GTPase, plays versatile roles in many aspects of cell function such as stress fiber formation, cytokinesis, and cell polarization. In this study, we investigated the subcellular localization of RhoA and its possible roles during oocyte maturation and fertilization. RhoA was localized in the cytoplasm of eggs from the germinal vesicle (GV) stage to 2-cell stage, especially concentrating in the midbody of telophase spindle when oocyte extruded PB1 and PB2. The RhoA kinases (ROCKs) specific inhibitor Y-27632 blocked GV breakdown (GVBD) and first polar body extrusion, but did not affect apparatus formation and anaphase/telophase I entry. Anti-RhoA antibody microinjection into the oocytes showed similar results. RhoA inhibitor caused abnormal organization of microfilaments, failure of spindle rotation, PB2 extrusion as well as cleavage furrow formation, while sister chromatid separation was not affected. Microinjection of RhoA antibody also blocked PB2 emission. Our findings indicate that RhoA, by regulating microfilament organization, regulates several important events including GVBD, polar body emission, spindle rotation, and cleavage.

Nucleocytoplasmic ratio of fully grown germinal vesicle oocytes is essential for mouse meiotic chromosome segregation and alignment, spindle shape and early embryonic development

BACKGROUND

This study examined the effect of nucleocytoplasmic ratio of fully grown germinal vesicle (GV) oocytes on meiotic chromosome segregation and alignment, spindle shape, Ca(2+) oscillations and capacity of early embryonic development in mouse.

METHODS

GV oocytes with reduced volume (equal to 1/5 to 4/5 of an intact oocyte) were made by micromanipulation to remove different amounts of cytoplasm, and then matured and fertilized in vitro.

RESULTS

When >1/2 of GV oocyte cytoplasm was removed, the time-course of GV breakdown (GVBD) was delayed and oocyte maturation rate decreased significantly. Abnormal chromosome segregation rate increased if >1/2 of the cytoplasm was removed from the oocyte. Length and structure of meiotic spindle and chromosome alignment were also impaired by the reduction of cytoplasmic volume. Once matured in vitro, the oocytes could undergo Sr(2+)-induced Ca(2+) oscillations and form pronuclei in a manner independent of nucleocytoplasmic ratio, but their ability to develop to 2-cell embryos was affected if >1/2 of their cytoplasm was removed from the GV oocytes.

CONCLUSIONS

These results suggest that nucleocytoplasmic ratio is essential for normal meiotic chromosome segregation, spindle formation and chromosome alignment over the metaphase spindle, and development to 2-cell stage, for which 1/2 of the volume of the GV oocyte appears to be a threshold.

Reprogrammationet épigenèse

The fact that the nucleus of a differentiated somatic cell can be reprogrammed in order to sustain embryonic development is now well established. Experiments of somatic cell nuclear transfer (cloning) have proved that a foreign nucleus introduced into an enucleated oocyte can give rise to physiologically normal offsprings, with a normal lifespan. Such evidence of genome expression plasticity is also observed experimentally with heterokaryons, created by the fusion or the nuclear transfer between two somatic cells, where differentiated nuclei are able to express genes characteristic of the host cell. However, the epigenetic mechanisms that permit nuclear plasticity remain poorly understood. In this paper we present the main evidences showing important modifications of the large scale organisation of chromosomal domains and of the DNA methylation pattern upon nuclear transfer and during the first cleavages. These modifications of epigenetic marks, brought by an intimate contact between the chromatin and the recipient oocyte cytoplasmic factors, appear essential for further development. They are established over the first cell cycles of development. The onset of embryonic genome activation and the first cellular differentiation events that occur over the implantation period are two additional check-points of reprogramming that appear to be also highly dependent on epigenetic alterations. Beyond those stages, defective placental functions might be directly responsible for the fetal and postnatal physiopathologies frequently observed in cloned animals. No direct link between preimplantation reprogramming defaults, placental dysfunctions and low development to term has been established yet. The epigenetics studies which are now used to characterise loci specific and probably genotype dependent alterations in cloned animals of different species will provide invaluable help to define the role of epigenesis in the achievement of a developmental program.