Developmental abnormalities of NT mouse embryos appear early after implantation

Inhibition of Wnt activity improves peri-implantation development of somatic cell nuclear transfer embryos

Somatic cell nuclear transfer (SCNT) can reprogram differentiated somatic cells into totipotency. Although pre-implantation development of SCNT embryos has greatly improved, most SCNT blastocysts are still arrested at the peri-implantation stage, and the underlying mechanism remains elusive. Here, we develop a 3D in vitro culture system for SCNT peri-implantation embryos and discover that persistent Wnt signals block the naïve-to-primed pluripotency transition of epiblasts with aberrant H3K27me3 occupancy, which in turn leads to defects in epiblast transformation events and subsequent implantation failure. Strikingly, manipulating Wnt signals can attenuate the pluripotency transition and H3K27me3 deposition defects in epiblasts and achieve up to a 9-fold increase in cloning efficiency. Finally, single-cell RNA-seq analysis reveals that Wnt inhibition markedly enhances the lineage developmental trajectories of SCNT blastocysts during peri-implantation development. Overall, these findings reveal diminished potentials of SCNT blastocysts for lineage specification and validate a critical peri-implantation barrier for SCNT embryos.

Loss of Slc38a4 imprinting is a major cause of mouse placenta hyperplasia in somatic cell nuclear transferred embryos at late gestation

Placenta hyperplasia is commonly observed in cloned animals and is believed to impede the proper development of cloned embryos. However, the mechanism underlying this phenomenon is largely unknown. Here, we show that placenta hyperplasia of cloned mouse embryos occurs in both middle and late gestation. Interestingly, restoring paternal-specific expression of an amino acid transporter Slc38a4, which loses maternal H3K27me3-dependent imprinting and becomes biallelically expressed in cloned placentae, rescues the overgrowth of cloned placentae at late gestation. Molecular analyses reveal that loss of Slc38a4 imprinting leads to over-activation of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway in cloned placentae, which is likely due to the increased amino acids transport by SLC38A4. Collectively, our study not only reveals loss of Slc38a4 imprinting is responsible for overgrowth of cloned placentae at late gestation but also suggests the underlying mechanism involves increased amino acid transport and over-activation of mTORC1.

Lessons Learned from Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) has been an area of interest in the field of stem cell research and regenerative medicine for the past 20 years. The main biological goal of SCNT is to reverse the differentiated state of a somatic cell, for the purpose of creating blastocysts from which embryonic stem cells (ESCs) can be derived for therapeutic cloning, or for the purpose of reproductive cloning. However, the consensus is that the low efficiency in creating normal viable offspring in animals by SCNT (1–5%) and the high number of abnormalities seen in these cloned animals is due to epigenetic reprogramming failure. In this review we provide an overview of the current literature on SCNT, focusing on protocol development, which includes early SCNT protocol deficiencies and optimizations along with donor cell type and cell cycle synchrony; epigenetic reprogramming in SCNT; current protocol optimizations such as nuclear reprogramming strategies that can be applied to improve epigenetic reprogramming by SCNT; applications of SCNT; the ethical and legal implications of SCNT in humans; and specific lessons learned for establishing an optimized SCNT protocol using a mouse model.

Atypical chemokine receptors 1, 2, 3 and 4: Expression and regulation in the endometrium during the estrous cycle and pregnancy and with somatic cell nucleus transfer-cloned embryos in pigs

Atypical chemokine receptor (ACKR) 1, ACKR2, ACKR3, and ACKR4, chemokine decoy receptors that lack G-protein-mediated signaling pathways, internalize and degrade chemokines to control their availability and function. Chemokines play important roles in the endometrium during the estrous cycle and pregnancy, but the expression and regulation of ACKRs have not been determined in pigs. Therefore, we examined the expression of ACKRs in the endometrium throughout the estrous cycle and pregnancy and in conceptus tissues in pigs. ACKR1, ACKR2, ACKR3, and ACKR4 mRNA was expressed in the endometrium, with higher levels of ACKR3 on day 12 of the estrous cycle than in pregnancy and higher levels of ACKR4 on day 15 of pregnancy than in the estrous cycle. ACKR1, ACKR2, and ACKR3, but not ACKR4, mRNA was detected in conceptus and chorioallantoic tissues during pregnancy. ACKR2 and ACKR3 mRNA and ACKR4 protein were mainly localized to luminal epithelial cells and weakly to glandular epithelial cells in the endometrium. Increasing doses of progesterone increased the expression of ACKR2 and ACKR4 and decreased the expression of ACKR3 in endometrial tissues. On day 12 of pregnancy, the expression of ACKR4 mRNA was lower in the endometria of gilts with somatic cell nucleus transfer-derived conceptuses than in the endometria of gilts carrying conceptuses derived from natural mating. These results indicate that the expression of ACKRs is dynamically regulated at the maternal-conceptus interface, suggesting that ACKR proteins might play critical roles in regulating endometrial chemokines to support the establishment and maintenance of pregnancy in pigs.

Electrofusion of 2-Cell Embryos for Porcine Tetraploid Embryo Production

The electrofusion of 2-cell embryos proves to be a simple and efficient way of generating mammalian tetraploid embryos. Many factors affect the fusion efficiency, such as fusion medium, electric field intensity, and fusion pulse length. In mice, production of tetraploid embryos by electrofusion has already been investigated; however, the investigation to produce porcine tetraploid embryos is seldom reported. In this chapter, we will describe oocytes in vitro maturation, in vitro fertilization, and the optimum conditions for electrofusion of 2-cell embryos to produce tetraploid embryos in pig.

