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

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.

Defective chromatin architectures in embryonic stem cells derived from somatic cell nuclear transfer impair their differentiation potentials

Nuclear transfer embryonic stem cells (ntESCs) hold enormous promise for individual-specific regenerative medicine. However, the chromatin states of ntESCs remain poorly characterized. In this study, we employed ATAC-seq and Hi-C techniques to explore the chromatin accessibility and three-dimensional (3D) genome organization of ntESCs. The results show that the chromatin accessibility and genome structures of somatic cells are re-arranged to ESC-like states overall in ntESCs, including compartments, topologically associating domains (TADs) and chromatin loops. However, compared to fertilized ESCs (fESCs), ntESCs show some abnormal openness and structures that have not been reprogrammed completely, which impair the differentiation potential of ntESCs. The histone modification H3K9me3 may be involved in abnormal structures in ntESCs, including incorrect compartment switches and incomplete TAD rebuilding. Moreover, ntESCs and iPSCs show high similarity in 3D genome structures, while a few differences are detected due to different somatic cell origins and reprogramming mechanisms. Through systematic analyses, our study provides a global view of chromatin accessibility and 3D genome organization in ntESCs, which can further facilitate the understanding of the similarities and differences between ntESCs and fESCs.

Livestock pluripotency is finally captured in vitro

Pluripotent stem cells (PSCs) have demonstrated great utility in improving our understanding of mammalian development and continue to revolutionise regenerative medicine. Thanks to the improved understanding of pluripotency in mice and humans, it has recently become feasible to generate stable livestock PSCs. Although it is unlikely that livestock PSCs will be used for similar applications as their murine and human counterparts, new exciting applications that could greatly advance animal agriculture are being developed, including the use of PSCs for complex genome editing, cellular agriculture, gamete generation and invitro breeding schemes.

Targeted histone demethylation improves somatic cell reprogramming into cloned blastocysts but not postimplantation bovine concepti†

Correct reprogramming of epigenetic marks in the donor nucleus is a prerequisite for successful cloning by somatic cell transfer (SCT). In several mammalian species, repressive histone (H) lysine (K) trimethylation (me3) marks, in particular H3K9me3, form a major barrier to somatic cell reprogramming into pluripotency and totipotency. We engineered bovine embryonic fibroblasts (BEFs) for the doxycycline-inducible expression of a biologically active, truncated form of murine Kdm4b, a demethylase that removes H3K9me3 and H3K36me3 marks. Upon inducing Kdm4b, H3K9me3 and H3K36me3 levels were reduced about 3-fold and 5-fold, respectively, compared with noninduced controls. Donor cell quiescence has been previously associated with reduced somatic trimethylation levels and increased cloning efficiency in cattle. Simultaneously inducing Kdm4b expression (via doxycycline) and quiescence (via serum starvation) further reduced global H3K9me3 and H3K36me3 levels by a total of 18-fold and 35-fold, respectively, compared with noninduced, nonstarved control fibroblasts. Following SCT, Kdm4b-BEFs reprogrammed significantly better into cloned blastocysts than noninduced donor cells. However, detrimethylated donors and sustained Kdm4b-induction during embryo culture did not increase the rates of postblastocyst development from implantation to survival into adulthood. In summary, overexpressing Kdm4b in donor cells only improved their reprogramming into early preimplantation stages, highlighting the need for alternative experimental approaches to reliably improve somatic cloning efficiency in cattle.

Porcine Fetal-Derived Fibroblasts Alter Gene Expression and Mitochondria to Compensate for Hypoxic Stress During Culture

The Warburg effect is characterized by decreased mitochondrial oxidative phosphorylation and increased glycolytic flux in adequate oxygen. The preimplantation embryo has been described to have characteristics of the Warburg effect, including similar changes in gene expression and mitochondria, which are more rudimentary in appearance. We hypothesized hypoxia would facilitate anaerobic glycolysis in fibroblasts thereby promoting gene expression and media metabolite production reflecting the Warburg effect hallmarks in early embryos. Additionally, we speculated that hypoxia would induce a rudimentary small mitochondrial phenotype observed in several cell types evidenced to demonstrate the Warburg effect. While many have examined the role hypoxia plays in pathological conditions, few studies have investigated changes in primary cells which could be used in somatic cell nuclear transfer. We found that cells grown in 1.25% O₂ had normal cell viability and more, but smaller mitochondria. Several hypoxia-inducible genes were identified, including seven genes for glycolytic enzymes. In conditioned media from hypoxic cells, the quantities of gluconolactone, cytosine, and uric acid were decreased indicating higher consumption than control cells. These results indicate that fibroblasts alter gene expression and mitochondria to compensate for hypoxic stress and maintain viability. Furthermore, the metabolic changes observed, making them more similar to preimplantation embryos, could be facilitating nuclear reprogramming making these cells more amendable to future use in somatic cell nuclear transfer.

