Production of Cloned Mice and ES Cells from Adult Somatic Cells by Nuclear Transfer: How to Improve Cloning Efficiency?

Effects of reprogramming on genomic imprinting and the application of pluripotent stem cells

Pluripotent stem cells are considered to be the ideal candidates for cell-based therapies in humans. In this regard, both nuclear transfer embryonic stem (ntES) cells and induced pluripotent stem (iPS) cells are particularly advantageous because patient-specific autologous ntES and iPS cells can avoid immunorejection and other side effects that may be present in the allogenic pluripotent stem cells derived from unrelated sources. However, they have been found to contain deleterious genetic and epigenetic changes that may hinder their therapeutic applications. Indeed, deregulation of genomic imprinting has been frequently observed in reprogrammed ntES and iPS cells. We will survey the recent studies on genomic imprinting in pluripotent stem cells, particularly in iPS cells. In a previous study published about six years ago, genomic imprinting was found to be variably lost in mouse iPS clones. Intriguingly, de novo DNA methylation also occurred at the previously unmethylated imprinting control regions (ICRs) in a high percentage of iPS clones. These unexpected results were confirmed by a recent independent study with a similar approach. Since dysregulation of genomic imprinting can cause many human diseases including cancer and neurological disorders, these recent findings on genomic imprinting in reprogramming may have some implications for therapeutic applications of pluripotent stem cells.

MC1568 Enhances Histone Acetylation During Oocyte Meiosis and Improves Development of Somatic Cell Nuclear Transfer Embryos in Pig

An increasing number of studies have revealed that histone deacetylase (HDAC) mediated histone deacetylation is important for mammalian oocyte development. However, nonselective HDAC inhibitors (HDACi) were applied in most studies; the precise functions of specific HDAC classes during meiosis are poorly defined. In this study, the class IIa-specific HDACi MC1568 was used to reveal a crucial role of class IIa HDACs in the regulation of histone deacetylation during porcine oocyte meiosis. Besides, the functions of HDACs and histone acetyltransferases in regulating the balance of histone acetylation/deacetylation were also confirmed during oocyte maturation. After the validation of nontoxicity of MC1568 in maturation rate, spindle morphology, and chromosome alignment, effects of MC1568 on developmental competence of porcine somatic cell nuclear transfer (SCNT) embryos were evaluated, and data indicated that treatment with 10 μM MC1568 for 12 hours following electrical activation significantly enhanced the blastocyst rate and cell numbers. Moreover, results showed that optimal MC1568 treatment increased the H4K12 acetylation level in SCNT one cells and two cells. In addition, MC1568 treatment stimulated expression of the development-related genes OCT4, CDX2, SOX2, and NANOG in SCNT blastocysts. Collectively, our investigation uncovered a critical role of class IIa HDACs in the regulation of histone deacetylation during oocyte meiosis. Furthermore, for the first time, we showed that MC1568 can improve the in vitro development of porcine SCNT embryos. These findings provide an alternative HDACi for improving animal cloning efficiency and may shed more light on nuclear reprogramming.

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.

Fam60al as a novel factor involved in reprogramming of somatic cell nuclear transfer in zebrafish (Danio rerio)

The main reason for abnormal development of cloned animals or embryos, and inefficient animal cloning, is a poor understanding of the reprogramming mechanism. To better comprehend reprogramming and subsequent generation of pluripotent stem cells, we must investigate factors related to reprogramming of somatic cells as nuclear donors. As we know, fam60al (family with sequence similarity 60, member A, like) is a coding gene only found in zebrafish and frog (Xenopus laevis) among vertebrates. However, until now, its functions have remained unknown. Here, we generated a zebrafish fam60al^-/- mutant line using transcription activator-like effector nucleases (TALENs), and found that both nanog and klf4b expression significantly decreased while myca expression significantly increased in fam60al^-/- mutant embryos. Concurrently, we also uncovered that in developmentally arrested embryos of somatic cell nuclear transfer, nanog, klf4b and myca expression was down-regulated, accompanying a decrease of fam60al expression. Interestingly, we identified a long noncoding RNA (lncRNA) of fam60al, named fam60al-AS, which negatively regulated fam60al by forming double-stranded RNA (dsRNA). RNase protection assay and real-time PCR confirmed these findings. Taken together, these results suggest that fam60al is a novel factor related to the reprogramming of somatic cell nuclear transfer in zebrafish, which is regulated by its reverse lncRNA.

Beneficial effects of diazepin-quinazolin-amine derivative (BIX-01294) on preimplantation development and molecular characteristics of cloned mouse embryos