Analysis of stage-specific expression of the toll-like receptor family in the porcine endometrium throughout the estrous cycle and pregnancy

Toll-like receptors (TLRs) play critical roles in innate immunity by regulating antimicrobial responses in mucosal tissues. The expression and function of TLRs in female reproductive tissues have been studied in several species, but the expression and function of TLRs and MYD88, an adaptor molecule in the TLR signaling pathway, at the maternal-conceptus interface are not well understood in pigs. Thus, we determined the expression of TLR1 - TLR10 and MYD88 in the endometrium, conceptus, and chorioallantoic tissues of pigs. TLR1 - TLR10 and MYD88 mRNAs were expressed in the endometrium during the estrous cycle and pregnancy in a stage-dependent manner. TLR and MYD88 mRNAs were also detected in early stage conceptuses and chorioallantoic tissues from Day 30 to term pregnancy. The expression of TLR2, TLR4, TLR5, and TLR7 was localized to epithelial and stromal cells in endometrial and chorioallantoic tissues. Increasing doses of P₄, but not E₂, induced the expression of TLR4, TLR5, TLR6, TLR7, and TLR8, while interferon-γ increased the expression of TLR2 and TLR7 in endometrial explant tissues. Expression of TLR3, TLR5, TLR6, TLR7, and MYD88 was higher in the endometrium with somatic cell nucleus transfer-derived conceptuses than conceptuses derived from natural mating on Day 12. These results indicate that the expression of TLR1 - TLR10 and MYD88 is dynamically regulated at the maternal-conceptus interface in pigs, suggesting that TLRs expressed in the endometrium and the placenta may play a critical role in regulating mucosal immune responses to support the establishment and maintenance of pregnancy.

Comparison of in vitro Neuronal Differentiation Capacity Between Mouse Epiblast Stem Cells Derived From Nuclear Transfer and Naturally Fertilized Embryos

Somatic cell nuclear transfer (SCNT) can give rise to fertile adults, but the successful perinatal and postnatal developmental rates are inefficient, including delayed developmental behaviors, and respiratory failure. However, the molecular and cellular mechanisms remain elusive. Mouse epiblast stem cells (mEpiSCs) from E5.5-6.5 epiblasts share defining features with human embryonic stem cells (hESCs), providing a new opportunity to study early mammalian development in vitro. In this study, mEpiSCs were established from naturally fertilized mouse embryos (F-mEpiSCs) and SCNT mouse embryos (NT-mEpiSCs). Also, the in vitro neuronal differentiation capacity of F-mEpiSCs and NT-mEpiSCs was compared. Morphology analysis showed less and smaller neurospheres formation and lower percentage of early neurons generation in NT-mEpiSCs. The immunocytochemical analysis and altered mRNA expression levels of the neuronal markers in differentiated cells further confirmed that neurogenesis was slower in NT-mEpiSCs than in F-mEpiSCs. Moreover, neuronal differentiation capacity was correlated with the basal expression levels of Atox1 and Vinculin but not Brachyury and Otx2, emphasizing that developmental aberrations in neurogenesis were associated with the NT technique but not random variations between clones. This study provided an important in vitro platform using mEpiSCs to study early epigenetic and developmental processes associated with neurogenesis.

Transcriptional defects and reprogramming barriers in somatic cell nuclear reprogramming as revealed by single-embryo RNA sequencing

Background

Nuclear reprogramming reinstates totipotency or pluripotency in somatic cells by changing their gene transcription profile. This technology is widely used in medicine, animal husbandry and other industries. However, certain deficiencies severely restrict the applications of this technology.

Results

Using single-embryo RNA-seq, our study provides complete transcriptome blueprints of embryos generated by cumulus cell (CC) donor nuclear transfer (NT), embryos generated by mouse embryonic fibroblast (MEF) donor NT and in vivo embryos at each stage (zygote, 2-cell, 4-cell, 8-cell, morula, and blastocyst). According to the results from further analyses, NT embryos exhibit RNA processing and translation initiation defects during the zygotic genome activation (ZGA) period, and protein kinase activity and protein phosphorylation are defective during blastocyst formation. Two thousand three constant genes are not able to be reprogrammed in CCs and MEFs. Among these constant genes, 136 genes are continuously mis-transcribed throughout all developmental stages. These 136 differential genes may be reprogramming barrier genes (RBGs) and more studies are needed to identify.

Conclusions

These embryonic transcriptome blueprints provide new data for further mechanistic studies of somatic nuclear reprogramming. These findings may improve the efficiency of somatic cell nuclear transfer.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5091-1) contains supplementary material, which is available to authorized users.

Assessment of cell lineages and cell death in blastocysts by immunostaining

During the last decade it has been shown that most mammalian blastocysts consisted of three cell lineages. Immunofluorescence with multiple antibodies enables to identify each cell type allowing an easy detection of eventual defects. It is complementary to RT-PCR experiments as this technique allows to look at cell position and to analyze and count the proportions between the different cell types. Thus after any kind of embryo manipulation such as nuclear transfer (NT), the analysis of the three cell lineages by immunofluorescence will provide criteria for good or poor development.