Improvement of in vitro and early in utero porcine clone development after somatic donor cells are cultured under hypoxia

Genetically engineered pigs serve as excellent biomedical and agricultural models. To date, the most reliable way to generate genetically engineered pigs is via somatic cell nuclear transfer (SCNT), however, the efficiency of cloning in pigs is low (1-3%). Somatic cells such as fibroblasts frequently used in nuclear transfer utilize the tricarboxylic acid cycle and mitochondrial oxidative phosphorylation for efficient energy production. The metabolism of somatic cells contrasts with cells within the early embryo, which predominately use glycolysis. We hypothesized that fibroblast cells could become blastomere-like if mitochondrial oxidative phosphorylation was inhibited by hypoxia and that this would result in improved in vitro embryonic development after SCNT. In a previous study, we demonstrated that fibroblasts cultured under hypoxic conditions had changes in gene expression consistent with increased glycolytic/gluconeogenic metabolism. The goal of this pilot study was to determine if subsequent in vitro embryo development is impacted by cloning porcine embryonic fibroblasts cultured in hypoxia. Here we demonstrate that in vitro measures such as early cleavage, blastocyst development, and blastocyst cell number are improved (4.4%, 5.5%, and 17.6 cells, respectively) when donor cells are cultured in hypoxia before nuclear transfer. Survival probability was increased in clones from hypoxic cultured donors compared to controls (8.5 vs. 4.0 ± 0.2). These results suggest that the clones from donor cells cultured in hypoxia are more developmentally competent and this may be due to improved nuclear reprogramming during somatic cell nuclear transfer.

Pharmacologic treatment of donor cells induced to have a Warburg effect-like metabolism does not alter embryonic development in vitro or survival during early gestation when used in somatic cell nuclear transfer in pigs

Somatic cell nuclear transfer is a valuable technique for the generation of genetically engineered animals, however, the efficiency of cloning in mammalian species is low (1-3%). Differentiated somatic cells commonly used in nuclear transfer utilize the tricarboxylic acid cycle and cellular respiration for energy production. Comparatively the metabolism of somatic cells contrasts that of the cells within the early embryos which predominately use glycolysis. Early embryos (prior to implantation) are evidenced to exhibit characteristics of a Warburg Effect (WE)-like metabolism. We hypothesized that pharmacologically driven fibroblast cells can become more blastomere-like and result in improved in vitro embryonic development after SCNT. The goals were to determine if subsequent in vitro embryo development is impacted by (1) cloning pharmacologically treated donor cells pushed to have a WE-like metabolism or (2) culturing non-treated donor clones with pharmaceuticals used to push a WE-like metabolism. Additionally, we investigated early gestational survival of the donor-treated clone embryos. Here we demonstrate that in vitro development of clones is not hindered by pharmacologically treating either the donor cells or the embryos themselves with CPI, PS48, or the combination of these drugs. Furthermore, these experiments demonstrate that early embryos (or at least in vitro produced embryos) have a low proportion of mitochondria which have high membrane potential and treatment with these pharmaceuticals does not further alter the mitochondrial function in early embryos. Lastly, we show that survival in early gestation was not different between clones from pharmacologically induced WE-like donor cells and controls.

Molecular features of cellular reprogramming and development

Differentiating somatic cells are progressively restricted to specialized functions during ontogeny, but they can be experimentally directed to form other cell types, including those with complete embryonic potential. Early nuclear reprogramming methods, such as somatic cell nuclear transfer (SCNT) and cell fusion, posed significant technical hurdles to precise dissection of the regulatory programmes governing cell identity. However, the discovery of reprogramming by ectopic expression of a defined set of transcription factors, known as direct reprogramming, provided a tractable platform to uncover molecular characteristics of cellular specification and differentiation, cell type stability and pluripotency. We discuss the control and maintenance of cellular identity during developmental transitions as they have been studied using direct reprogramming, with an emphasis on transcriptional and epigenetic regulation.

