Nuclear transfer in cattle using colostrum-derived mammary gland epithelial cells and ear-derived fibroblast cells

Using a modified piggyBac transposon-combined Cre/loxP system to produce selectable reporter-free transgenic bovine mammary epithelial cells for somatic cell nuclear transfer

Transposon systems are widely used for genetic engineering in various model organisms. PiggyBac (PB) has recently been confirmed to have highly efficient transposition in the mouse germ line and mammalian cell lines. In this study, we used a modified PB transposon system mediated by PB transposase (PBase) mRNA carrying the human lactoferrin gene driven by bovine β-casein promoter to transfect bovine mammary epithelial cells (BMECs), and the selectable reporter in two stable transgenic BMEC clones was removed using cell-permeant Cre recombinase. These reporter-free transgenic BMECs were used as donor cells for somatic cell nuclear transfer (SCNT) and exhibited a competence of SCNT embryos similar to stable transgenic BMECs and nontransgenic BMECs. The comprehensive information from this study provided a modified approach using an altered PB transposon system mediated by PBase mRNA in vitro and combined with the Cre/loxP system to produce transgenic and selectable reporter-free donor nuclei for SCNT. Consequently, the production of safe bovine mammary bioreactors can be promoted.

Live births from urine derived cells

Here we report urine-derived cell (UDC) culture and subsequent use for cloning which resulted in the successful development of cloned canine pups, which have remained healthy into adulthood. Bovine UDCs were used in vitro to establish comparative differences between cell sources. UDCs were chosen as a readily available and noninvasive source for obtaining cells. We analyzed the viability of cells stored in urine over time and could consistently culture cells which had remained in urine for 48hrs. Cells were shown to be viable and capable of being transfected with plasmids. Although primarily of epithelial origin, cells were found from multiple lineages, indicating that they enter the urine from more than one source. Held in urine, at 4°C, the majority of cells maintained their membrane integrity for several days. When compared to in vitro fertilization (IVF) derived embryos or those from traditional SCNT, UDC derived embryos did not differ in total cell number or in the number of DNA breaks, measured by TUNEL stain. These results indicate that viable cells can be obtained from multiple species' urine, capable of being used to produce live offspring at a comparable rate to other cell sources, evidenced by a 25% pregnancy rate and 2 live births with no losses in the canine UDC cloning trial. This represents a noninvasive means to recover the breeding capacity of genetically important or infertile animals. Obtaining cells in this way may provide source material for human and animal studies where cells are utilized.

Generation of cloned mice and nuclear transfer embryonic stem cell lines from urine-derived cells

Cloning animals by nuclear transfer provides the opportunity to preserve endangered mammalian species. However, there are risks associated with the collection of donor cells from the body such as accidental injury to or death of the animal. Here, we report the production of cloned mice from urine-derived cells collected noninvasively. Most of the urine-derived cells survived and were available as donors for nuclear transfer without any pretreatment. After nuclear transfer, 38–77% of the reconstructed embryos developed to the morula/blastocyst, in which the cell numbers in the inner cell mass and trophectoderm were similar to those of controls. Male and female cloned mice were delivered from cloned embryos transferred to recipient females, and these cloned animals grew to adulthood and delivered pups naturally when mated with each other. The results suggest that these cloned mice had normal fertility. In additional experiments, 26 nuclear transfer embryonic stem cell lines were established from 108 cloned blastocysts derived from four mouse strains including inbreds and F1 hybrids with relatively high success rates. Thus, cells derived from urine, which can be collected noninvasively, may be used in the rescue of endangered mammalian species by using nuclear transfer without causing injury to the animal.

Effect of synchronization of donor cells in early G1-phase using shake-off method on developmental potential of somatic cell nuclear transfer embryos in cattle

In this study, we compared the developmental ability of somatic cell nuclear transfer (SCNT) embryos reconstructed with three bovine somatic cells that had been synchronized in G0-phase (G0-SCNT group) or early G1-phase (eG1-SCNT group). Furthermore, we investigated the production efficiency of cloned offspring for NT embryos derived from these donor cells. The G0-phase and eG1-phase cells were synchronized, respectively, using serum starvation and antimitotic reagent treatment combined with shaking of the plate containing the cells (shake-off method). The fusion rate in the G0-SCNT groups (64.2 ± 1.8%) was significantly higher than that of eG1-SCNT groups (39.2 ± 1.9%) (P < 0.05), but the developmental rates to the blastocyst stage of SCNT embryos per fused oocytes were similar for all groups. The overall production efficiency of the clone offspring in eG1-SCNT groups (12.7%) per recipient cow was higher than that in G0-SCNT groups (3%) (P < 0.05). The mean birth weight of cloned calves and the average calving score in the G0-SCNT groups (48.1 ± 3.4 kg and 3.3 ± 0.3, respectively) was significantly higher (P < 0.05) than those of eG1-SCNT groups (37.2 ± 2.1 kg and 2.3 ± 0.2, respectively). Results of this study indicate that synchronization of donor cells in eG1-phase using the shake-off method improved the overall production efficiency of the clone offspring per transferred embryo.