Whole-mount in situ hybridization to assess advancement of development and embryo morphology

Whole-mount in situ hybridization (WISH) is widely used to visualize the site and dynamics of gene expression during embryonic development. Various methods of probe labeling and hybridization detection are available nowadays. Meanwhile the technique was adapted to be used on many different species and has evolved from a manual to a larger scale and automated procedure. Standardized automated protocols improve the chance to compare different experimental settings reliably. The high resolution of this method is ideally suited for examination of manipulated (e.g., cloned) embryos often displaying subtle changes only. Embedding and sectioning of in situ hybridized specimen further enhance the detailed examination of their gene expression and morphology.

Regulation of S100G Expression in the Uterine Endometrium during Early Pregnancy in Pigs

Calcium ions play an important role in the establishment and maintenance of pregnancy, but molecular and cellular regulatory mechanisms of calcium ion action in the uterine endometrium are not fully understood in pigs. Previously, we have shown that calcium regulatory molecules, transient receptor potential vanilloid type 5 (TRPV6) and calbindin-D9k (S100G), are expressed in the uterine endometrium during the estrous cycle and pregnancy in a pregnancy status- and stage-specific manner, and that estrogen of conceptus origin increases endometrial TRPV6 expression. However, regulation of S100G expression in the uterine endometrium and conceptus expression of S100G has been not determined during early pregnancy. Thus, we investigated regulation of S100G expression by estrogen and interleukin-1β (IL1B) in the uterine endometrium and conceptus expression of S100G during early pregnancy in pigs. We obtained uterine endometrial tissues from day (D) 12 of the estrous cycle and treated with combinations of steroid hormones, estradiol-17β (E2) and progesterone (P4), and increasing doses of IL1B. Real-time RT-PCR analysis showed that E2 and IL1B increased S100G mRNA levels in the uterine endometrium, and conceptuses expressed S100G mRNA during early pregnancy, as determined by RT-PCR analysis. To determine if endometrial expression of S100G mRNA during the implantation period was affected by the somatic cell nuclear transfer (SCNT) procedure, we compared S100G mRNA levels in the uterine endometrium from gilts with SCNT-derived conceptuses with those from gilts with conceptuses derived from natural mating on D12 of pregnancy. Real-time RT-PCR analysis showed that levels of S100G mRNA in the uterine endometrium from gilts carrying SCNT-derived conceptuses was significantly lower than those from gilts carrying conceptuses derived from natural mating. These results showed that S100G expression in the uterine endometrium was regulated by estrogen and IL1B of conceptus origin, and affected by the SCNT procedure during early pregnancy. These suggest that conceptus signals regulate S100G, an intracellular calcium transport protein, for the establishment of pregnancy in pigs.

Analysis of ENPP2 in the Uterine Endometrium of Pigs Carrying Somatic Cell Nuclear Transfer Cloned Embryos

Somatic cell nuclear transfer (SCNT) is a useful tool for animal cloning, but the efficiency of producing viable offspring by SCNT is very low. To improve this efficiency in the production of cloned pigs, it is critical to understand the interactions between uterine function and cloned embryos during implantation. Lysophosphatidic acid (LPA) is a lipid mediator that plays an important role in the establishment of pregnancy in pigs; however, LPA production in the uterine endometrium of pigs carrying SCNT-cloned conceptuses has not been determined. Therefore, we investigated expression of ENPP2, an LPA-generating enzyme, in the uterine endometrium of gilts with conceptuses derived from SCNT during the implantation period. Uterine endometrial tissue and uterine flushing were obtained from gilts carrying SCNT-derived conceptuses and from gilts carrying conceptuses resulting from natural mating on d 12 of pregnancy. Our results demonstrated no difference in the level of ENPP2 mRNA expression in the uterine endometrium between gilts carrying SCNT-derived conceptuses and gilts carrying naturally-conceived conceptuses, but secretion of ENPP2 protein into the uterine lumen did decrease significantly in pigs with SCNT-derived conceptuses. These results indicate that expression and secretion of ENPP2, which are critical for appropriate LPA production and successful pregnancy, are dysregulated in the uterine endometrium of pigs carrying SCNT-derived conceptuses.

Massive deregulation of miRNAs from nuclear reprogramming errors during trophoblast differentiation for placentogenesis in cloned pregnancy

Background

Low efficiency of Somatic Cell Nuclear Transfer (NT) has been widely addressed with high incidence of placental abnormalities due to genetic and epigenetic modifications. MiRNAs are shown to be major regulators of such modifications. The present study has been carried out to identify the expression patterns of 377 miRNAs, their functional associations and mechanism of regulation in bovine placentas derived from artificial insemination (AI), in vitro production (IVP) and NT pregnancies.

Results

This study reveals a massive deregulation of miRNAs as chromosomal cluster or miRNA families without sex-linkage in NT and in-vitro derived IVP placentas. Cell specific localization miRNAs in blastocysts and expression profiling of embryos and placentas at different developmental stages identified that the major deregulation of miRNAs exhibited in placentas at day 50 of pregnancies is found to be less dependent on global DNA methylation, rather than on aberrant miRNA biogenesis molecules. Among them, aberrant AGO2 expression due to hypermethylation of its promoter was evident. Along with other factors, aberrant AGO2 expression was observed to be associated with multiple defects in trophoblast differentiation through deregulation of miRNAs mediated mechanisms.

Conclusion

These aberrant miRNA activities might be associated with genetic and epigenetic modifications in abnormal placentogenesis due to maldifferentiation of early trophoblast cell lineage in NT and IVP pregnancies. This study provides the first insight into genome wide miRNA expression, their role in regulation of trophoblast differentiation as well as abnormal placental development in Somatic Cell Nuclear Transfer pregnancies to pave the way to improve the efficiency of cloning by nuclear transfer.