Somatic cell nuclear transfer is frequently associated with abnormal epigenetic modifications that may lead to the developmental failure of cloned embryos. BIX-01294 (a diazepine–quinazoline–amine derivative) is a specific inhibitor of the histone methyltransferase G9a. The aim of the present study was to investigate the effects of BIX-01294 on development, dimethylation of histone H3 at lysine 9 (H3K9), DNA methylation and the expression of imprinted genes in cloned mouse preimplantation embryos. There were no significant differences in blastocyst rates of cloned embryos treated with or without 0.1 μM BIX-01294. Relative to clone embryos treated without 0.1 μM BIX-01294, exposure of embryos to BIX-01294 decreased histone H3K9 dimethylation and DNA methylation in cloned embryos to levels that were similar to those of in vivo-fertilised embryos at the 2-cell and blastocyst stages. Cloned embryos had lower expression of octamer-binding transcription factor 4 (Oct4) and small nuclear ribonucleoprotein N (Snrpn), but higher expression of imprinted maternally expressed transcript (non-protein coding) (H19) and growth factor receptor-bound protein 10 (Grb10) compared with in vivo-fertilised counterparts. The addition of 0.1 μM BIX-01294 to the activation and culture medium resulted in lower H19 expression and higher cyclin dependent kinase inhibitor 1C (Cdkn1c) and delta-like 1 homolog (Dlk1) expression, but had no effect on the expression of Oct4, Snrpn and Grb10. The loss of methylation at the Grb10 cytosine–phosphorous–guanine (CpG) islands in cloned embryos was partially corrected by BIX-01294. These results indicate that BIX-01294 treatment of cloned embryos has beneficial effects in terms of correcting abnormal epigenetic modifications, but not on preimplantation development.

Techniques of Human Embryonic Stem Cell and Induced Pluripotent Stem Cell Derivation

Developing procedures for the derivation of human pluripotent stem cells (PSCs) gave rise to novel pathways into regenerative medicine research. For many years, stem cells have attracted attention as a potentially unlimited cell source for cellular therapy in neurodegenerative disorders, cardiovascular diseases, and spinal cord injuries, for example. In these studies, adult stem cells were insufficient; therefore, many attempts were made to obtain PSCs by other means. This review discusses key issues concerning the techniques of pluripotent cell acquisition. Technical and ethical issues hindered the medical use of somatic cell nuclear transfer and embryonic stem cells. Therefore, induced PSCs (iPSCs) emerged as a powerful technique with great potential for clinical applications, patient-specific disease modelling and pharmaceutical studies. The replacement of viral vectors or the administration of analogous proteins or chemical compounds during cell reprogramming are modifications designed to reduce tumorigenesis risk and to augment the procedure efficiency. Intensified analysis of new PSC lines revealed other barriers to overcome, such as epigenetic memory, disparity between human and mouse pluripotency, and variable response to differentiation of some iPSC lines. Thus, multidimensional verification must be conducted to fulfil strict clinical-grade requirements. Nevertheless, the first clinical trials in patients with spinal cord injury and macular dystrophy were recently carried out with differentiated iPSCs, encouraging alternative strategies for potential autologous cellular therapies.

Induction of autophagy improves embryo viability in cloned mouse embryos

Autophagy is an essential cellular mechanism that degrades cytoplasmic proteins and organelles to recycle their components. Moreover, autophagy is essential for preimplantation development in mammals. Here we show that autophagy is also important for reprogramming in somatic cell nuclear transfer (SCNT). Our data indicate that unlike fertilized oocytes, autophagy is not triggered in SCNT embryos during 6 hours of activation. Mechanistically, the inhibited autophagic induction during SCNT activation is due to the cytochalasin B (CB) caused depolymerization of actin filaments. In this study, we induced autophagy during SCNT activation by rapamycin and pp242, which could restore the expected level of autophagy and significantly enhance the development of SCNT embryos to the blastocyst stage when compared with the control (68.5% and 68.7% vs. 41.5%, P < 0.05). Furthermore, the treatment of rapamycin and pp242 accelerates active DNA demethylation indicated by the conversion of 5 mC to 5 hmC, and treatment of rapamycin improves degradation of maternal mRNA as well. Thus, our findings reveal that autophagy is important for development of SCNT embryos and inhibited autophagic induction during SCNT activation might be one of the serious causes of low efficiency of SCNT.

The Induction Effect of Am80 and TSA on ESC Differentiation via Regulation of Stra8 in Chicken

Stra8 encodes stimulated by retinoic acid gene 8, a protein that is important for initiation of meiosis in mammals and birds. This study was aimed at identifying the active control area of chicken STRA8 gene core promoter, to screen optimum inducers of the STRA8 gene, thus to enhance the differentiation of embryonic stem cells (ESCs) into spermatogonial stem cells. Fragments of chicken STRA8 gene promoter were cloned into fluorescent reporter plasmids and transfected into DF-1 cells. Then Dual-Luciferase® Reporter Assay System was used to identify the activity of the STRA8 gene under different inducers. Our studies showed that the promoter fragment -1055 bp to +54 bp of Suqin chicken Stra8 revealed the strongest activity. The dual-luciferase® reporter showed that Tamibarotene (Am80) and TrichostatinA (TSA) could significantly enhance STRA8 transcription. The in vitro inductive culture of chicken ESCs demonstrated that spermatogonial stem cells (SSC)-like cells appeared and Integrinβ1 protein was expressed on day 10, indicating that Am80 and TSA can promote ESCs differentiation into SSCs via regulation of Stra8.