Recent progress in bovine somatic cell nuclear transfer

Bovine somatic cell nuclear transfer (SCNT) embryos can develop to the blastocyst stage at a rate similar to that of embryos produced by in vitro fertilization. However, the full-term developmental rate of SCNT embryos is very low, owing to the high embryonic and fetal losses after embryo transfer. In addition, increased birth weight and postnatal mortality are observed at high rates in cloned calves. The low efficiency of SCNT is probably attributed to incomplete reprogramming of the donor nucleus and most of the developmental problems of clones are thought to be caused by epigenetic defects. Applications of SCNT will depend on improvement in the efficiency of production of healthy cloned calves. In this review, we discuss problems and recent progress in bovine SCNT.

Production of cloned embryos from caprine mammary epithelial cells expressing recombinant human β-defensin-3

Transgenic animals that express antimicrobial agents in their milk can inhibit bacterial pathogens that cause mastitis. Our objective was to produce human β-defensin-3 (HBD3) transgenic embryos by nuclear transfer using goat mammary epithelial cells (GMECs) as donor cells. Three GMEC lines (GMEC1, GMEC2, and GMEC3) were transfected with a HBD3 mammary-specific expression vector by electroporation. There was a difference (P < 0.05) in the rate of geneticin-resistant colony formation among cell lines GMEC1, GMEC2, and GMEC3 (39 and 47 vs. 19 colonies per 3 × 10(6) cells, respectively). After inducing expression, the mRNA and protein of HBD3 were detected by reverse transcription polymerase chain reaction and Western blot analysis in transgenic cells. Transgenic clonal cells expressing HBD3 were used as donor cells to investigate development of cloned embryos. There were no significant differences in rates of cleavage or blastocyst formation of cloned embryos from transgenic (GMEC1T2 and GMEC2T3) and nontransgenic (GMEC1 and GMEC2) GMECs (72.3 ± 5.0%, 69.5 ± 2.3%, 61.8 ± 4.8%, and 70.0 ± 2%; and 16.8 ± 0.5%, 17.5 ± 0.7%, 16.7 ± 0.9%, and 17.5 ± 0.6%, respectively). However, the fusion rate, cleavage rate, and blastocyst formation rate of cloned embryos from a transgenic clonal cell line (GMEC2T6, 50.7 ± 2.1%, 55.5 ± 2.0%, and 11.1 ± 0.6%) were lower than those of other groups (P < 0.05). We concluded that genetic modification of GMECs might not influence the in vitro development of cloned embryos, but that some of the transgenic clonal cells were not suitable for nuclear transfer to produce transgenic goats, because of low developmental rates. However, transgenic GMECs expressing HBD3 might be used as donor cells for producing transgenic goats that express increased concentrations of β-defensins in their milk.

Aberrant placenta gene expression pattern in bovine pregnancies established after transfer of cloned or in vitro produced embryos

In the present study, we used the global transcriptome profile approach to identify dysregulated genes, molecular pathways, and molecular functional alterations in bovine placentas derived from somatic cell nuclear transfer (SCNT) and in vitro embryo production (IVP) pregnancies compared with their artificial insemination (AI) counterparts at day 50 of gestation. For this, day 7 blastocysts derived from AI, IVP, or SCNT were transferred to oestrus-synchronized cows. The pregnant animals were slaughtered at day 50 of gestation, and the placentas were then recovered and used for transcriptome analysis using Affymetrix GeneChip bovine genome array. Results showed the SCNT placenta to be different from its AI counterpart in the expression of 1,196 transcripts. These genes were found to be associated with alterations in key biological processes and molecular pathways in SCNT placenta, and the dysregulation of 9% (n = 110) of these genes was due to transcriptional reprogramming error. IVP placenta also displayed alterations in the expression of 72 genes, of which 58 were common to SCNT placenta. Gene enrichment analysis revealed that the expression of genes involved in organ development, blood vessel development, extracellular matrix organization, and the immune system was affected in both SCNT and IVP placentas. However, 96% of the affected genes in SCNT were not significantly altered in IVP groups. Thus, the higher transcriptome dysregulation in SCNT placenta followed by IVP would reflect the degree of placental abnormality in SCNT and IVP pregnancies at day 50 of the gestation, which may have a profound effect on subsequent fetal development and health of the offspring.