Factors affecting the development of somatic cell nuclear transfer embryos in Cattle

Nuclear transfer is a complex multistep procedure that includes oocyte maturation, cell cycle synchronization of donor cells, enucleation, cell fusion, oocyte activation and embryo culture. Therefore, many factors are believed to contribute to the success of embryo development following nuclear transfer. Numerous attempts to improve cloning efficiency have been conducted since the birth of the first sheep by somatic cell nuclear transfer. However, the efficiency of somatic cell cloning has remained low, and applications have been limited. In this review, we discuss some of the factors that affect the developmental ability of somatic cell nuclear transfer embryos in cattle.

Application of a novel population of multipotent stem cells derived from skin fibroblasts as donor cells in bovine SCNT

Undifferentiated stem cells are better donor cells for somatic cell nuclear transfer (SCNT), resulting in more offspring than more differentiated cells. While various stem cell populations have been confirmed to exist in the skin, progress has been restricted due to the lack of a suitable marker for their prospective isolation. To address this fundamental issue, a marker is required that could unambiguously prove the differentiation state of the donor cells. We therefore utilized magnetic activated cell sorting (MACS) to separate a homogeneous population of small SSEA-4⁺ cells from a heterogeneous population of bovine embryonic skin fibroblasts (BEF). SSEA-4⁺ cells were 8-10 μm in diameter and positive for alkaline phosphatase (AP). The percentage of SSEA-4⁺ cells within the cultured BEF population was low (2-3%). Immunocytochemistry and PCR analyses revealed that SSEA-4⁺ cells expressed pluripotency-related markers, and could differentiate into cells comprising all three germ layers in vitro. They remained undifferentiated over 20 passages in suspension culture. In addition, cloned embryos derived from SSEA-4 cells showed significant differences in cleavage rate and blastocyst development when compared with those from BEF and SSEA-4⁻ cells. Moreover, blastocysts derived from SSEA-4⁺ cells showed a higher total cell number and lower apoptotic index as compared to BEF and SSEA-4^– derived cells. It is well known that nuclei from pluripotent stem cells yield a higher cloning efficiency than those from adult somatic cells, however, pluripotent stem cells are relatively difficult to obtain from bovine. The SSEA-4⁺ cells described in the current study provide an attractive candidate for SCNT and a promising platform for the generation of transgenic cattle.

Ovum pick up, intracytoplasmic sperm injection and somatic cell nuclear transfer in cattle, buffalo and horses: from the research laboratory to clinical practice

Assisted reproductive techniques developed for cattle in the last 25 years, like ovum pick up (OPU), intracytoplasmic sperm injection (ICSI), and somatic cell nuclear transfer, have been transferred and adapted to buffalo and horses. The successful clinical applications of these techniques require both the clinical skills specific to each animal species and an experienced laboratory team to support the in vitro phase of the work. In cattle, OPU can be considered a consolidated technology that is rapidly outpacing conventional superovulation for embryo transfer. In buffalo, OPU represents the only possibility for embryo production to advance the implementation of embryo-based biotechnologies in that industry, although it is still mainly in the developmental phase. In the horse, OPU is now an established procedure for breeding from infertile and sporting mares throughout the year. It requires ICSI that in the horse, contrary to what happens in cattle and buffalo, is very efficient and the only option because conventional IVF does not work. Somatic cell nuclear transfer is destined to fill a very small niche for generating animals of extremely high commercial value. The efficiency is low, but because normal animals can be generated it is likely that advancing our knowledge in that field might improve the technology and reduce its cost.

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.

Positive correlation between the efficiency of induced pluripotent stem cells and the development rate of nuclear transfer embryos when the same porcine embryonic fibroblast lines are used as donor cells