Treatment of donor cell/embryo with different approaches to improve development after nuclear transfer

The successful production of cloned animals by somatic cell nuclear transfer (SCNT) is a promising technology with many potential applications in basic research, medicine, and agriculture. However, the low efficiency and the difficulty of cloning are major obstacles to the widespread use of this technology. Since the first mammal cloned from an adult donor cell was born, many attempts have been made to improve animal cloning techniques, and some approaches have successfully improved its efficiency. Nuclear transfer itself is still difficult because it requires an accomplished operator with a practiced technique. Thus, it is very important to find simple and reproducible methods for improving the success rate of SCNT. In this chapter, we will review our recent protocols, which seem to be the simplest and most reliable method to date to improve development of SCNT embryos.

Oxamflatin treatment enhances cloned porcine embryo development and nuclear reprogramming

Faulty epigenetic reprogramming of somatic nuclei is thought to be the main reason for low cloning efficiency by somatic cell nuclear transfer (SCNT). Histone deacetylase inhibitors (HDACi), such as Scriptaid, improve developmental competence of SCNT embryos in several species. Another HDACi, Oxamflatin, is about 100 times more potent than Scriptaid in the ability to inhibit nuclear-specific HDACs. The present study determined the effects of Oxamflatin treatment on embryo development, DNA methylation, and gene expression. Oxamflatin treatment enhanced blastocyst formation of SCNT embryos in vitro. Embryo transfer produced more pigs born and fewer mummies from the Oxamflatin-treated group compared to the Scriptaid-treated positive control. Oxamflatin also decreased DNA methylation of POU5F1 regulatory elements and centromeric repeat elements in day-7 blastocysts. When compared to in vitro-fertilized (IVF) embryos, the methylation status of POU5F1, NANOG, and centromeric repeat was similar in the cloned embryos, indicating these genes were successfully reprogrammed. However, compared to the lack of methylation of XIST in day-7 IVF embryos, a higher methylation level in day-7 cloned embryos was observed, implying that X chromosomes were activated in day-7 IVF blastocysts, but were not fully activated in cloned embryos, i.e., reprogramming of XIST was delayed. A time-course analysis of XIST DNA methylation on day-13, -15, -17, and -19 in vivo embryos revealed that XIST methylation initiated at about day 13 and was not completed by day 19. The methylation of the XIST gene in day-19 control cloned embryos was delayed again when compared to in vivo embryos. However, methylation of XIST in Oxamflatin-treated embryos was comparable with in vivo embryos, which further demonstrated that Oxamflatin could accelerate the delayed reprogramming of XIST gene and thus might improve cloning efficiency.

Progress and prospects for genetic modification of nonhuman primate models in biomedical research

The growing interest of modeling human diseases using genetically modified (transgenic) nonhuman primates (NHPs) is a direct result of NHPs (rhesus macaque, etc.) close relation to humans. NHPs share similar developmental paths with humans in their anatomy, physiology, genetics, and neural functions; and in their cognition, emotion, and social behavior. The NHP model within biomedical research has played an important role in the development of vaccines, assisted reproductive technologies, and new therapies for many diseases. Biomedical research has not been the primary role of NHPs. They have mainly been used for safety evaluation and pharmacokinetics studies, rather than determining therapeutic efficacy. The development of the first transgenic rhesus macaque (2001) revolutionized the role of NHP models in biomedicine. Development of the transgenic NHP model of Huntington's disease (2008), with distinctive clinical features, further suggested the uniqueness of the model system; and the potential role of the NHP model for human genetic disorders. Modeling human genetic diseases using NHPs will continue to thrive because of the latest advances in molecular, genetic, and embryo technologies. NHPs rising role in biomedical research, specifically pre-clinical studies, is foreseeable. The path toward the development of transgenic NHPs and the prospect of transgenic NHPs in their new role in future biomedicine needs to be reviewed. This article will focus on the advancement of transgenic NHPs in the past decade, including transgenic technologies and disease modeling. It will outline new technologies that may have significant impact in future NHP modeling and will conclude with a discussion of the future prospects of the transgenic NHP model.

Intracytoplasmic sperm injection (ICSI)-mediated transgenesis in mice

Over the years many well-described techniques for the introduction of transgene DNA into host organisms have been used, including pronuclear injection, in vitro fertilization-mediated transgenesis, transfection of ES and spermatogenic cells, nuclear transfer of somatic cell nuclei, and lentiviral vectors. The application of these techniques has been limited however either by the time and effort to be executed or by their narrow efficiency with large transgenes. The greatest advantage of intracytoplasmic sperm injection (ICSI)-mediated transgenesis is precisely its ability to stably introduce large DNA molecules into the genome of host organisms with relatively high efficiency, as compared to alternative procedures. In mice, this procedure has been shown to be a reproducible method to generate transgenic offspring with a high efficiency. Recently, it proved also to be a viable method to generate transgenic rats and pigs, and as such, it is foreseen with great interest for the production of transgenic farm animals, where it would constitute an important tool for the production of recombinant proteins and livestock improvement.

Generation of five human lactoferrin transgenic cloned goats using fibroblast cells and their methylation status of putative differential methylation regions of IGF2R and H19 imprinted genes

BACKGROUND

Somatic cell nuclear transfer (SCNT) is a promising technique to produce transgenic cloned mammalian, including transgenic goats which may produce Human Lactoferrin (hLF). However, success percentage of SCNT is low, because of gestational and neonatal failure of transgenic embryos. According to the studies on cattle and mice, DNA methylation of some imprinted genes, which plays a vital role in the reprogramming of embryo in NT maybe an underlying mechanism.