Somatic donor cell type correlates with embryonic, but not extra-embryonic, gene expression in postimplantation cloned embryos

The great majority of embryos generated by somatic cell nuclear transfer (SCNT) display defined abnormal phenotypes after implantation, such as an increased likelihood of death and abnormal placentation. To gain better insight into the underlying mechanisms, we analyzed genome-wide gene expression profiles of day 6.5 postimplantation mouse embryos cloned from three different cell types (cumulus cells, neonatal Sertoli cells and fibroblasts). The embryos retrieved from the uteri were separated into embryonic (epiblast) and extraembryonic (extraembryonic ectoderm and ectoplacental cone) tissues and were subjected to gene microarray analysis. Genotype- and sex-matched embryos produced by in vitro fertilization were used as controls. Principal component analysis revealed that whereas the gene expression patterns in the embryonic tissues varied according to the donor cell type, those in extraembryonic tissues were relatively consistent across all groups. Within each group, the embryonic tissues had more differentially expressed genes (DEGs) (>2-fold vs. controls) than did the extraembryonic tissues (P<1.0×10^–26). In the embryonic tissues, one of the common abnormalities was upregulation of Dlk1, a paternally imprinted gene. This might be a potential cause of the occasional placenta-only conceptuses seen in SCNT-generated mouse embryos (1–5% per embryos transferred in our laboratory), because dysregulation of the same gene is known to cause developmental failure of embryos derived from induced pluripotent stem cells. There were also some DEGs in the extraembryonic tissues, which might explain the poor development of SCNT-derived placentas at early stages. These findings suggest that SCNT affects the embryonic and extraembryonic development differentially and might cause further deterioration in the embryonic lineage in a donor cell-specific manner. This could explain donor cell-dependent variations in cloning efficiency using SCNT.

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.

Analysis of legumain and cystatin 6 expression at the maternal-fetal interface in pigs

Cathepsins (CTSs), a family of lysosomal cysteine proteases, and their inhibitors, cystatins (CSTs), play a critical role in endometrial and placental tissue remodeling during the establishment and maintenance of pregnancy in many species including rodents, sheep, cow, and pigs. In this study, we determined expression of legumain (LGMN), a cathepsinmember, and its inhibitor, CST6, at the maternal-fetal interface in pigs. Expression of both LGMN and CST6 mRNAs increased during mid- to late pregnancy in the uterine endometrium. LGMN and CST6 mRNAs localized to luminal epithelial cells (LE) and glandular epithelial cells (GE) and to the chorionic membrane (CM), with a strong intensity in GE and the CM for LGMN and in the CM for CST6 during pregnancy. LGMN protein was detected at molecular weights (MW) of approximately 50,000 and 37,000, and the abundance of the37,000-MW LGMN protein increased during mid- to latepregnancy. CST6 protein was also highly expressed in the uterine endometrium in mid- to latepregnancy. LGMN protein localized to LE, GE, and the CM during pregnancy. LGMN and CST6 were aberrantly expressed in the uterine endometrium from gilts with somatic cell nuclear transfer-derived conceptuses at term compared to those of gilts carrying conceptuses derived from natural mating. These results demonstrated that LGMN and CST6 were expressed in the uterine endometrium in a cell-type and stage-specific manner, suggesting that the LGMN and CST6 system at the maternal-fetal interface may play an important role in the establishment and maintenance of pregnancy in pigs.

Recent advancements in cloning by somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) cloning is the sole reproductive engineering technology that endows the somatic cell genome with totipotency. Since the first report on the birth of a cloned sheep from adult somatic cells in 1997, many technical improvements in SCNT have been made by using different epigenetic approaches, including enhancement of the levels of histone acetylation in the chromatin of the reconstructed embryos. Although it will take a considerable time before we fully understand the nature of genomic programming and totipotency, we may expect that somatic cell cloning technology will soon become broadly applicable to practical purposes, including medicine, pharmaceutical manufacturing and agriculture. Here we review recent progress in somatic cell cloning, with a special emphasis on epigenetic studies using the laboratory mouse as a model.

Mice embryology: a microscopic overview

In this work, we studied the embryology of mice of 12, 14, and 18 days of gestation by gross observation, light microscopy, and scanning electron microscopy. Grossly, the embryos of 12 days were observed in C-shaped region of the brain, eye pigmentation of the retina, first, second, and third pharyngeal arches gill pit nasal region on the fourth ventricle brain, cervical curvature, heart, liver, limb bud thoracic, spinal cord, tail, umbilical cord, and place of the mesonephric ridge. Microscopically, the liver, cardiovascular system and spinal cord were observed. In the embryo of 14 days, we observed structures that make up the liver and heart. At 18 days of gestation fetuses, it was noted the presence of eyes, mouth, and nose in the cephalic region, chest and pelvic region with the presence of well-developed limbs, umbilical cord, and placenta. Scanning electron microscopy in 18 days of gestation fetuses evidenced head, eyes closed eyelids, nose, vibrissae, forelimb, heart, lung, kidney, liver, small bowel, diaphragm, and part of the spine. The results obtained in this work describe the internal and external morphology of mice, provided by an integration of techniques and review of the morphological knowledge of the embryonic development of this species, as this animal is of great importance to scientific studies.

