Effects of Nuclear Transfer Procedures on ES Cell Cloning Efficiency in the Mouse

Double cytoplast embryonic cloning improves in vitro but not in vivo development from mitotic pluripotent cells in cattle

Cloning multiple animals from genomically selected donor embryos is inefficient but would accelerate genetic gain in dairy cattle breeding. To improve embryo cloning efficiency, we explored the idea that epigenetic reprogramming improves when donor cells are in mitosis. We derived primary cultures from bovine inner cell mass (ICM) cells of in vitro fertilized (IVF) embryos. Cells were grown feeder-free in a chemically defined medium with increased double kinase inhibition (2i⁺). Adding recombinant bovine interleukin 6 to 2i⁺ medium improved plating efficiency, outgrowth expansion, and expression of pluripotency-associated epiblast marker genes (NANOG, FGF4, SOX2, and DPPA3). For genotype multiplication by embryonic cell transfer (ECT) cloning, primary colonies were treated with nocodazole, and single mitotic donors were harvested by mechanical shake-off. Immunofluorescence against phosphorylated histone 3 (P-H3) showed 37% of nocodazole-treated cells in metaphase compared to 6% in DMSO controls (P < 1 × 10⁻⁵), with an average of 53% of P-H3-positive cells expressing the pluripotency marker SOX2. We optimized several parameters (fusion buffer, pronase treatment, and activation timing) for ECT with mitotic embryonic donors. Sequential double cytoplast ECT, whereby another cytoplast was fused to the first cloned reconstruct, doubled cloned blastocyst development and improved morphological embryo quality. However, in situ karyotyping revealed that over 90% of mitotic ECT-derived blastocysts were tetraploid or aneuploid with extra chromosomes, compared to less than 2% in the original ICM donor cells. Following the transfer of single vs. double cytoplast embryos, there was no difference between the two methods in pregnancy establishment at D35 (1/22 = 5% vs. 4/53 = 8% for single vs. double ECT, respectively). Overall, post-implantation development was drastically reduced from embryonic mitotic clones when compared to somatic interphase clones and IVF controls. We conclude that mitotic donors cause ploidy errors during in vitro development that cannot be rescued by enhanced epigenetic reprogramming through double cytoplast cloning.

Mitosis gives a brief window of opportunity for a change in gene transcription

In mitotic nuclei transplant experiments, many genes undergo major changes in gene expression. This supports the idea that mitosis facilitates new cell fate decisions during normal development.

Cell differentiation is remarkably stable but can be reversed by somatic cell nuclear transfer, cell fusion, and iPS. Nuclear transfer to amphibian oocytes provides a special opportunity to test transcriptional reprogramming without cell division. We show here that, after nuclear transfer to amphibian oocytes, mitotic chromatin is reprogrammed up to 100 times faster than interphase nuclei. We find that, as cells traverse mitosis, their genes pass through a temporary phase of unusually high responsiveness to oocyte reprogramming factors (mitotic advantage). Mitotic advantage is not explained by nuclear penetration, DNA modifications, histone acetylation, phosphorylation, methylation, nor by salt soluble chromosomal proteins. Our results suggest that histone H2A deubiquitination may account, at least in part, for the acquisition of mitotic advantage. They support the general principle that a temporary access of cytoplasmic factors to genes during mitosis may facilitate somatic cell nuclear reprogramming and the acquisition of new cell fates in normal development.

Cells are dividing very actively at a time in development when new gene expression and new cell lineages arise. At mitosis, most transcription factors are temporarily displaced from chromosomes. We show that, after transplantation to oocytes, somatic cell nuclei that have been synchronized in mitosis can be reprogrammed to pluripotency gene expression up to 100 times faster than interphase nuclei. We find that, as cells traverse mitosis, their genes pass through a temporary phase of unusually high responsiveness to oocyte reprogramming factors (mitotic advantage). Many other genes in the genome have also shown a mitotic advantage, which affects the rate rather than the final level of transcriptional enhancement. This is attributable to a chromatin state rather than to more rapid passage of reprogramming factors through the nuclear membrane. Histone H2A deubiquitination at mitosis is required for the acquisition of mitotic advantage. Our results support the general principle that a temporary access of cytoplasmic factors to genes during mitosis facilitates somatic cell nuclear reprogramming and the acquisition of new cell fates in normal development.