METHODOLOGY/PRINCIPAL FINDINGS

Fibroblast cells were derived from the ear of a two-month-old goat. The vector expressing hLF was constructed and transfected into fibroblasts. G418 selection, EGFP expression, PCR, and cell cycle distribution were applied sequentially to select transgenic cells clones. After NT and embryo transfer, five transgenic cloned goats were obtained from 240 cloned transgenic embryos. These transgenic goats were identified by 8 microsatellites genotyping and southern blot. Of the five transgenic goats, 3 were lived after birth, while 2 were dead during gestation. We compared differential methylation regions (DMR) pattern of two paternally imprinted genes (H19 and IGF2R) of the ear tissues from the lived transgenic goats, dead transgenic goats, and control goats from natural reproduction. Hyper-methylation pattern appeared in cloned aborted goats, while methylation status was relatively normal in cloned lived goats compared with normal goats.

CONCLUSIONS/SIGNIFICANCE

In this study, we generated five hLF transgenic cloned goats by SCNT. This is the first time the DNA methylation of lived and dead transgenic cloned goats was compared. The results demonstrated that the methylation status of DMRs of H19 and IGF2R were different in lived and dead transgenic goats and therefore this may be potentially used to assess the reprogramming status of transgenic cloned goats. Understanding the pattern of gene imprinting may be useful to improve cloning techniques in future.

Nuclear reprogramming of sperm and somatic nuclei in eggs and oocytes

Eggs and oocytes have a prominent ability to reprogram sperm nuclei for ensuring embryonic development. The reprogramming activity that eggs/oocytes intrinsically have towards sperm is utilised to reprogram somatic nuclei injected into eggs/oocytes in nuclear transfer (NT) embryos. NT embryos of various species can give rise to cloned animals, demonstrating that eggs/oocytes can confer totipotency even to somatic nuclei. However, many studies indicate that reprogramming of somatic nuclei is not as efficient as that of sperm nuclei. In this review, we explain how and why sperm and somatic nuclei are differentially reprogrammed in eggs/oocytes. Recent studies have shown that sperm chromatin is epigenetically modified to be adequate for early embryonic development, while somatic nuclei do not have such modifications. Moreover, epigenetic memories encoded in sperm chromatin are transgenerationally inherited, implying unique roles of sperm. We also discuss whether somatic nuclei can be artificially modified to acquire sperm-like chromatin states in order to increase the efficiency of nuclear reprogramming.

Long-term live-cell imaging of mammalian preimplantation development and derivation process of pluripotent stem cells from the embryos

Mammalian fertilization is a process in which two highly specialized haploid gametes unite and endow totipotency to the resulting diploid zygote. This is followed by cell proliferation and the onset of differentiation during the brief period leading up to implantation. In these processes, a number of cellular components and structures are regulated spatially and temporally, as seen in repeated cell division, cell cycle progression, and epigenetic reprogramming. In mammals, the numbers of oocytes and embryos that can be collected are very limited. Therefore, analyses of molecular mechanisms are hampered because of difficulties in conducting biochemical analyses on such limited material. Furthermore, immunostaining methods require cell fixation and are insufficient for understanding ontogeny, because the processes observed in fertilization and early embryonic development progress in time-dependent manners and each phenomenon is connected with others by cause-and-effect relationships. Consequently, it is important to develop an experimental system that enables molecular imaging without affecting embryonic development. To achieve the above advantages, especially retrospective and prospective analyses, we have established a live-cell imaging system that enables observations under minimally invasive conditions. Using this approach, we have succeeded in visualizing and predicting the developmental potential of embryos after various perturbations. We also succeeded in imaging embryonic stem (ES) cell derivation in natural conditions. In this review, we describe a brief history of embryonic imaging and detailed protocols. We also discuss promising aspects of imaging in the fields of developmental and stem cell biology.

Valproic acid treatment from the 4-cell stage improves Oct4 expression and nuclear distribution of histone H3K27me3 in mouse cloned blastocysts

We examined effects of treatment with valproic acid (0, 0.2, 1 or 2 mM, VPA), an inhibitor of class I and IIa histone deacetylases (HDACs), of mouse somatic cell nuclear transfer (SCNT) embryos for 24 h from 48 h (4-cell stage), 24 h (2-cell stage) or immediately after oocyte activation on blastocyst formation rates and qualities of the resultant blastocysts. Blastocyst formation rates (33.4–37.0%) were not improved by VPA treatments compared with the untreated control (35.1–36.4%). However, immunofluorescence staining revealed that Oct4 expression levels, evaluated from percentages of embryos expressing Oct4 strongly and having more than 10 Oct4-positive cells, in blastocysts from SCNT embryos treated with 1 mM VPA for 24 h from the 4-cell stage (VPA-4C) were highest among all the groups and that the proportion of cells with a normal nuclear distribution of histone H3 trimethylated at lysine 27 (H3K27me3), a marker of the state of X-chromosome inactivation, significantly increased in the VPA-4C group (36.6%) compared with the control group (12.4%, P<0.05). Treatments with scriptaid and sodium butyrate, inhibitors of class I and IIa/b HDACs, for 24 h from the 4-cell stage also had beneficial effects on SCNT blastocysts. These findings indicate that treatment with 1 mM VPA from the 4-cell stage improves the Oct4 expression and nuclear distribution of H3K27me3 in mouse SCNT blastocysts and suggest that the inhibition of class I and IIa HDACs from the 4-cell stage plays an important role in these effects.

