Somatic cell nuclear transfer: Infinite reproduction of a unique diploid genome

Extranuclear Inheritance of Mitochondrial Genome and Epigenetic Reprogrammability of Chromosomal Telomeres in Somatic Cell Cloning of Mammals

The effectiveness of somatic cell nuclear transfer (SCNT) in mammals seems to be still characterized by the disappointingly low rates of cloned embryos, fetuses, and progeny generated. These rates are measured in relation to the numbers of nuclear-transferred oocytes and can vary depending on the technique applied to the reconstruction of enucleated oocytes. The SCNT efficiency is also largely affected by the capability of donor nuclei to be epigenetically reprogrammed in a cytoplasm of reconstructed oocytes. The epigenetic reprogrammability of donor nuclei in SCNT-derived embryos appears to be biased, to a great extent, by the extranuclear (cytoplasmic) inheritance of mitochondrial DNA (mtDNA) fractions originating from donor cells. A high frequency of mtDNA heteroplasmy occurrence can lead to disturbances in the intergenomic crosstalk between mitochondrial and nuclear compartments during the early embryogenesis of SCNT-derived embryos. These disturbances can give rise to incorrect and incomplete epigenetic reprogramming of donor nuclei in mammalian cloned embryos. The dwindling reprogrammability of donor nuclei in the blastomeres of SCNT-derived embryos can also be impacted by impaired epigenetic rearrangements within terminal ends of donor cell-descended chromosomes (i.e., telomeres). Therefore, dysfunctions in epigenetic reprogramming of donor nuclei can contribute to the enhanced attrition of telomeres. This accelerates the processes of epigenomic aging and replicative senescence in the cells forming various tissues and organs of cloned fetuses and progeny. For all the above-mentioned reasons, the current paper aims to overview the state of the art in not only molecular mechanisms underlying intergenomic communication between nuclear and mtDNA molecules in cloned embryos but also intrinsic determinants affecting unfaithful epigenetic reprogrammability of telomeres. The latter is related to their abrasion within somatic cell-inherited chromosomes.

Effect of season on the in-vitro maturation and developmental competence of buffalo oocytes after somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) is a valuable technology tool with various uses in transgenic animals, regenerative medicine, and stem cell research. However, the efficiency of SCNT embryos appears to have poor developmental competency. Environmental issues may adversely affect SCNT embryos in buffalo. Thereafter, the present study aimed to explore the effect of season on the maturation of buffalo oocytes and subsequent developmental capability after parthenogenetic activation and SCNT in buffalo. Buffalo oocytes (n = 6353) were collected from local slaughterhouse at various seasons; spring (March-April), summer (May-August), autumn (September-November), and winter (December-January). A significant increase (p < 0.05) was recorded in the maturation rate (57.07%) at autumn compared with spring, summer, and winter (50.46, 50.93, and 50.66%, respectively). No significant differences were recorded in the fusion and the cleavage rates among all seasons. Blastocyst development rate was higher (p < 0.05) in autumn and winter (16.52 ± 8.45% and 15.98 ± 7.17%, respectively) than in spring and summer (9.47 ± 6.71% and 10.84 ± 6.58%, respectively) seasons. It could be concluded that the season had a significant effect on oocyte development competence which can be used for SCNT in buffalo.

A newly developed cloning technique in sturgeons; an important step towards recovering endangered species

Several steps of sturgeon somatic cell nuclear transfer (SCNT) have been recently established, but improvements are needed to make it a feasible tool to preserve the natural populations of this group of endangered species. The donor cell position inside the recipient egg seems to be crucial for its reprogramming; therefore by injecting multiple donor somatic cells instead of a single cell with a single manipulation, we increased the potential for embryo development. Using the Russian sturgeon Acipenser gueldenstaedtii as a multiple cell donor and sterlet Acipenser ruthenus as the non-enucleated egg recipient, we obtained higher proportion of eggs developing into embryos than previously reported with single-SCNT. Molecular data showed the production of a specimen (0.8%) contained only the donor genome with no contribution from the recipient, while two specimens (1.6%) showed both recipient and donor genome. These findings are the first report of donor DNA integration into a sturgeon embryo after interspecific cloning. In all, we provide evidence that cloning with the multiple donor somatic cells can be feasible in the future. Despite the fact that the sturgeon cloning faces limitations, to date it is the most promising technique for their preservation.