Uncoupled embryonic and extra-embryonic tissues compromise blastocyst development after somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) is the most efficient cell reprogramming technique available, especially when working with bovine species. Although SCNT blastocysts performed equally well or better than controls in the weeks following embryo transfer at Day 7, elongation and gastrulation defects were observed prior to implantation. To understand the developmental implications of embryonic/extra-embryonic interactions, the morphological and molecular features of elongating and gastrulating tissues were analysed. At Day 18, 30 SCNT conceptuses were compared to 20 controls (AI and IVP: 10 conceptuses each); one-half of the SCNT conceptuses appeared normal while the other half showed signs of atypical elongation and gastrulation. SCNT was also associated with a high incidence of discordance in embryonic and extra-embryonic patterns, as evidenced by morphological and molecular “uncoupling”. Elongation appeared to be secondarily affected; only 3 of 30 conceptuses had abnormally elongated shapes and there were very few differences in gene expression when they were compared to the controls. However, some of these differences could be linked to defects in microvilli formation or extracellular matrix composition and could thus impact extra-embryonic functions. In contrast to elongation, gastrulation stages included embryonic defects that likely affected the hypoblast, the epiblast, or the early stages of their differentiation. When taking into account SCNT conceptus somatic origin, i.e. the reprogramming efficiency of each bovine ear fibroblast (Low: 0029, Med: 7711, High: 5538), we found that embryonic abnormalities or severe embryonic/extra-embryonic uncoupling were more tightly correlated to embryo loss at implantation than were elongation defects. Alternatively, extra-embryonic differences between SCNT and control conceptuses at Day 18 were related to molecular plasticity (high efficiency/high plasticity) and subsequent pregnancy loss. Finally, because it alters re-differentiation processes in vivo, SCNT reprogramming highlights temporally and spatially restricted interactions among cells and tissues in a unique way.

RNAi-mediated knockdown of Xist can rescue the impaired postimplantation development of cloned mouse embryos

Cloning mammals by somatic cell nuclear transfer (SCNT) is highly inefficient. Most SCNT-generated embryos die after implantation because of unidentified, complex epigenetic errors in the process of postimplantation embryonic development. Here we identify the most upstream level of dysfunction leading to impaired development of clones by using RNAi against Xist, a gene responsible for X chromosome inactivation (XCI). A prior injection of Xist-specific siRNA into reconstructed oocytes efficiently corrected SCNT-specific aberrant Xist expression at the morula stage, but failed to do so thereafter at the blastocyst stage. However, we found that shortly after implantation, this aberrant XCI status in cloned embryos had been corrected autonomously in both embryonic and extraembryonic tissues, probably through a newly established XCI control for postimplantation embryos. Embryo transfer experiments revealed that siRNA-treated embryos showed 10 times higher survival than controls as early as embryonic day 5.5 and this high survival persisted until term, resulting in a remarkable improvement in cloning efficiency (12% vs. 1% in controls). Importantly, unlike control clones, these Xist-siRNA clones at birth showed only a limited dysregulation of their gene expression, indicating that correction of Xist expression in preimplantation embryos had a long-term effect on their postnatal normality. Thus, contrary to the general assumption, our results suggest that the fate of cloned embryos is determined almost exclusively before implantation by their XCI status. Furthermore, our strategy provides a promising breakthrough for mammalian SCNT cloning, because RNAi treatment of oocytes is readily applicable to most mammal species.

Defects in trophoblast cell lineage account for the impaired in vivo development of cloned embryos generated by somatic nuclear transfer

The low success rate of somatic nuclear transfer (NT) is hypothesized to be mainly due to functional defects in the trophoblast cell lineage rather than the inner cell mass (ICM); this hypothesis, however, remains to be tested directly. Here we separated the ICMs from cloned blastocysts and aggregated the cloned ICM with two fertilization-derived (FD) tetraploid (4N) embryos. We found that the full-term development of cloned ICMs was dramatically improved after the trophoblast cells in the cloned blastocysts were replaced by cells from tetraploid embryos, thus providing direct evidence that defects in trophoblast cell lineage underlie the low success rate of somatic NT.

Mouse chimeras as a system to investigate development, cell and tissue function, disease mechanisms and organ regeneration

Chimeras are organisms composed of at least two genetically distinct cell lineages originating from different zygotes. In the laboratory, mouse chimeras can be produced experimentally; various techniques allow combining different early stage mouse embryos with each other or with pluripotent stem cells. Identification of the progeny of the different lineages in chimeras permits to follow cell fate and function, enabling correlation of genotype with phenotype. Mouse chimeras have become a tool to investigate critical developmental processes, including cell specification, differentiation, patterning, and the function of specific genes. In addition, chimeras can also be generated to address biological processes in the adult, including mechanisms underlying diseases or tissue repair and regeneration. This review summarizes the different types of chimeras and how they have been generated and provides examples of how mouse chimeras offer a unique and powerful system to investigate questions pertaining to cell and tissue function in the developing and adult organism.

Epigenetic control of development and expression of quantitative traits

In recent years, it has become increasingly clear that epigenetic regulation of gene expression is critical during embryo development and subsequently during pre- and post-natal life. The phenotype of an individual is the result of complex interactions between genotype and current, past and ancestral environment leading to a lifelong remodelling of its epigenome. Practically, if the genome was compared with the hardware in a computer, the epigenome would be the software that directs the computer's operation. This review points to the importance of epigenetic processes for genome function in various biological processes, such as embryo development and the expression of quantitative traits.