Mitochondrial DNA transmission and confounding mitochondrial influences in cloned cattle and pigs

Although somatic cell nuclear transfer (SCNT) is a powerful tool for production of cloned animals, SCNT embryos generally have low developmental competency and many abnormalities. The interaction between the donor nucleus and the enucleated ooplasm plays an important role in early embryonic development, but the underlying mechanisms that negatively impact developmental competency remain unclear. Mitochondria have a broad range of critical functions in cellular energy supply, cell signaling, and programmed cell death; thus, affect embryonic and fetal development. This review focuses on mitochondrial considerations influencing SCNT techniques in farm animals. Donor somatic cell mitochondrial DNA (mtDNA) can be transmitted through what has been considered a "bottleneck" in mitochondrial genetics via the SCNT maternal lineage. This indicates that donor somatic cell mitochondria have a role in the reconstructed cytoplasm. However, foreign somatic cell mitochondria may affect the early development of SCNT embryos. Nuclear-mitochondrial interactions in interspecies/intergeneric SCNT (iSCNT) result in severe problems. A major biological selective pressure exists against survival of exogenous mtDNA in iSCNT. Yet, mtDNA differences in SCNT animals did not reflect transfer of proteomic components following proteomic analysis. Further study of nuclear-cytoplasmic interactions is needed to illuminate key developmental characteristics of SCNT animals associated with mitochondrial biology.

Nuclear transfer in the mouse oocyte

The nuclear transfer (NT) technique in the mouse has enabled us to generate cloned mice and to establish NT embryonic stem (ntES) cells. Direct nuclear injection into mouse oocytes with a piezo impact drive unit can aid in the bypass of several steps of the original cell fusion procedure. It is important to note that only the NT approach can reveal dynamic and global modifications in the epigenome without using genetic modification as well as generating live animals from single cells. Thus, these techniques could also be applied to the preservation of genetic material from any mouse strain instead of preserving embryos or gametes. Moreover, with this technique, we can use not only living cells but also the nuclei of dead cells from frozen mouse carcasses for NT. This chapter describes our most recent protocols of NT into the mouse oocyte for cloning mice and for the establishment of ntES cells from cloned embryos.

Reprogramming mammalian somatic cells

Somatic cell nuclear transfer (SCNT), the technique commonly known as cloning, permits transformation of a somatic cell into an undifferentiated zygote with the potential to develop into a newborn animal (i.e., a clone). In somatic cells, chromatin is programmed to repress most genes and express some, depending on the tissue. It is evident that the enucleated oocyte provides the environment in which embryonic genes in a somatic cell can be expressed. This process is controlled by a series of epigenetic modifications, generally referred to as "nuclear reprogramming," which are thought to involve the removal of reversible epigenetic changes acquired during cell differentiation. A similar process is thought to occur by overexpression of key transcription factors to generate induced pluripotent stem cells (iPSCs), bypassing the need for SCNT. Despite its obvious scientific and medical importance, and the great number of studies addressing the subject, the molecular basis of reprogramming in both reprogramming strategies is largely unknown. The present review focuses on the cellular and molecular events that occur during nuclear reprogramming in the context of SCNT and the various approaches currently being used to improve nuclear reprogramming. A better understanding of the reprogramming mechanism will have a direct impact on the efficiency of current SCNT procedures, as well as iPSC derivation.

Chromatin and epigenetic modifications during early mammalian development

In mammals, the embryonic genome is transcriptionally inactive after fertilization and embryonic gene expression is initiated during the preimplantation developmental period, during so-called "embryonic genome activation (EGA)". EGA is dependent on the presence of the basal transcriptional machinery components but also on the parental genome reorganization after fertilization. Indeed, during the first cell cycles, the embryonic nuclei undergo intense remodelling that participates in the regulation of embryonic development. Among the mechanisms of this remodeling, it appears that modifications of epigenetic marks are essential especially at the time of embryonic genome activation. This review will focus on DNA methylation and histone modifications such as acetylation or methylation which are important to produce healthy embryos. We will also consider nuclear higher-order structures, such as chromosomes territories and pericentric heterochromatin clusters. The relevance of these chromatin epigenetic modifications has been sustained by the work performed on cloned embryos produced through nuclear transfer of somatic donor cells. It is indeed believed that incomplete reprogramming of the somatic nucleus, in other words, the incomplete re-establishment of the embryonic epigenetic patterns and peculiar nuclear organization may be among the causes of development failure of cloned animals. This will also be discussed in this review.