Strategies for improvement of cloning by somatic cell nuclear transfer

Somatic cell nuclear transfer (SCNT) is a powerful tool that is being applied in a variety of fields as diverse as the cloning and production of transgenic animals, rescue of endangered species and regenerative medicine. However, cloning efficiency is still very low and SCNT embryos generally show poor developmental competency and many abnormalities. The low efficiency is probably due to incomplete reprogramming of the donor nucleus and most of the developmental problems are thought to be caused by epigenetic defects. Applications of SCNT will, therefore, depend on improvements in the efficiency of production of healthy clones. This review has summarised the progress and strategies that have been used to make improvements in various animal species, especially over the period 2010–2017, including strategies based on histone modification, embryo aggregation and mitochondrial function. There has been considerable investiagation into the mechanisms that underpin each strategy, helping us better understand the nature of genomic reprogramming and nucleus–cytoplasm interactions.

Effects of Embryo Aggregation and PXD101 on the In Vitro Development of Mouse Somatic Cell Nuclear Transfer Embryos

To improve the cloning efficiency of somatic cell nuclear transfer (SCNT) and to establish nuclear transfer embryonic stem cells (NT-ESCs) reliably, it is necessary to produce high-quality blastocysts derived from mice SCNT embryos. Therefore, the present study aims to investigate an optimal method for mouse SCNT embryo production and NT-ESCs derivation by comparing the effects of two methods: the treatment of histone deacetylase inhibitor PXD101 after SCNT, embryo aggregation and their combination treatment. The results suggest that embryo aggregation at four-cell stage and 50 nM PXD101 treated for 10 hours during and after activation could improve both mouse SCNT embryos' development (PXD101: 40.0% vs. 18.5%; p < 0.05; aggregation: 40.2% vs. 18.5%; p < 0.05) and also enhance the isolation rate of NT-ESCs (PXD101: 38.2% vs. 12.5%; p < 0.05; aggregation: 39.0% vs. 12.5%; p < 0.05). The combination of their treatments had a higher development rate (43.6%) and significantly higher NT-ESCs isolation rate (54.7%), therefore, we concluded that the combination of these two methods (50 nM PXD101 treated for 10 hours after SCNT and then aggregated at four-cell stage) is considered as the optimal way for the in vitro development of SCNT embryo and subsequent NT-ESCs isolation in mice, providing a new approach for the practical improvement of mouse cloning techniques and opening new opportunities to improve cloning efficiencies in other species.

Mitogen-Activated Protein Kinase Activity Is Not Essential for the First Step of Nuclear Reprogramming in Bovine Somatic Cell Nuclear Transfer

For reprogramming a somatic nucleus during mammalian cloning, metaphase of the second meiotic division (MII) oocytes has been widely used as recipient cytoplasm. High activity of maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK) is believed to accelerate the remodeling and/or reprogramming of a somatic nucleus introduced into the ooplasm by somatic cell nuclear transfer. We demonstrated previously that the first step in nuclear reprogramming is not directly regulated by MPF and MAPK because activated oocytes in which MPF activity is diminished and MAPK activity is maintained can develop to the blastocyst stage after receiving an M phase somatic nucleus in bovine cloning. In this study, our aim was to test whether MAPK activity is necessary for the first step in nuclear reprogramming and/or chromatin remodeling (phosphorylation of histone H3 at Ser3, trimethylation of histone H3 at Lys 9, and acetylation of histone H3 at Lys14) in bovine somatic cloning. We found that it was not necessary, and neither was MPF activity.

Histone deacetylases inhibitors (HDACis) as novel therapeutic application in various clinical diseases

Accumulating evidence suggests that histone hypoacetylation which is partly mediated by histone deacetylase (HDAC), plays a causative role in the etiology of various clinical disorders such as cancer and central nervous diseases. HDAC inhibitors (HDACis) are natural or synthetic small molecules that can inhibit the activities of HDACs and restore or increase the level of histone acetylation, thus may represent the potential approach to treating a number of clinical disorders. This manuscript reviewed the progress of the most recent experimental application of HDACis as novel potential drugs or agents in a large number of clinical disorders including various brain disorders including neurodegenerative and neurodevelopmental cognitive disorders and psychiatric diseases like depression, anxiety, fear and schizophrenia, and cancer, endometriosis and cell reprogramming in somatic cell nuclear transfer in human and animal models of disease, and concluded that HDACis as potential novel therapeutic agents could be used alone or in adjunct to other pharmacological agents in various clinical diseases.