Induced pluripotent stem cells (iPSCs) and nuclear transfer (NT) are two of the primary routes to reprogram differentiated cells back to the pluripotent state. However, it is still unknown whether there is any correlation between the reprogramming efficiency of iPSCs and NT if the same donor cells are employed. In this study, six porcine embryonic fibroblast (PEF) lines from Landrace (L1, L6, L9) or Congjiang local pigs (C4, C5, C6) were used for iPSC induction and NT. Furthermore, the resultant iPSCs from four PEF lines (L1, L6, C4, and C5) were used for NT (iPSC-NT), and the expression of exogenous genes was detected in iPSC-NT embryos by real-time PCR. The results showed that the efficiency of iPSC lines established from different PEF lines were significantly different. When the same PEF lines were used as donor cells for NT, the blastocysts rates were also different among different PEF lines and positively related with iPSCs induction efficiency. When the iPSCs were used as donor cells for NT, compared with the source PEFs, the blastocysts rates were significantly decreased. Real-time PCR results indicated that exogenous genes (Oct4, c-Myc) continued to be expressed in iPSC-NT embryos. In summary, our results demonstrate that there was a positive correlation between iPSCs and NT reprogramming efficiency, although the mechanism of these two routes is different. This may provide a new method to select the appropriate donor cells for inducing iPSCs.

Keith's MAGIC: Cloning and the Cell Cycle

Abstract Professor Keith Campbell's critical contribution to the discovery that a somatic cell from an adult animal can be fully reprogrammed by oocyte factors to form a cloned individual following nuclear transfer (NT)(Wilmut et al., 1997 ) overturned a dogma concerning the reversibility of cell fate that many scientists had considered to be biologically impossible. This seminal experiment proved the totipotency of adult somatic nuclei and finally confirmed that adult cells could differentiate without irreversible changes to the genetic material.

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.

Somatic cell nuclear transfer and transgenesis in large animals: current and future insights

Somatic cell nuclear transfer (SCNT) was first developed in livestock for the purpose of accelerating the widespread use of superior genotypes. Although many problems still exist now after fifteen years of research owing to the limited understanding of genome reprogramming, SCNT has provided a powerful tool to make copies of selected individuals in different species, to study genome pluripotency and differentiation, opening new avenues of research in regenerative medicine and representing the main route for making transgenic livestock. Besides well-established methods to deliver transgenes, recent development in enzymatic engineering to edit the genome provides more precise and reproducible tools to target-specific genomic loci especially for producing knockout animals. The interest in generating transgenic livestock lies in the agricultural and biomedical areas and it is, in most cases, at the stage of research and development, with few exceptions that are making the way into practical applications.

Reprogramming mammalian somatic cells

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

The risk of introduction of equine infectious anemia virus into USA via cloned horse embryos imported from Canada

Deriving horse oocytes in the USA is hampered by the lack of abattoirs processing horse carcasses which could provide abundant quantities of ovaries from slaughtered mares. Therefore, several cloning industries in the USA are attempting to import cloned horse embryos from Canada. Like any agricultural commodity, cloned embryos pose a risk of introduction of exotic animal diseases into the importing country. Under such circumstances, risk assessment could provide an objective, transparent, and internationally accepted means for evaluating the risk. This quantitative risk assessment (QRA) was initiated to determine the risk of introduction of Equine infectious anemia virus (EIAV) into the USA via cloned horse embryos imported from Canada. In assessing the risk, a structured knowledge base regarding cloning in relation to Equine infectious anemia (EIA) was first developed. Based on the knowledge base, a scenario tree was developed to determine conditions (with mathematical probabilities) that could lead to the introduction and maintenance of EIAV along the cloning pathway. Parameters for the occurrence of the event at each node were estimated using published literature. Using @Risk software and setting Monte Carlo simulation at 50,000 iterations, the probability of importing an EIAV-infected cloned horse embryo was 1.8 × 10(-9) (R = 1.5 × 10(-12) to 2.9 × 10(-8)). Taking into account the current protocol for equine cloning and assuming the yield of 5 to 30 clones per year, the possible number of EIAV-infected cloned horse embryos ranged from 2.0 × 10(-10) to 9.1 × 10(-5) (Mean = 1.4×10(-6)) per year. Consequently, it would take up to 1.5 × 10(7) (R = 1.6 × 10(4) to 5.1 × 10(10)) years for EIAV to be introduced into the USA. Based on the knowledge base and our critical pathway analysis, the biological plausibility of introducing EIAV into USA via cloned horse embryos imported from Canada is extremely low.