Early patterning of cloned mouse embryos contributes to post-implantation development

Several research groups have suggested that the embryonic-abembryonic (Em-Ab) axis in the mouse can be predicted by the first cleavage plane of the early embryo. Currently, it is not known whether this early patterning occurs in cloned embryos produced by nuclear transfer and whether it affects development to term. In this work, the relationship between the first cleavage plane and the Em-Ab axis was determined by the labeling of one blastomere in cloned mouse embryos at the 2-cell stage, followed by ex-vivo tracking until the blastocyst stage. The results demonstrate that approximately half of the cloned blastocysts had an Em-Ab axis perpendicular to the initial cleavage plane of the 2-cell stage. These embryos were classified as "orthogonal" and the remainder as "deviant". Additionally, we report here that cloned embryos were significantly more often orthogonal than their naturally fertilized counterparts and overexpressed Sox2. Orthogonal cloned embryos demonstrated a higher rate of post-implantation embryonic development than deviant embryos, but cloned pups did not all survive. These results reveal that the angular relationship between the Em-Ab axis and the first cleavage plane can influence later development and they support the hypothesis that proper early patterning of mammalian embryos is required after nuclear transfer.

Comparative analysis of nuclear transfer embryo-derived mouse embryonic stem cells. Part II: gene regulation

In a mouse model nuclear transfer embryo-derived embryonic stem cell lines (ntESCs) of various genetic backgrounds and donor cell types were compared with reference ESCs and analyzed comprehensively at molecular level as a second part of a larger study. Expression profiles of ntESCs established by different NT-methods (piezoelectric microinjection or zona-free) were indistinguishable. However, expression profiling analyses identified differentially regulated genes between reference ESCs and ntESCs from different genetic backgrounds. A number of pluripotency and stemness marker genes significantly differed at the mRNA level between the cell lines. However, cluster and lineage analyses revealed that such differences had no effect on cell differentiation and cell fate. Regardless of the donor cell type, gene expression profiles of ntESCs were more similar to each other than to their counterpart fertilized embryo-derived ESCs of the same genotype. Overall, the results indicated that expression profile differences may be related to the genotype rather than to technical variations.

Comparative analysis of nuclear transfer embryo-derived mouse embryonic stem cells. Part I: cellular characterization

Embryonic stem cells derived from nuclear transfer embryos (ntESCs) are particularly valuable for regenerative medicine, as they are a patient-specific and histocompatible cell source for the treatment of varying diseases. However, currently, little is known about their cellular and molecular profile. In the present study, in a mouse model different donor cell-derived ntESCs from various genetic backgrounds were compared with reference ESCs and analyzed comprehensively at the cellular level. A number of pluripotency marker genes were compared by flow cytometry and immunocytochemistry analysis. Significant differences at the protein level were observed for POU5F1, SOX2, FGF4, NANOG, and SSEA-1. However, such differences had no effect on in vitro cell differentiation and cell fate: derivatives of the three germ layers were detected in all ntESC lines. The neural and cardiac in vitro differentiation revealed minor differences between the cell lines, both at the mRNA and protein level. Karyotype analyses and cell growth studies did not reveal any significant variations. Despite some differences observed, the present study revealed that ntESC lines had similar differentiation competences compared to other ESCs. The results indicate that the observed differences may be related to the genotype rather than to the nuclear transfer technology.

Cytochalasin B treatment of mouse oocytes during intracytoplasmic sperm injection (ICSI) increases embryo survival without impairment of development

Intracytoplasmic sperm injection (ICSI) is a technique commonly used in clinical and research settings. In mouse oocytes, conventional ICSI has a poor survival rate caused by a high level of lysis. Cytochalasin B (CB) is a toxic microfilament-inhibiting agent that is known to relax the cytoskeleton and enhance the flexibility of oocytes. CB has been used widely in nuclear transfer experiments to improve the success rate of the micromanipulation, however information describing the use of CB in ICSI is limited. Here, we demonstrated that the addition of 5 μg/ml CB to the manipulation medium of ICSI procedure significantly improved the survival rate of the ICSI embryos (80.74% vs. 89.50%, p < 0.05), and that there was no harm for the in vitro or in vivo development. The birth rates and birth weights were not significantly different between the CB-treated and -untreated groups. Interestingly, the microfilaments of the ICSI embryos were almost undetectable immediately after CB treatment; however, they gradually re-appeared and had fully recovered to the normal level 2 h later. Moreover, CB did not disturb spindle rotation, second polar body formation or pronuclei migration, and had no effect on the microtubules. We thus conclude that ICSI manipulation in CB-containing medium results in significantly improved survival rate of mouse ICSI embryos, and that short-term treatment with CB during ICSI manipulation does not have adverse effects on the development of ICSI embryos.

Interspecies somatic cell nuclear transfer: a salvage tool seeking first aid

Much emphasis is currently given to the use of Interspecific Somatic Cell Nuclear Transfer (ISCNT) as a potential salvage tool for endangered animals. In this short review we present a survey on all data published so far on ISCNT, including abstract communication in international meetings. From the analysis of these data it appears that the results obtained are very preliminary and often confusing on the real stage of the embryonic development obtained. Moreover, the acronym ISCNT is improperly used because in many reports the nuclei and oocyte donor are not within the same species, but belong to different order and sometimes taxa, therefore, we classified all the ISCNT reports by allocating cell and oocyte donors to their respective order/species/class. The efficiency of cloning is low in all species owing to incomplete nuclear reprogramming of differentiated cells under the current procedures. ISCNT, however, poses additional hurdles which are rarely addressed in previously published work, and on which we focus in this review: mt/genomic DNA compatibility; embryonic genome activation of the donor nucleus by the recipient oocyte; availability of suitable foster mothers for ISCNT embryos. All these issues are discussed here, and possible solutions for the successful application of somatic cell nuclear transfer to endangered animals are also put forth.