Disruption of Mitochondrion-To-Nucleus Interaction in Deceased Cloned Piglets

Most animals produced by somatic cell nuclear transfer (SCNT) are heteroplasmic for mitochondrial DNA (mtDNA). Oxidative phosphorylation (OXPHOS) in clones therefore requires the coordinated expression of genes encoded by the nuclear DNA and the two sources of mitochondria. Such interaction is rarely studied because most clones are generated using slaughterhouse oocytes of unrecorded origin. Here we traced the maternal lineages of seven diseased and five one-month-old live cloned piglets by sequencing their mtDNA. Additionally by using a 13K oligonucleotide microarray, we compared the expression profiles of nuclear and mtDNA-encoded genes that are involved in mitochondrial functions and regulation between the cloned groups and their age-matched controls (n=5 per group). We found that the oocytes used to generate the cloned piglets were of either the Large White or Duroc background, and oocyte genetic background was not related to the clones’ survival. Expression profiles of mtDNA-encoded genes in clones and controls showed intermixed clustering patterns without treatment or maternal lineage-dependency. In contrast, clones and controls clustered separately for their global and nuclear DNA-encoded mitochondrial genes in the lungs for both the deceased and live groups. Functional annotation of differentially expressed genes encoded by both nuclear and mtDNA revealed abnormal gene expression in the mitochondrial OXPHOS pathway in deceased clones. Among the nine differentially expressed genes of the OXPHOS pathway, seven were down-regulated in deceased clones compared to controls, suggesting deficiencies in mitochondrial functions. Together, these data demonstrate that the coordination of expression of mitochondrial genes encoded by nuclear and mtDNA is disrupted in the lung of diseased clones.

Using somatic-cell nuclear transfer to study aging

In mammals, a diploid genome following fertilization of haploid cells, an egg, and a spermatozoon is unique and irreproducible. This implies that the generated unique diploid genome is doomed with the individual's inevitable demise. Since it was first reported in 1997 that Dolly the sheep had been cloned, many mammalian species have been cloned successfully using somatic-cell nuclear transfer (SCNT). The success of SCNT in mammals enables us not only to reproduce offspring without germ cells, that is, to "passage" a unique diploid genome, but also to address valuable biological questions on development, nuclear reprogramming, and epigenetic memory. Successful cloning can also support epigenetic reprogramming where the aging clock is reset or reversed. Recent work using iPS cell technology has explored the practicality and led to the recapitulation of premature aging with iPSCs from progeroid laminopathies. As a result, reprogramming tools are also expected to contribute to studying biological age. However, the efficiency of animal cloning is still low in most cases and the mechanism of reprogramming in cloned embryos is still largely unclear. Here, based on recent advances, we describe an improved, more efficient mouse cloning protocol using histone deacetylase inhibitors (HDACis) and latrunculin A, which increases the success rates of producing cloned mice or establishing ES cells fivefold. This improved method of cloning will provide a strong tool to address many issues including biological aging more easily and with lower cost.

The state of the art for pluripotent stem cells derivation in domestic ungulates

Since the successful isolation, characterization and long-term culture of embryonic stem cells (ESCs) from mice in the early 1980s and from humans a decade later, considerable effort has been made to establish ESCs lines from livestock. The derivation of validated ESCs lines is a necessary step if the generation of economically relevant transgenic animals is to be achieved. However, this is still elusive, as the isolation of true ESCs lines for livestock has not been accomplished to date. It has been demonstrated that by forced expression of a defined set of transcription factors, it is possible to reprogram somatic cells to cells that closely resemble an ES-like state. These cells were termed induced pluripotent stem cells (iPSCs). We introduce the basic concepts relating to stem cell biology and give an overview of the various attempts to isolate and generate pluripotent stem cells (PSCs) from species relevant to livestock production. Further, we point out the issues to be addressed and hurdles to be overcome to realize the promise of stem cells in agriculture.

Aging in the mouse and perspectives of rejuvenation through induced pluripotent stem cells (iPSCs)

The mouse is a perfect model to study aging in mammals. It has a relatively short life span and genetic manipulations in this species are well established. Most interestingly, the mouse is a fantastic tool to produce stem cells. Forced expression of only four transcription factors (Oct3/4, Sox2, Klf4, and c-Myc) in murine and human somatic cells resets the expression of genes that are characteristic of differentiated cells and consequently induces the formation of pluripotent stem cells (iPSCs). This technology opens new and exciting possibilities in medical research, especially personalized cell therapies for treating human disease. To treat damaged tissues or repair organs in elderly patients, it will be necessary to establish iPSCs from their tissues. To determine the feasibility of using this technology with elderly patients, we asked whether it was indeed possible to establish iPSCs from the tissues of aged mice and to differentiate them to tissue cells. We succeeded in establishing iPSC clones using bone marrow (BM) from 21-month-old EGFP-C57BL/6 mice, which had been cultured for 4 days in the presence of granulocyte macrophage-colony stimulating factor (GM-CSF). Our iPSCs from aged mice (aged iPSCs) and those from mouse embryonic fibroblasts (MEFs) strongly expressed SSEA-1 and Pou5f1, and showed strong alkaline phosphatase (AP) activity. Our aged iPSCs made teratomas when injected into the back skin of syngeneic mice, and differentiated to tissue cells of three germ lines in vitro. Further experiments to make chimeric mice and germ line cells will determine whether the aged iPSCs possess the properties of much younger cells and are capable of regenerating aged mice.