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

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

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

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

Pluripotent stem cells and reprogrammed cells in farm animals

Pluripotent cells are unique because of their ability to differentiate into the cell lineages forming the entire organism. True pluripotent stem cells with germ line contribution have been reported for mice and rats. Human pluripotent cells share numerous features of pluripotentiality, but confirmation of their in vivo capacity for germ line contribution is impossible due to ethical and legal restrictions. Progress toward derivation of embryonic stem cells from domestic species has been made, but the derived cells were not able to produce germ line chimeras and thus are termed embryonic stem-like cells. However, domestic animals, in particular the domestic pig (Sus scrofa), are excellent large animals models, in which the clinical potential of stem cell therapies can be studied. Reprogramming technologies for somatic cells, including somatic cell nuclear transfer, cell fusion, in vitro culture in the presence of cell extracts, in vitro conversion of adult unipotent spermatogonial stem cells into germ line derived pluripotent stem cells, and transduction with reprogramming factors have been developed with the goal of obtaining pluripotent, germ line competent stem cells from domestic animals. This review summarizes the present state of the art in the derivation and maintenance of pluripotent stem cells in domestic animals.

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.

Melanoma: Stem cells, sun exposure and hallmarks for carcinogenesis, molecular concepts and future clinical implications

Background:

The classification and prognostic assessment of melanoma is currently based on morphologic and histopathologic biomarkers. Availability of an increasing number of molecular biomarkers provides the potential for redefining diagnostic and prognostic categories and utilizing pharmacogenomics for the treatment of patients. The aim of the present review is to provide a basis that will allow the construction–or reconstruction–of future melanoma research.

Methods:

We critically review the common medical databases (PubMed, EMBASE, Scopus and Cochrane CENTRAL) for studies reporting on molecular biomarkers for melanoma. Results are discussed along the hallmarks proposed for malignant transformation by Hanahan and Weinberg. We further discuss the genetic basis of melanoma with regard to the possible stem cell origin of melanoma cells and the role of sunlight in melanoma carcinogenesis.

Results:

Melanocyte precursors undergo several genome changes –UV-induced or not– which could be either mutations or epigenetic. These changes provide stem cells with abilities to self-invoke growth signals, to suppress antigrowth signals, to avoid apoptosis, to replicate without limit, to invade, proliferate and sustain angiogenesis. Melanocyte stem cells are able to progressively collect these changes in their genome. These new potential functions, drive melanocyte precursors to the epidermis were they proliferate and might cause benign nevi. In the epidermis, they are still capable of acquiring new traits via changes to their genome. With time, such changes could add up to transform a melanocyte precursor to a malignant melanoma stem cell.

Conclusions:

Melanoma cannot be considered a “black box” for researchers anymore. Current trends in the diagnosis and prognosis of melanoma are to individualize treatment based on molecular biomarkers. Pharmacogenomics constitute a promising field with regard to melanoma patients' treatment. Finally, development of novel monoclonal antibodies is expected to complement melanoma patient care while a number of investigational vaccines could find their way into everyday oncology practice.

Developmental ability of miniature pig embryos cloned with mesenchymal stem cells

The present study compared the developmental ability of miniature pig embryos cloned with fetal fibroblasts (FFs), bone marrow-derived mesenchymal stem cells (MSCs) and differentiated (osteocytes, adipocytes and chondrocytes) MSCs. MSCs were isolated from an approximately 1-month-old female miniature pig (T-type, PWG Micro-pig((R)), PWG Genetics Korea). MSCs were differentiated into osteocytes, adipocytes and chondrocytes under controlled conditions and characterized by cell surface antigen profile using specific markers. These differentiated or undifferentiated MSCs, as well as FFs of miniature pig, were transferred into enucleated oocytes of domestic pigs. Data from 10 replicates involving 1567 cloned embryos were assessed in terms of developmental rates. The in vitro development rate to the blastocyst stage of embryos cloned with undifferentiated MSCs was significantly (P<0.05) higher than that of embryos cloned with differentiated MSCs and FFs. Surgical transfer of 523 two-cell stage embryos cloned with undifferentiated MSCs into five synchronized domestic pig recipients resulted in 5 cloned miniature pig offspring (1 stillborn and 4 viable) from 2 pregnant recipients. The results imply that MSCs might be multipotent and that they can be used to produce viable cloned miniature pigs that cannot be easily reproduced with differentiated somatic cells.