Comparison of liver mitochondrial proteins derived from newborn cloned calves and from cloned adult cattle by two-dimensional differential gel electrophoresis

Aberrant reprogramming of donor somatic cell nuclei may result in many severe problems in animal cloning. The inability to establish functional interactions between donor nucleus and recipient mitochondria is also likely responsible for such a developmental deficiency. However, detailed knowledge of protein expression during somatic cell nuclear transfer (SCNT) in cattle is lacking. In the present study, variations in mitochondrial protein levels between SCNT-derived and control cattle, and from calves derived by artificial insemination were investigated. Mitochondrial fractions were prepared from frozen liver samples and subjected to two-dimensional (2-D) fluorescence differential gel electrophoresis (DIGE) using CyDye™ dyes. Protein expression changes were confirmed with a volume ratio greater than 2.0 (P < 0.05). 2D-DIGE analysis revealed differential expression of three proteins for SCNT cattle (n = 4) and seven proteins for SCNT calves (n = 6) compared to controls (P < 0.05). Different protein patterning was observed among SCNT animals even if animals were generated from the same donor cell source. No differences were detected in two of the SCNT cattle. Moreover, there was no novel protein identified in any of the SCNT cattle or calves. In conclusion, variation in mitochondrial protein expression concentrations was observed in non-viable, neonatal SCNT calves and among SCNT individuals. This result implicates mitochondrial-related gene expression in early developmental loss of SCNT embryos. Comparative proteomic analysis represents an important tool for further studies on SCNT animals.

Effects of downregulating DNA methyltransferase 1 transcript by RNA interference on DNA methylation status of the satellite I region and in vitro development of bovine somatic cell nuclear transfer embryos

For the successful production of cloned animals by somatic cell nuclear transfer (NT), the epigenetic status of the differentiated donor cell is reversed to an embryonic totipotent status. However, in NT embryos, this process is aberrant, with genomic hypermethylation consistently observed. Here, we investigated the effects of silencing DNA methyltransferase 1 (DNMT1) mRNA by small interfering RNA (siRNA) on the DNA methylation status of the satellite I region and in vitro development of bovine NT embryos. First, the levels of DNMT1 expression were analyzed at 0, 24, 48, 72, 120 and 192 h after in vitro culture. Real-time PCR and western blotting analyses detected a significant decrease in DNMT1 mRNA in the siRNA-injected NT (siRNA-NT) group up to 72 h after in vitro culture. Next, the levels of DNA methylation of the satellite I region were analyzed at several time points after in vitro culture. The level of DNA methylation detected in siRNA-NT embryos was significantly less than those in NT embryos throughout in vitro development. Moreover, the developmental rate of embryos to blastocysts in the siRNA-NT group was significantly higher than that of NT embryos. Our data suggest that knockdown of DNMT1 mRNA in NT embryos can induce DNA demethylation, which may enhance reprogramming efficiency.

Inhibition of class IIb histone deacetylase significantly improves cloning efficiency in mice

Since the first mouse clone was produced by somatic cell nuclear transfer, the success rate of cloning in mice has been extremely low. Some histone deacetylase inhibitors, such as trichostatin A and scriptaid, have improved the full-term development of mouse clones significantly, but the mechanisms allowing for this are unclear. Here, we found that two other specific inhibitors, suberoylanilide hydroxamic acid and oxamflatin, could also reduce the rate of apoptosis in blastocysts, improve the full-term development of cloned mice, and increase establishment of nuclear transfer-generated embryonic stem cell lines significantly without leading to obvious abnormalities. However, another inhibitor, valproic acid, could not improve cloning efficiency. Suberoylanilide hydroxamic acid, oxamflatin, trichostatin A, and scriptaid are inhibitors for classes I and IIa/b histone deacetylase, whereas valproic acid is an inhibitor for classes I and IIa, suggesting that inhibiting class IIb histone deacetylase is an important step for reprogramming mouse cloning efficiency.

Functional full-term placentas formed from parthenogenetic embryos using serial nuclear transfer

Mammalian parthenogenetic embryos invariably die in mid-gestation from imprinted gene defects and placental hypoplasia. Based on chimera experiments, trophoblastic proliferation is supposed to be inhibited in the absence of a male genome. Here, we show that parthenogenetic mouse embryonic cell nuclei can be reprogrammed by serial rounds of nuclear transfer without using any genetic modification. The durations of survival in uteri of cloned foetuses derived from green fluorescent protein (GFP)-labelled parthenogenetic cell nuclei were extended with repeated nuclear transfers. After five repeats, live cloned foetuses were obtained up to day 14.5 of gestation; however, they did not survive longer even when we repeated nuclear transfer up to nine times. All foetuses showed intestinal herniation and possessed well-expanded large placentas. When embryonic stem (ES) cells derived from fertilised embryos were aggregated with the cloned embryos, full-term offspring with large placentas were obtained from the chimeric embryos. Those placentas were derived from parthenogenetic cell nuclei, judging from GFP expression. The patterns of imprinted gene expression and methylation status were similar to their parthenogenetic origin, except for Peg10, which showed the same level as in the normal placenta. These results suggest that there is a limitation for foetal development in the ability to reprogramme imprinted genes by repeated rounds of nuclear transfer. However, the placentas of parthenogenetic embryos can escape epigenetic regulation when developed using nuclear transfer techniques and can support foetal development to full gestation.