Effect of human chorionic gonadotropin and progesterone administration on the developmental potential of mouse somatic cell nuclear-transferred oocytes

Somatic cell nuclear-transferred (SCNT) oocytes have a relatively high potential to develop into blastocysts in vitro, but a large proportion embryos die at various pre- and postimplantation stages after transfer to recipients. Although the reason for the high mortality of SCNT embryos at the peri- and postimplantation stages is not clear, epigenetic abnormalities of SCNT embryos are considered to be the main cause. Such abnormalities of SCNT embryos may decrease their ability to maintain the corpora lutea, which is necessary for initiating implantation and maintaining fetal development. To examine this hypothesis, human chorionic gonadotropin (hCG) and progesterone were administered at different times to recipients that received SCNT embryos. When hCG was administered daily from day 3.5 to day 6.5 of pregnancy, the implantation and fetal development rates increased significantly compared to those of controls. The potential of SCNT embryos to develop to full term, however, was not greater than that of controls, even if hCG administration was continued to day 11.5 or day 17.5 and progesterone was administered from day 7.5 to day 17.5 after hCG injection. These findings demonstrated that administering hCG to recipients protects the in vivo development of SCNT embryos until day 10.5, but other treatment is necessary to support the progression of the embryos to full-term development.

The developmental potential of mouse somatic cell nuclear-transferred oocytes treated with trichostatin A and 5-aza-2′-deoxycytidine

To facilitate nuclear reprogramming, somatic cells or somatic cell nuclear-transferred (SCNT) oocytes have been treated with the histone deacetylase inhibitor trichostatin A (TSA), or the DNA methyltransferase inhibitor, 5-aza-2′-deoxycytidine (5-aza-dC), to relax epigenetic marks of differentiated somatic cells. TSA-treated SCNT oocytes have increased developmental potential, but the optimal treatment period is unknown. Reduced methylation levels in somatic cells have no positive effect on SCNT oocytes, but the treatment of SCNT embryos with 5-aza-dC has not been investigated. We examined the effect of TSA treatment duration on the developmental potential of mouse SCNT oocytes and the effect of 5-aza-dC treatment on their in vitro and in vivo developmental potential. To determine the effects of TSA treatment duration, nuclear-transferred (NT) oocytes were cultured for 0 to 26 h with 100 nM TSA. SCNT oocytes treated with TSA for 8 to 12 h had the higher rate of development to blastocysts and full-term fetuses were obtained after treatment for 8 to 12 h. When oocytes were treated for 14 h and 26 h, blastocyst rates were significantly decreased and fetuses were not obtained. To examine the effect of 5-aza-dC, 2-cell stage SCNT embryos were cultured with 10 or 100 nM 5-aza-dC for 48 h to the morula stage and transferred. The potential of embryos treated with 5-aza-dC to develop into blastocysts was decreased and no fetuses were obtained after transfer. The findings demonstrated that long-term TSA treatment of SCNT mouse oocytes and treatment with 5-aza-dC inhibit the potential to develop into blastocysts and to fetuses after transfer.

Efficient production of nuclear transferred rat embryos by modified methods of reconstruction

In this study we investigated spontaneous oocyte activation and developmental ability of rat embryos of the SD-OFA substrain. We also tried to improve the somatic cell nuclear transfer (SCNT) technique in the rat by optimizing methods for the production of reconstructed embryos. About 20% of oocytes extruded the second polar body after culture for 3 hr in vitro and 84% of oocytes were at the MII stage. MG132 blocked spontaneous activation but decreased efficiency of parthenogenetic activation. Pronuclear formation was more efficient in strontium-activated oocytes (66.1-80.9%) compared to roscovitine activation (24.1-54.5%). Survival rate after enucleation was significantly higher (89.4%) after slitting the zona pellucida and then pressing the oocyte with a holding pipette in medium without cytochalasin B (CB) compared to the conventional protocol using aspiration of the chromosomes after CB treatment (67.7%). Exposure of rat ova to UV light for 30 sec did not decrease their in vitro developmental capacity. Intracytoplasmic cumulus cell injection dramatically decreased survival rate of oocytes (42%). In contrast, 75.9% of oocytes could be successfully electrofused. Development to the 2-cell stage was reduced after SCNT (24.6% compared 94.6% in controls) and none from 244 reconstructed embryos developed in vitro beyond this stage. After overnight in vitro culture, 74.4% of the SCNT embryos survived and 56.1% formed pronuclei. The pregnancy rate of 33 recipients after the transfer of 695 of these cloned embryos was, however, very low (18.2%) and only six implantation sites could be detected (0.9%) without any live fetuses and offspring.