Epigenetic reprogramming by somatic cell nuclear transfer in primates

We recently demonstrated that somatic cells from adult primates could be reprogrammed into a pluripotent state by somatic cell nuclear transfer. However, the low efficiency with donor cells from one monkey necessitated the need for large oocyte numbers. Here, we demonstrate nearly threefold higher blastocyst development and embryonic stem (ES) cell derivation rates with different nuclear donor cells. Two ES cell lines were isolated using adult female rhesus macaque skin fibroblasts as nuclear donors and oocytes retrieved from one female, following a single controlled ovarian stimulation. In addition to routine pluripotency tests involving in vitro and in vivo differentiation into various somatic cell types, primate ES cells derived from reprogrammed somatic cells were also capable of contributing to cells expressing markers of germ cells. Moreover, imprinted gene expression, methylation, telomere length, and X-inactivation analyses were consistent with accurate and extensive epigenetic reprogramming of somatic cells by oocyte-specific factors.

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.

Derivation of induced pluripotent stem cells from pig somatic cells

For reasons that are unclear the production of embryonic stem cells from ungulates has proved elusive. Here, we describe induced pluripotent stem cells (iPSC) derived from porcine fetal fibroblasts by lentiviral transduction of 4 human (h) genes, hOCT4, hSOX2, hKLF4, and hc-MYC, the combination commonly used to create iPSC in mouse and human. Cells were cultured on irradiated mouse embryonic fibroblasts (MEF) and in medium supplemented with knockout serum replacement and FGF2. Compact colonies of alkaline phosphatase-positive cells emerged after approximately 22 days, providing an overall reprogramming efficiency of approximately 0.1%. The cells expressed porcine OCT4, NANOG, and SOX2 and had high telomerase activity, but also continued to express the 4 human transgenes. Unlike human ESC, the porcine iPSC (piPSC) were positive for SSEA-1, but negative for SSEA-3 and -4. Transcriptional profiling on Affymetrix (porcine) microarrays and real time RT-PCR supported the conclusion that reprogramming to pluripotency was complete. One cell line, ID6, had a normal karyotype, a cell doubling time of approximately 17 h, and has been maintained through >220 doublings. The ID6 line formed embryoid bodies, expressing genes representing all 3 germ layers when cultured under differentiating conditions, and teratomas containing tissues of ectoderm, mesoderm, and endoderm origin in nude mice. We conclude that porcine somatic cells can be reprogrammed to form piPSC. Such cell lines derived from individual animals could provide a means for testing the safety and efficacy of stem cell-derived tissue grafts when returned to the same pigs at a later age.

Cloning from stem cells: different lineages, different species, same story

Following nuclear transfer (NT), the most stringent measure of extensive donor cell reprogramming is development into viable offspring. This is referred to as cloning efficiency and quantified as the proportion of cloned embryos transferred into surrogate mothers that survive into adulthood. Cloning efficiency depends on the ability of the enucleated recipient cell to carry out the reprogramming reactions (‘reprogramming ability’) and the ability of the nuclear donor cell to be reprogrammed (‘reprogrammability’). It has been postulated that reprogrammability of the somatic donor cell epigenome is inversely proportional to its differentiation status. In order to test this hypothesis, reprogrammability was compared between undifferentiated stem cells and their differentiated isogenic progeny. In the mouse, cells of divergent differentiation status from the neuronal, haematopoietic and skin epithelial lineage were tested. In cattle and deer, skeletal muscle and antler cells, respectively, were used as donors. No conclusive correlation between differentiation status and cloning efficiency was found, indicating that somatic donor cell type may not be the limiting factor for cloning success. This may reflect technical limitations of the NT-induced reprogramming assay. Alternatively, differentiation status and reprogrammability may be unrelated, making all cells equally difficult to reprogramme once they have left the ground state of pluripotency.

Climbing Mount Efficiency--small steps, not giant leaps towards higher cloning success in farm animals

Despite more than a decade of research efforts, farm animal cloning by somatic cell nuclear transfer (SCNT) is still frustratingly inefficient. Inefficiency manifests itself at different levels, which are currently not well integrated. At the molecular level, it leads to widespread genetic, epigenetic and transcriptional aberrations in cloned embryos. At the organismal level, these genome-wide abnormalities compromise development of cloned foetuses and offspring. Specific molecular defects need to be causally linked to specific cloned phenotypes, in order to design specific treatments to correct them. Cloning efficiency depends on the ability of the nuclear donor cell to be fully reprogrammed into an embryonic state and the ability of the enucleated recipient cell to carry out the reprogramming reactions. It has been postulated that reprogrammability of the somatic donor cell epigenome is influenced by its differentiation status. However, direct comparisons between cells of divergent differentiation status within several somatic lineages have found no conclusive evidence for this. Choosing somatic stem cells as donors has not improved cloning efficiency, indicating that donor cell type may be less critical for cloning success. Different recipient cells, on the other hand, vary in their reprogramming ability. In bovine, using zygotes instead of oocytes has increased cloning success. Other improvements in livestock cloning efficiency include better coordinating donor cell type with cell cycle stage and aggregating cloned embryos. In the future, it will be important to demonstrate if these small increases at every step are cumulative, adding up to an integrated cloning protocol with greatly improved efficiency.