Spontaneous reversal of the developmental aging of normal human cells following transcriptional reprogramming

AIM

To determine whether transcriptional reprogramming is capable of reversing the developmental aging of normal human somatic cells to an embryonic state.

MATERIALS & METHODS

An isogenic system was utilized to facilitate an accurate assessment of the reprogramming of telomere restriction fragment (TRF) length of aged differentiated cells to that of the human embryonic stem (hES) cell line from which they were originally derived. An hES-derived mortal clonal cell strain EN13 was reprogrammed by SOX2, OCT4 and KLF4. The six resulting induced pluripotent stem (iPS) cell lines were surveyed for telomere length, telomerase activity and telomere-related gene expression. In addition, we measured all these parameters in widely-used hES and iPS cell lines and compared the results to those obtained in the six new isogenic iPS cell lines.

RESULTS

We observed variable but relatively long TRF lengths in three widely studied hES cell lines (16.09-21.1 kb) but markedly shorter TRF lengths (6.4-12.6 kb) in five similarly widely studied iPS cell lines. Transcriptome analysis comparing these hES and iPS cell lines showed modest variation in a small subset of genes implicated in telomere length regulation. However, iPS cell lines consistently showed reduced levels of telomerase activity compared with hES cell lines. In order to verify these results in an isogenic background, we generated six iPS cell clones from the hES-derived cell line EN13. These iPS cell clones showed initial telomere lengths comparable to the parental EN13 cells, had telomerase activity, expressed embryonic stem cell markers and had a telomere-related transcriptome similar to hES cells. Subsequent culture of five out of six lines generally showed telomere shortening to lengths similar to that observed in the widely distributed iPS lines. However, the clone EH3, with relatively high levels of telomerase activity, progressively increased TRF length over 60 days of serial culture back to that of the parental hES cell line.

CONCLUSION

Prematurely aged (shortened) telomeres appears to be a common feature of iPS cells created by current pluripotency protocols. However, the spontaneous appearance of lines that express sufficient telomerase activity to extend telomere length may allow the reversal of developmental aging in human cells for use in regenerative medicine.

Chromosomal and telomeric reprogramming following ES-somatic cell fusion

Chromosomal and telomeric reprogramming was assessed in intraspecies hybrids obtained by fusion of embryonic stem (ES) cells and mouse embryonic fibroblasts. Evaluation of the ploidy of ES-somatic hybrids revealed that 21 of 59 clones had a tetraploid DNA profile while the remaining clones showed deviations from the expected profile of fusion between two diploid cells. Microsatellite polymerase chain reaction analysis of four of these clones demonstrated no random loss of somatic chromosome pairs in the ES-somatic cell hybrids. Pluripotential of ES-somatic hybrids was assessed by gene expression analysis, antibody staining for Oct4 and SSEA-1 and teratoma formation containing derivatives of the three germ layers. Reprogramming of telomeric maintenance was observed with ES-somatic hybrids showing high telomerase activity and increased telomere lengths. However, we detected no significant increase in the expression of the three critical telomerase subunits: telomerase reverse transcriptase (TERT), telomerase RNA component (TERC), and dyskerin. This indicates that activation of telomerase and telomere maintenance is not reliant on changes in gene expression of TERT, TERC, and dyskerin following ES-somatic cell fusion or sister chromatid recombination and may arise through elimination of negative regulation of telomerase activity. This is the first demonstration of telomere lengthening following cell fusion and offers a new model for studying and identifying new regulators of telomere maintenance.

[Immunofluorescence patterns of histone H3 lysine 4 dimethylation on X chromosomes of different cloned cattle]

Using immunofluorescence staining, the patterns of histone H3 K4m2 of metaphase X chromosomes were measured in cloned cattle derived from FFB and FOV donor cells. The results demonstrated that the modification pattern of H3 K4m2 in the ear tissue of all clones was largely consistent with that of the donor cell line FFB and conventionally produced cattle, but different from that of the donor cell line FOV.

Altered gene expression in cloned piglets

Studies on cloned pigs are scant compared with those in mice and cattle. Expression profiles of cloned pig embryos on full-term cloned pigs are even more limited owing to the limited availability of DNA microarray technology in the pig. We have conducted expression profile comparisons between pigs from somatic cell nuclear transfer and pigs from conventional breeding at birth and 1 month of age. Differentially expressed genes that are subjected to DNA methylation were also examined for their DNA methylation status. These data will be presented in the 2009 Annual Meeting of the International Embryo Transfer Society in San Diego. In the present review, we focus on summarising existing findings on epigenetic and other changes in cloned embryo, cloned pigs and their offspring by conventional breeding.