Pluripotency maintenance in mouse somatic cell nuclear transfer embryos and its improvement by treatment with the histone deacetylase inhibitor TSA

Reprogramming of somatic cells to pluripotency can be achieved by nuclear transfer into enucleated oocytes (SCNT). A key event of this process is the demethylation of the Oct4 gene and its temporally and spatially regulated expression. Different studies have shown that it occurs abnormally in some SCNT embryos. TSA is a histone deacetylase inhibitor known to increase the efficiency of development to term of SCNT embryos, but its impact on the developmental features of SCNT embryos is poorly understood. Here, we have followed the fate of the pluripotent cells within SCNT embryos, from the late blastocyst to the early epiblast prior to gastrulation. Our data show a delay in development correlated with a defect in forming and maintaining a correct number of Oct4 expressing ICM and epiblast cells in SCNT embryos. As a consequence, during the outgrowth phase of embryonic stem cell derivation as well as during diapause in vivo, part of the SCNT blastocysts completely lose their ICM cells. Meanwhile, the others display a correctly reprogrammed ICM compatible with the derivation of ES cells and development of the epiblast. Our data also indicate that TSA favors the establishment of pluripotency in SCNT embryos.

Somatic cell nuclear transfer efficiency: how can it be improved through nuclear remodeling and reprogramming?

Fertile offspring from somatic cell nuclear transfer (SCNT) is the goal of most cloning laboratories. For this process to be successful, a number of events must occur correctly. First the donor nucleus must be in a state that is amenable to remodeling and subsequent genomic reprogramming. The nucleus must be introduced into an oocyte cytoplasm that is capable of facilitating the nuclear remodeling. The oocyte must then be adequately stimulated to initiate development. Finally the resulting embryo must be cultured in an environment that is compatible with the development of that particular embryo. Much has been learned about the incredible changes that occur to a nucleus after it is placed in the cytoplasm of an oocyte. While we think that we are gaining an understanding of the reorganization that occurs to proteins in the donor nucleus, the process of cloning is still very inefficient. Below we will introduce the procedures for SCNT, discuss nuclear remodeling and reprogramming, and review techniques that may improve reprogramming. Finally we will briefly touch on other aspects of SCNT that may improve the development of cloned embryos.

Trophoblast cell lineage in cloned mouse embryos

Most conceptuses derived by somatic cell nuclear transfer (SCNT) in mice undergo developmental arrest as a result of embryonic or extraembryonic defects. Even when fetuses survive to term, prominent placental overgrowth or placentomegaly is often present, indicating that SCNT affects the development of trophoblast cell lineage. The trophoblast cell lineage is established at the blastocyst stage when the stem cell population of the trophoblast cell lineage resides in the polar trophectoderm. Therefore, it is possible that the developmental arrest and placentomegaly that accompany SCNT are induced by insufficient reprogramming of the donor somatic nucleus to enable the cells to acquire full potency as stem cells of the trophoblast cell lineage. Despite the abnormalities of the extraembryonic tissues of SCNT embryos, trophoblast stem (TS) cell lines have been successfully isolated from SCNT blastocysts and their properties appear to be indistinguishable from those of TS cells derived from native blastocysts. This suggests that SCNT does not affect the emergence and autonomous properties of TS cells. In this review, we discuss specification of cell lineage and the extent of reprogramming of TS cells in SCNT blastocysts.

Governing cell lineage formation in cloned mouse embryos

Blastomeres of the pre-implantation mouse embryo form trophectoderm and inner cell mass via a process that requires the transcription factors Tead4, Cdx2, Oct4 and Nanog. In mouse morulae cloned by somatic cell nuclear transfer, we observed that the trophectoderm transcription factor Cdx2 is expressed very differently at the protein level compared to time- and stage-matched fertilized counterparts. Protein levels of Cdx2 in cloned embryos appear 'erratic,' i.e. are widely distributed, when plotted as histograms. In contrast to Cdx2, protein levels of the upstream factor Tead4 and of inner cell mass transcription factors Oct4 and Nanog are similar in cloned and fertilized embryos. These observations suggest that trophectoderm formation is initiated but not maintained correctly in cloned mouse morulae, which is consistent with cloned blastocysts' limited implantation and post-implantation success. Because a cell's ability to differentiate is greatly enhanced if it is surrounded by more cells differentiating the same way, a concept designated community effect by Gurdon, we reasoned that the insufficient cell numbers often observed in cloned embryos might lead to premature Cdx2 expression and differentiation of blastomeres into trophectoderm. Therefore, we created larger cloned embryos by aggregating them at the 4-cell stage. Homologous aggregation stimulates expression of multiple signaling pathways' components and results in cloned embryos with levels of Cdx2 similar to fertilized embryos. Most of the resultant morulae and blastocysts consist of cells of all three founders, indicating that aggregation increases stability of all of the individual components. We conclude that the induction of pluripotency in cloned embryos is more efficient than previously assumed, and we propose that a minimum cell number is necessary to stabilize pluripotency and inhibit premature expression of Cdx2 in cloned mouse embryos.