The effects of trichostatin A on mRNA expression of chromatin structure-, DNA methylation-, and development-related genes in cloned mouse blastocysts

Trichostatin A (TSA) is the most potent histone deacetylase (HDAC) inhibitor known. We previously reported that treatment of mouse somatic cell nuclear-transferred (SCNT) oocytes with TSA significantly increased the blastocyst rate, blastocyst cell number, and full-term development. How TSA enhances the epigenetic remodeling ability of somatic nuclei and the expression of development-related genes, however, is not known. In the present study, we compared the expression patterns of nine genes involved in chromatin structure and DNA methylation, and seven development-related genes in blastocysts developed from SCNT oocytes treated with and without TSA, and in blastocysts developed in vivo and in vitro using real-time reverse transcription-polymerase chain reaction. In vivo-recovered blastocysts and blastocysts developed from TSA-treated SCNT oocytes exhibited similar expression patterns for Hdac1, 2, and 3, CBP, PCAF, and Dnmt3b genes compared with in vitro-developed blastocysts and blastocysts developed from SCNT oocytes without TSA treatment. There were significantly lower expression levels of Hdac1 and Hdac2 transcripts in TSA-treated and in vivo-recovered blastocysts than in TSA-untreated and in vitro-developed blastocysts. The finding that TSA treatment of SCNT oocytes significantly upregulated Sox2 and cMyc transcripts in blastocysts indicated that both transcripts are TSA-responsive genes. Thus, TSA treatment of mouse SCNT oocytes decreased the expression of chromatin structure- and DNA methylation-related genes, and increased the expression of Sox2 and cMyc genes in blastocysts. Such modifications might be a reason for the high developmental potential of mouse SCNT oocytes treated with TSA.

Identification of genes aberrantly expressed in mouse embryonic stem cell-cloned blastocysts

During development, cloned embryos often undergo embryonic arrest at any stage of embryogenesis, leading to diverse morphological abnormalities. The long-term effects resulting from embryo cloning procedures would manifest after birth as early death, obesity, various functional disorders, and so forth. Despite extensive studies, the parameters affecting the developmental features of cloned embryos remain unclear. The present study carried out extensive gene expression analysis to screen a cluster of genes aberrantly expressed in embryonic stem cell-cloned blastocysts. Differential screening of cDNA subtraction libraries revealed 224 differentially expressed genes in the cloned blastocysts: eighty-five were identified by the BLAST search as known genes performing a wide range of functions. To confirm their differential expression, quantitative gene expression analyses were performed by real-time PCR using single blastocysts. The genes Skp1a, Canx, Ctsd, Timd2, and Psmc6 were significantly up-regulated, whereas Aqp3, Ak3l1, Rhot1, Sf3b3, Nid1, mt-Rnr2, mt-Nd1, mt-Cytb, and mt-Co2 were significantly down-regulated in the majority of embryonic stem cell-cloned embryos. Our results suggest that an extraordinarily high frequency of multiple functional disorders caused by the aberrant expression of various genes in the blastocyst stage is involved in developmental arrest and various other disorders in cloned embryos.