Effect of trichostatin A on chromatin remodeling, histone modifications, DNA replication, and transcriptional activity in cloned mouse embryos

Our group and others have found that the treatment of embryos with trichostatin A (TSA) after cloning by somatic cell nuclear transfer (SCNT) results in a significant improvement in efficiency. We believe that TSA treatment improves nuclear remodeling via histone modifications, which are important in the epigenetic regulation of gene silencing and expression. Some studies found that treatment of SCNT-generated embryos with TSA improved lysine acetylation of core histones in a manner similar to that seen in normally fertilized embryos. However, how histone methylation is modified in TSA-treated cloned embryos is not completely understood. In the present study, we found that TSA treatment caused an increase in chromosome decondensation and nuclear volume in SCNT-generated embryos similar to that in embryos produced by intracytoplasmic sperm injection. Histone acetylation increased in parallel with chromosome decondensation. This was associated with a more effective formation of DNA replication complexes in treated embryos. We also found a differential effect of TSA on the methylation of histone H3 at positions K4 and K9 in SCNT-generated embryos that could contribute to genomic reprogramming of the somatic cell nuclei. In addition, using 5-bromouridine 5'-triphosphate-labeled RNA, we showed that TSA enhanced the levels of newly synthesized RNA in 2-cell embryos. Interestingly, the amount of SCNT-generated embryos showing asymmetric expression of nascent RNA was reduced significantly in the TSA-treated group compared with the nontreated group at the 2-cell stage. We conclude that the incomplete and inaccurate genomic reprogramming of SCNT-generated embryos was improved by TSA treatment. This could enhance the reprogramming of somatic nuclei in terms of chromatin remodeling, histone modifications, DNA replication, and transcriptional activity.

How to improve the success rate of mouse cloning technology

It has now been 13 years since the first cloned mammal Dolly the sheep was generated from somatic cells using nuclear transfer (SCNT). Since then, this technique has been considered an important tool not only for animal reproduction but also for regenerative medicine. However, the success rate is still very low and the mechanisms involved in genomic reprogramming are not yet clear. Moreover, the NT technique requires donated fresh oocyte, which raises ethical problems for production of human cloned embryo. For this reason, the use of induced pluripotent stem cells for genomic reprogramming and for regenerative medicine is currently a hot topic in this field. However, we believe that the NT approach remains the only valid way for the study of reproduction and basic biology. For example, only the NT approach can reveal dynamic and global modifications in the epigenome without using genetic modification, and it can generate offspring from a single cell or even a frozen dead body. Thanks to much hard work by many groups, cloning success rates are increasing slightly year by year, and NT cloning is now becoming a more applicable method. This review describes how to improve the efficiency of cloning, the establishment of clone-derived embryonic stem cells and further applications.

Optimization of embryo culture conditions for increasing efficiency of cloning in buffalo (Bubalus bubalis) and generation of transgenic embryos via cloning

Cloning in bovine species is marred by low efficiency of blastocyst formation. Any increase in the efficiency of blastocyst formation upon nuclear transfer will greatly enhance the efficiency of cloning. In the present study, the effect of various media, protein sources, and growth factors on the development of cloned buffalo embryos was evaluated. Among various combinations tested, culture of cloned embryos in TCM-199 media on the feeder layer of Buffalo Oviductal Epithelial Cells (BOEC) in the presence of bovine serum albumin-free fatty acid (BSA-FFA) and leukemia inhibitory factor (LIF) provided most suitable environment for efficient development of cloned blastocysts. Under these conditions, we achieved a blastocyst formation rate of 43%, which is better than those reported previously. Because preimplantation embryonic development, in vivo, occurs in an environment of oviductal cells, the blastocysts generated by this method may presumably be more suitable for implantation and further development. Additionally, we generated green blastocysts from enucleated oocytes by transfer of nuclei from cells transfected with EGFP transgene, showing possibility of transgenesis via cloning in this species. To our knowledge, this is the first report regarding the production of transgenic cloned buffalo embryos and their developmental competence with respect to various media, cocultures, and supplements.

Active demethylation of paternal genome in mammalian zygotes

Epigenetic reprogramming in early preimplantation embryos, that refers to erasing and remodeling epigenetic marks such as DNA methylation, is essential for differentiation and development. In many species, paternal genome is subjected to genome-wide active demethylation before the DNA replication commences, while maternal genome maintains its methylation status until being demethylated passively during the subsequent cleavage divisions. The purpose of this manuscript was to review the available knowledge about the paternal genome active demethylation process concerning the possible mechanisms, species variation and the factors affecting the active demethylation dynamics such as in vitro protocols for production of pronuclear-stage zygotes. Better understanding the mechanisms by which the epigenetic reprogramming is occurred may contribute to clarify the biological significance of this process.