Establishment of trophoblast stem cell lines from somatic cell nuclear-transferred embryos

Placental abnormalities occur frequently in cloned animals. Here, we attempted to isolate trophoblast stem (TS) cells from mouse blastocysts produced by somatic cell nuclear transfer (NT) at the blastocyst stage (NT blastocysts). Despite the predicted deficiency of the trophoblast cell lineage, we succeeded in isolating cell colonies with typical morphology of TS cells and cell lines from the NT blastocysts (ntTS cell lines) with efficiency as high as that from native blastocysts. The established 10 ntTS cell lines could be maintained in the undifferentiated state and induced to differentiate into several trophoblast subtypes in vitro. A comprehensive analysis of the transcriptional and epigenetic traits demonstrated that ntTS cells were indistinguishable from control TS cells. In addition, ntTS cells contributed exclusively to the placenta and survived until term in chimeras, indicating that ntTS cells have developmental potential as stem cells. Taken together, our data show that NT blastocysts contain cells that can produce TS cells in culture, suggesting that proper commitment to the trophoblast cell lineage in NT embryos occurs by the blastocyst stage.

Gene expression and development of early pig embryos produced by serial nuclear transfer

During nuclear transfer, reprogramming makes the donor nucleus capable of directing development of the reconstructed embryo. In most cases reprogramming is incomplete, which leads to abnormal expression of early embryonic genes and subsequently, to reduced developmental potential. In the present study, we monitored the expression of Oct4, Nanog, and Sox2 in cloned porcine embryos and evaluated whether serial nuclear transfer, the transfer of nuclei of cloned embryos into enucleated oocytes, has the potential to provide a more complete reprogramming of the donor genome. The data suggested that Nanog and Sox2 expression is properly reactivated after nuclear transfer, but the relative abundance of Oct4 transcripts is abnormally low in cloned porcine blastocysts compared to control embryos produced by in vitro fertilization. When the nuclei of 8- to 16-cell stage cloned embryos were introduced into enucleated oocytes to expose the chromosomes repeatedly to the ooplasmic factors, the resulting embryos showed poor developmental potential: a significantly lower percentage of embryos developed to the 4-cell (12.0% vs. 31.8%), 8-cell (3.1% vs. 15.0%) and blastocyst (0% vs. 8.7%) stages compared to those produced following a single round of nuclear transfer (P < 0.05). The additional time for reprogramming also did not improve gene expression. By the late 4-cell stage, Oct4 and Sox2 expression levels were low in serial nuclear transfer embryos compared to those in embryos generated by in vitro fertilization or nuclear transfer. Overall, both developmental and gene expression data indicated that reprogramming of the donor nucleus could not be improved by serial nuclear transfer in the pig.

Embryonic rather than extraembryonic tissues have more impact on the development of placental hyperplasia in cloned mice

Somatic cell cloning by nuclear transfer (NT) in mice is associated with hyperplastic placentas at term. To dissect the effects of embryonic and extraembryonic tissues on this clone-associated phenotype, we constructed diploid (2n) fused with (<-->) tetraploid (4n) chimeras from NT- and fertilization-derived (FD) embryos. Generally, the 4n cells contributed efficiently to all the extraembryonic tissues but not to the embryo itself. Embryos constructed by 2n NT<-->4n FD aggregation developed hyperplastic placentas (0.33+/-0.22 g) with a predominant contribution by NT-derived cells. Even when the population of FD-derived cells in placentas was increased using multiple FD embryos (up to four) for aggregation, most placentas remained hyperplastic (0.36+/-0.13 g). By contrast, placentas of the reciprocal combination, 2n FD<-->4n NT, were less hyperplastic (0.15+/-0.02 g). These nearly normal-looking placentas had a large proportion of NT-derived cells. Thus, embryonic rather than extraembryonic tissues had more impact on the onset of placental hyperplasia, and that the abnormal placentation in clones occurs in a noncell-autonomous manner. These findings suggest that for improvement of cloning efficiency we should understand the mechanisms regulating placentation, especially those of embryonic origin that might control the proliferation of trophoblastic lineage cells.

Cotransfer of parthenogenetic embryos improves the pregnancy and implantation of nuclear transfer embryos in mouse

The majority of somatic cell nuclear transfer (SCNT) clones dies in the peri- or postimplantation period. Improvement of the full-term healthy pregnancy rates is a key issue for the economical viability and animal welfare profile of SCNT technology. In this study the effects of cotransfer of parthenogenetic or fertilized embryos on the pregnancy and implantation of SCNT mouse embryos have been investigated. SCNT embryos were produced by transferring cumulus cell nuclei into enucleated B6D2F1 mouse oocytes, whereas parthenogenetically activated (PA) and fertilized embryos were derived from ICR mice by artificial activation with strontium and in vivo fertilization, respectively. SCNT embryos were inferior in their developmental capacity to blastocyst compared to either PA or fertilized embryos. SCNT embryos were transferred alone (SCNT), or cotransferred with two to three PA (SCNT + PA) or fertilized (SCNT + Fert) embryos into the oviducts of an ICR recipient. Both pregnancy and implantation rates originating from clones in the SCNT + PA group were significantly higher than those of SCNT group (p < 0.05). The weight of placentas of clones derived from SCNT, SCNT + PA, or SCNT + Fert was in all cases significantly higher than that of fertilized controls (p < 0.001). Most of the clones derived from SCNT embryos cotransferred with PA or fertilized embryos survived to adulthood and were fertile and healthy according to histopathological observations. Our results demonstrate in mouse that cotransfer of PA embryos improves the pregnancy and implantation of SCNT embryos without compromising the overall health of the resulting clones.

Stem cell sources for regenerative medicine

Tissue-resident stem cells or primitive progenitors play an integral role in homeostasis of most organ systems. Recent developments in methodologies to isolate and culture embryonic and somatic stem cells have many new applications poised for clinical and preclinical trials, which will enable the potential of regenerative medicine to be realized. Here, we overview the current progress in therapeutic applications of various stem cells and discuss technical and social hurdles that must be overcome for their potential to be realized.