Piezo-assisted nuclear transfer affects cloning efficiency and may cause apoptosis

Even though it generates healthy adults, nuclear transfer in mammals remains an inefficient process. Mainly attributed to abnormal reprograming of the donor chromatin, this inefficiency may also be caused at least partly by a specific effect of the cloning technique which has not yet been well investigated. There are two main procedures for transferring nuclei into enucleated oocytes: fusion and piezoelectric microinjection, the latter being used mostly in mice. We have, therefore, decided to compare the quality and the developmental ability, both in vivo and in vitro, of embryos reconstructed with electrofusion or piezoelectric injection. In addition, the effect of piezo setups of differing electric strengths was investigated. Along with the record of the rate of development, we compared the nuclear integrity in the blastomeres during the first cleavages as well as the morphological and cellular quality of the blastocysts. Our results show that the piezo-assisted micromanipulation can induce DNA damage in the reconstructed embryos, apoptosis, and reduced cell numbers in blastocysts as well as a lower rate of development to term. Even if piezo-driven injection facilitates a faster and more efficient rate of reconstruction, it should be used with precaution and with as low parameters as possible.

Donor cell differentiation, reprogramming, and cloning efficiency: elusive or illusive correlation?

Compared to other assisted reproductive technologies, mammalian nuclear transfer (NT) cloning is inefficient in generating viable offspring. It has been postulated that nuclear reprogramming and cloning efficiency can be increased by choosing less differentiated cell types as nuclear donors. This hypothesis is mainly supported by comparative mouse cloning experiments using early blastomeres, embryonic stem (ES) cells, and terminally differentiated somatic donor cells. We have re-evaluated these comparisons, taking into account different NT procedures, the use of donor cells from different genetic backgrounds, sex, cell cycle stages, and the lack of robust statistical significance when post-blastocyst development is compared. We argue that while the reprogrammability of early blastomeres appears to be much higher than that of somatic cells, it has so far not been conclusively determined whether differentiation status affects cloning efficiency within somatic donor cell lineages.

Aging of recipient oocytes reduces the development of cloned embryos receiving cumulus cells

Parthenogenetic activation is an important factor in successful production of cloned mammals. Because it has been reported that aged oocytes are more sensitive to parthenogenetic activation than young oocytes, the present study examined the effects of oocyte aging on the in vitro and in vivo developmental potential of nuclear-transferred (NT) mouse oocytes receiving cumulus cells. The potentials of young NT oocytes (14 h after human chorionic gonadotrophin [hCG] injection) to develop into blastocysts was, however, significantly higher than that of aged oocytes (20 h after hCG injection; 16% vs 6%). When the nuclei of NT oocytes at the 2-cell stage were fused with enucleated fertilized 2-cell embryos, the potentials of the serial NT embryos to develop into blastocysts were no different for both young and aged oocytes (74% vs 74%). Live young, however, were obtained only after transfer of serial NT blastocysts developed from young NT oocytes (2%). In contrast to a report using embryonic nuclei as the nuclear donors, the results of the present study indicate that young oocytes are superior to aged oocytes as a source of recipient cytoplasm for mouse somatic cell cloning.

Cloning cattle: the methods in the madness

Somatic cell nuclear transfer (SCNT) is much more widely and efficiently practiced in cattle than in any other species, making this arguably the most important mammal cloned to date. While the initial objective behind cattle cloning was commercially driven--in particular to multiply genetically superior animals with desired phenotypic traits and to produce genetically modified animals-researchers have now started to use bovine SCNT as a tool to address diverse questions in developmental and cell biology. In this paper, we review current cattle cloning methodologies and their potential technical or biological pitfalls at any step of the procedure. In doing so, we focus on one methodological parameter, namely donor cell selection. We emphasize the impact of epigenetic and genetic differences between embryonic, germ, and somatic donor cell types on cloning efficiency. Lastly, we discuss adult phenotypes and fitness of cloned cattle and their offspring and illustrate some of the more imminent commercial cattle cloning applications.

The role of early embryonic environment on epigenotype and phenotype

The influence of epigenetic modifications to the genome on the phenotype of the adult organism is now a tractable problem in biology. This has come about through the development of methods that enable us to study the methylation state of the DNA and the packaging of the chromatin at specific gene loci. It is becoming clear that early embryogenesis is a critical period for the establishment of the epigenotype. Furthermore, it appears that this process is sensitive to environmental conditions. This is a concern in light of the increasing use of artificial reproductive technologies throughout the world.