Melanoma, nevogenesis, and stem cell biology

It is now well established that a subpopulation of tumor stem cells (TSCs) are present within cancer tissues. This suggests that tumors evolve from stem cells; however, the exact cell of tumor origin, the potential role of dedifferentiation, and the role of plasticity in tumor development are largely unknown. A model cancer for the study of the oncologic process is melanoma. The developmental biology of melanocytes is relatively well understood, the cells pigment as they differentiate making them easy to identify, and benign and malignant tumors develop on the skin surface allowing direct observation of growth features, early detection, and removal. This ready access to early-stage tumors will facilitate study of the early oncologic processes and the role of tissue stem cells. Melanomas, like other cancers, include a subpopulation of TSCs. These TSCs have access to embryologic developmental programs, including the capacity to differentiate along multiple cell lineages. For example, melanomas can activate germ-cell pathways with major implications for TSC self-renewal through the activation of telomerase and genomic instability through the collision of meiotic and mitotic pathways (meiomitosis). The TSC model is still evolving, but the existence of TSCs has significant ramifications for tumor development, diagnosis, prognosis, and treatment of melanoma and other cancers.

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.

Conditioning of karyoplasts for producing somatic nuclear transferred gonadal germ cells in domestic chickens

The objective of this study was to establish a protocol for generating karyoplasts that can be used to produce somatic nuclear transferred gonadal germ cells (snt-GGCs) in domestic chickens. Karyoplasts were produced by centrifuging cultured fibroblasts from 10-day-old chick embryos at 10,000 x g in the presence of 1.0 microg/ml cytochalasin B. The number of karyoplasts was significantly (P<0.05) higher and the diameters of the karyoplasts were significantly (P<0.05) smaller when fibroblasts were centrifuged for 60 min than for 10 or 30 min. It was possible to generate snt-GGCs by electrofusion of GGCs with karyoplasts produced from cryopreserved or serum-starved fibroblasts. These results indicate that karyoplasts generated from 10-day-old chick embryos can be used to produce snt-GGCs even after cryopreservation and serum starvation of the fibroblasts.

Stem cells, the molecular circuitry of pluripotency and nuclear reprogramming

Reprogramming of somatic cells to a pluripotent embryonic stem cell-like state has been achieved by nuclear transplantation of a somatic nucleus into an enucleated egg and most recently by introducing defined transcription factors into somatic cells. Nuclear reprogramming is of great medical interest, as it has the potential to generate a source of patient-specific cells. Here, we review strategies to reprogram somatic cells to a pluripotent embryonic state and discuss our understanding of the molecular mechanisms of reprogramming based on recent insights into the regulatory circuitry of the pluripotent state.

Cattle Cloned from Increasingly Differentiated Muscle Cells1

It has been postulated that mammalian nuclear transfer (NT) cloning efficiency is inversely correlated with donor cell differentiation status. To test this hypothesis, we compared genetically identical and increasingly differentiated donors within the myogenic lineage. Bovine male fetal muscle cells were cultured for 1-6 days in vitro. The proportion of cells displaying the following antigens was quantified by immunofluorescence microscopy: MYOD1, MYF5, PAX7, MYOG, DES, MYH, and 5-Bromo-2-deoxyuridine. Based on the antigen profile of both bulk populations and individually size-selected cells prepared for NT, donors serum-starved for 1, 4, and 5 days were classified as myogenic precursors (MPCs), myotubes (MTs), and muscle-derived fibroblasts (MFs) with purities of 92%, 85%, and 99%, respectively. Expression of the following transcripts was measured by RT-PCR in 1) cells selected for NT, 2) metaphase II oocytes, 3) NT couplets, 4) NT reconstructs, 5) NT two-cell embryos, and 6) NT blastocysts: MYOD1, MYF5, PAX7, MYOG, MYF6, ACTB, and 18S rRNA. Muscle-specific genes were silenced and remained undetectable up to the blastocyst stage, whereas housekeeping genes 18S and ACTB continued to be expressed. Differentiation status affected development to transferable embryos (118 [23%] of 520 vs. 93 [11%] of 873 vs. 66 [38%] of 174 for MPC vs. MT vs. MF, respectively, P < 0.001). However, there were no significant differences in pregnancy rate and development to weaning between the cell types (pregnancy rate: 14 [64%] of 22 vs. 8 [35%] of 23 vs. 10 [45%] of 22, and development: 4 [18%] of 22 vs. 2 [9%] of 23 vs. 3 [14%] of 22 for MPC vs. MT vs. MF, respectively).