Epigenetic regulation of foetal development in nuclear transfer animal models

Somatic cell nuclear transfer (SCNT, 'cloning') holds great potential for agricultural applications, generation of medical model animals, transgenic farm animals or by 'therapeutic cloning' for generating human embryonic stem cells for the treatment of human diseases. However, the low survival rate of SCNT-derived pregnancies represents a serious limitation of the current technology. In order to overcome this hurdle, a deeper understanding of the epigenetic reprogramming of the somatic cell nuclei and its effect on the pregnancy is needed. Here we review the literature on nuclear reprogramming by SCNT, including studies of gene expression, DNA methylation, chromatin remodelling, genomic imprinting and X chromosome inactivation. Reprogramming of genes expressed in the inner cell mass, from which the body of the foetus is formed, seems to be highly efficient. Defects in the extra-embryonic tissues are probably the major cause of the low success rate of reproductive cloning. Methods to partially overcome such problems exist, yet more future research is needed to find practical and efficient methods to remedy this problem. Improvement of the survival of foetuses is a central issue for the future of agricultural SCNT not only for its economic viability, but also because in lack of improvements in animal welfare current regulations can block the use of the method in the EU and several other countries.

Review Paper: A Review of the Pathology of Abnormal Placentae of Somatic Cell Nuclear Transfer Clone Pregnancies in Cattle, Sheep, and Mice

Cloning of cattle, sheep, and mice by somatic cell nuclear transfer (SCNT) can result in apparently healthy offspring, but the probability of a successful and complete pregnancy is less than 5%. Failures of SCNT pregnancy are associated with placental abnormalities, such as placentomegaly, reduced vascularisation, hypoplasia of trophoblastic epithelium, and altered basement membrane. The pathogenesis of these changes is poorly understood, but current evidence implicates aberrant reprogramming of donor nuclei by the recipient oocyte cytoplast, resulting in epigenetic modifications of key regulatory genes essential for normal placental development. The purpose of this review is to provide an overview of the anatomic pathology of abnormal placentae of SCNT clones and to summarize current knowledge concerning underlying pathogenetic mechanisms.

Gene expression profiling of the uterus with embryos cloned by somatic cell nuclear transfer on day 30 of pregnancy

Cloning by somatic cell nuclear transfer (SCNT) in pigs has great value for research and biomedical applications. However, cloning pigs is inefficient, and cloning procedures often lead to the birth of abnormal offspring because of the inadequate nuclear remodeling of donor cells as well as inadequate subsequent development. To understand the problems of the cloning process, it is necessary to understand how the uterus interacts with cloned embryo during pregnancy and supports placentation and fetal development. In this study, we compared gene expression profiles of the uterus with SCNT embryos to those of the uterus with normal embryos by natural mating. We obtained the uterine endometrial tissues on day 30 of pregnancy and conducted gene expression profiling using the Platinum Pig 13K oligonucleotide microarrays. Of the 13,610 genes analyzed, expression of 351 genes significantly increased or decreased in the uterine tissues with SCNT embryos compared to those with normal embryos. The differentially regulated genes included enzymes involved in steroidogenesis and extracellular matrix remodeling and uterine secretory proteins. Analyses of real-time reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization of selected genes confirmed the validity of the gene expression patterns observed in the microarray analysis. Results of this study showed that the transcriptional profile of the genes in the uterus with SCNT embryos was regulated differently indicating that the maternal responsiveness to the SCNT embryos was impaired, resulting in the altered gene expression in the uterus and, in turn, abnormal placental and fetal development and increased embryonic loss.

Nuclear reprogramming: what has been done and potential avenues for improvements

A major challenge for reproductive biologists is the development of novel strategies to improve cloning efficiency. Even in species for which cloning is relatively successful, like cattle, the efficiency is still unacceptably low. In this review article we critically analyse all approaches that have been suggested by different laboratories in the field so far. As will be discussed below, so far none of these gives rise to a dramatic increase in cloning efficiency. Possibly, a multi-step approach including a pre-treatment of donor cells to modify their chromatin, along with improved culture system for cloned embryos would be the most promising.

Aggregation of bovine cloned embryos at the four-cell stage stimulated gene expression and in vitro embryo development

Pre-implantation embryos produced by somatic cell nuclear transfer (SCNT) have varied developmental potentials. The majority of SCNT blastocysts do not develop to term, and the mechanisms inhibiting development are still largely unknown. Aggregation of cloned embryos has been attempted to compensate for the developmental deficiency of individual cloned embryos. In this report, we investigated the impact of aggregation of bovine cloned embryos at the four-cell stage on in vitro development and gene expression of the embryos. Cell numbers and development rate of aggregated (NTagg) and non-aggregated (NT) blastocysts were characterized and compared. The blastocyst formation after aggregation was modeled using the binominal distribution. The results indicate that aggregation enhances the blastocyst formation but does not increase the overall blastocyst rate. Additionally, utilizing microarray gene chip analysis 8.8% of 8,059 genes analyzed were differentially expressed between NTagg and NT blastocysts, with more than 80% of the differentially expressed genes up-regulated in the NTagg blastocysts. Up-regulated genes include those involved in transcription, biosynthesis and signaling such as TDGF1, HNFA, CAV1, GLU5, and CD81. Our results indicate that aggregation of bovine cloned embryos at an early stage promotes the in vitro development of the resulting pre-implantation embryos.