Irreversible barrier to the reprogramming of donor cells in cloning with mouse embryos and embryonic stem cells

Somatic cloning does not always result in ontogeny in mammals, and development is often associated with various abnormalities and embryo loss with a high frequency. This is considered to be due to aberrant gene expression resulting from epigenetic reprogramming errors. However, a fundamental question in this context is whether the developmental abnormalities reported to date are specific to somatic cloning. The aim of this study was to determine the stage of nuclear differentiation during development that leads to developmental abnormalities associated with embryo cloning. In order to address this issue, we reconstructed cloned embryos using four- and eight-cell embryos, morula embryos, inner cell mass (ICM) cells, and embryonic stem cells as donor nuclei and determined the occurrence of abnormalities such as developmental arrest and placentomegaly, which are common characteristics of all mouse somatic cell clones. The present analysis revealed that an acute decline in the full-term developmental competence of cloned embryos occurred with the use of four- and eight-cell donor nuclei (22.7% vs. 1.8%) in cases of standard embryo cloning and with morula and ICM donor nuclei (11.4% vs. 6.6%) in serial nuclear transfer. Histological observation showed abnormal differentiation and proliferation of trophoblastic giant cells in the placentae of cloned concepti derived from four-cell to ICM cell donor nuclei. Enlargement of placenta along with excessive proliferation of the spongiotrophoblast layer and glycogen cells was observed in the clones derived from morula embryos and ICM cells. These results revealed that irreversible epigenetic events had already started to occur at the four-cell stage. In addition, the expression of genes involved in placentomegaly is regulated at the blastocyst stage by irreversible epigenetic events, and it could not be reprogrammed by the fusion of nuclei with unfertilized oocytes. Hence, developmental abnormalities such as placentomegaly as well as embryo loss during development may occur even in cloned embryos reconstructed with nuclei from preimplantation-stage embryos, and these abnormalities are not specific to somatic cloning.

Comparison of in vitro development of porcine nuclear-transferred oocytes receiving fetal somatic cells by injection and fusion methods

The present study compared the potential of nuclear-transferred porcine oocytes receiving fetal somatic cells by direct injection and cell fusion procedures to develop into blastocysts. After brief treatment of in vitro matured oocytes with demecolcine and sucrose, the protrusion containing the condensed chromosome mass was mechanically removed. Single donor cells were fused with enucleated oocytes following electric pulses or injected into oocytes by piezo-actuated microinjection. The reconstruction rate by direct injection was significantly higher than that following cell fusion (89 vs. 48%). The potential of nuclear-transferred oocytes to develop into blastocysts, however, was not different between injection and fusion methods (13% vs. 18%). Total cell number, inner cell mass, and trophectoderm cell numbers of cloned blastocysts were also not different between the two groups.

Comparative analysis of development-related gene expression in mouse preimplantation embryos with different developmental potential

The potential of embryonic and somatic cell nuclear-transferred (NT) mouse oocytes to develop into young is low compared with bovine NT oocytes. To examine the reasons for the low developmental potential of NT mouse oocytes, we analyzed the gene expression patterns of six development-related genes (Oct4, Nanog, Stat3, stella, FGF4, and Sox2) during preimplantation development in manipulated oocytes with different potentials to develop into young using real-time polymerase chain reaction (PCR) methods. The manipulated oocytes were parthenogenetically activated oocytes and embryonic stem cell, cumulus cell, morula blastomere NT oocytes, and in vitro-cultured and in vivo-recovered embryos. The mRNA expression patterns in mouse NT-derived embryos markedly differed from in vivo and in vitro counterparts. Some transcript expression patterns in embryonic stem-cell NT oocytes resembled those of parthenogenetic oocytes. Of the six developmentally important transcripts examined in NT embryos, four had a downregulated expression pattern at the blastocyst stage. Our findings indicate that abnormal expression patterns of development-related genes during preimplantation development correlate with the low potential of NT oocytes to develop into young. Although more detailed information is required, Sox2 mRNA expression pattern in blastocysts seems to closely correlate with the developmental potential of NT embryos.