Red Deer Cloned from Antler Stem Cells and Their Differentiated Progeny1

The significance of donor cell differentiation status for successful cloning by somatic cell nuclear transfer (SCNT) is unclear. Here, we cloned a new species, red deer (Cervus elaphus), from multipotent antler stem cells and their differentiated progeny. Cultured donor cell lines from male antlerogenic periosteum (AP) were left undifferentiated or chemically induced to initiate osteogenesis or adipogenesis. Based on their morphology and marker gene expression profile, donor cells were classified as undifferentiated AP cells, presumptive osteoblasts, or adipocytes. Adipocytes upregulated adipogenic markers procollagen type I alpha 2 (COL1A2), peroxisome proliferator-activated receptor gamma 2 (PPARG), and gylceraldehyde-3-phosphate dehydrogenase (GAPDH), and downregulated antlerogenic transcripts POU-domain class 5 transcription factor (POU5F1) and parathyroid hormone (PTH)-like hormone (PTHLH). Despite differences prior to NT, transcript abundance of donor-specific markers COL1A2, PPARG, GAPDH, and POU5F1 did not differ significantly in cloned blastocysts (P = 0.10, 0.50, 0.61, and 0.16, respectively). However, donor cell and blastocyst expression levels were completely different for most genes analyzed, indicating their successful reprogramming. The type of donor cell used for NT (AP, bone, and fat cells), had no effect on in vitro development to blastocysts (93 [38%] of 248 vs. 32 [44%] of 73 vs. 59 [32%] of 183, respectively). Likewise, development to weaning was not significantly different between the three cell types (2 [4%] of 46 vs. 2 [29%] of 7 vs. 4 [13%] of 31, for AP vs. bone vs. fat, respectively). Microsatellite DNA analysis confirmed that the eight cloned red deer calves were genetically identical to the cells used for NT.

Differential developmental ability of embryos cloned from tissue-specific stem cells

Although cloning animals by somatic cell nuclear transfer is generally inefficient, the use of certain nuclear donor cell types may significantly improve or deteriorate outcomes. We evaluated whether two multipotent stem cell lines produced in vitro--neural stem cells (NSCs) and mesenchymal stem cells (MSCs)--could serve as nuclear donors for nuclear transfer cloning. Most (76%) NSC-derived embryos survived the two-cell-to-four-cell transition, the stage when the major zygotic gene activation occurs. Consistent with this observation, the expression patterns of zygotically active genes were better in NSC-derived embryos than in fibroblast clone embryos, which arrested at the two-cell stage more frequently. Embryo transfer experiments demonstrated that at least some of these NSC embryos had the ability to develop to term fetuses (1.6%, 3/189). In contrast, embryos reconstructed using MSCs showed a low rate of in vitro development and never underwent implantation in vivo. Chromosomal analysis of the donor MSCs revealed very frequent aneuploidy, which probably impaired the potential for development of their derived clones. This is the first demonstration that tissue-specific multipotent stem cells produced in vitro can serve as donors of nuclei for cloning mice; however, these cells may be prone to chromosomal aberrations, leading to high embryonic death rates. We found previously that hematopoietic stem cells (HSCs) are very inefficient donor cells because of their failure to activate the genes essential for embryonic development. Taken together, our data led us to conclude that tissue-specific stem cells in mice, namely NSCs, MSCs, and HSCs, exhibited marked variations in the ability to produce cloned offspring and that this ability varies according to both the epigenetic and genetic status of the original genomes. Disclosure of potential conflicts of interest is found at the end of this article.