Hand-made cloned goat (Capra hircus) embryos—a comparison of different donor cells and culture systems

Nuclear transfer is a very effective method for propagation of valuable, extinct, and endangered animals. Hand-made cloning (HMC) is an efficient alternative to the conventional micromanipulator-based technique in some domestic species. The present study was carried out for the selection of suitable somatic cells as a nuclear donor and development of an optimum culture system for in vitro culture of zona-free goat cloned embryos. Cleavage and blastocyst rates were observed 72.06 ± 2.94% and 0% for fresh cumulus cells, 81.95 ± 3.40% and 12.74 ± 2.12% for cultured cumulus cells, and 92.94 ± 0.91% and 23.78 ± 3.33% for fetal fibroblast cells, respectively. There was a significant (p < 0.05) increase in blastocyst production in goats when cultured on a flat surface (FS) (23.78 ± 3.33 %) than well of wells (WOW) (15.84 ± 2.12 %) and microdrops (MD) (0.7 ± 0.7%). Furthermore, cleavage and blastocyst production rates were significantly (p < 0.05) more in the WOW (15.84 ± 2.12%) than the MD (0.7 ± 0.7%) system. The quality of HMC blastocysts was studied by differential staining. Genetic similarity was confirmed by polymerase chain reaction (PCR)-based amplification of the second exon of the MHC class II DRB gene, which gave similar bands in electrophoresis (286 bp) both in cloned embryos and donor cells. In conclusion, the present study describes that the fetal fibroblast cell is a suitable candidate as nuclear donor, and the flat surface culture system is suitable for zona-free blastocyst development by the hand-made cloning technique in the goat.

Delivery of cloned offspring: experience in Zebu cattle (Bos indicus)

The production of a healthy cloned calf is dependent on a multitude of successful steps, including reprogramming mediated by the oocyte, the development of a functional placenta, adequate maternal–fetal interaction, the establishment of a physiological metabolic setting and the formation of a complete set of well-differentiated cells that will eventually result in well-characterised and fully competent tissues and organs. Although the efficiency of nuclear transfer has improved significantly since the first report of a somatic cell nuclear transfer-derived animal, there are many descriptions of anomalies concerning cloned calves leading to high perinatal morbidity and mortality. The present article discusses some our experience regarding perinatal and neonatal procedures for cloned Zebu cattle (B. indicus) that has led to improved survival rates in Nellore cloned calves following the application of such ‘labour-intensive technology’.

Phytohemagglutinin improves efficiency of electrofusing mammary gland epithelial cells into oocytes in goats

The objective was to investigate the effect of phytohemagglutinin (PHA) on the fusion of mammary gland epithelial (MGE) cells into enucleated oocytes in goats. The toxicity of PHA was evaluated by testing its effect on the development of parthenogenetic caprine oocytes. The effective dose and duration of PHA treatment (100 microg/mL, 20 min incubation) was selected and used to compare fusion efficiency and embryo development following nuclear transfer. Two electrofusion protocols, chamber fusion (CF) and pressurized microelectrode fusion (pMEF), were also compared, when couplets were treated with and without PHA (100 microg/mL, 20 min). Fusion rate of couplets increased from 52.8 to 74.0% for the CF protocol (P<0.05), but was not significantly different for the pMEF protocol (72.7% vs. 78.1%) after PHA treatment. There were no significant differences between treated group and control in rates of subsequent cleavage or blastocyst development. Following transfer of the cloned blastocysts derived from the PHA-treated group and the control group into synchronized recipients, pregnancy rates (Day 30) were not significantly different between treated group and control (28.6% vs. 25.0%). However, all recipients aborted within 120d, microsatellite DNA analyses confirmed that the aborted fetuses were genetically identical to the donor goat. In conclusion, the fusion rate of caprine MGE cell couplets was improved by pre-incubating couplets in medium containing 100 microg/mL PHA prior to electrical pulsing, and embryos derived from PHA treatment established early pregnancies.

Somatic cell nuclear transfer: Past, present and future perspectives

It is now over a decade since the birth, in 1996, of Dolly the first animal to be produced by nuclear transfer using an adult derived somatic cell as nuclear donor. Since this time similar techniques have been successfully applied to a range of species producing live offspring and allowing the development of transgenic technologies for agricultural, biotechnological and medical uses. However, though applicable to a range of species, overall, the efficiencies of development of healthy offspring remain low. The low frequency of successful development has been attributed to incomplete or inappropriate reprogramming of the transferred nuclear genome. Many studies have demonstrated that such reprogramming occurs by epigenetic mechanisms not involving alterations in DNA sequence, however, at present the molecular mechanisms underlying reprogramming are poorly defined. Since the birth of Dolly many studies have attempted to improve the frequency of development, this review will discuss the process of animal production by nuclear transfer and in particular changes in the methodology which have increased development and survival, simplified or increased robustness of the technique. Although much of the discussion is applicable across species, for simplicity we will concentrate primarily on published data for cattle, sheep, pigs and mice.

Isolation, culture and characterisation of somatic cells derived from semen and milk of endangered sheep and eland antelope

Semen and milk are potential sources of somatic cells for genome banks. In the present study, we cultured and characterised cells from: (1) cooled sheep milk; (2) fresh, cooled and frozen–thawed semen from Gulf Coast native (GCN) sheep (Ovis aries); and (3) fresh eland (Taurotragus oryx) semen. Cells attached to the culture surface from fresh (29%), cooled (43%) and slow-frozen (1°C/min; 14%) ram semen, whereas no attachment occurred in the fast-frozen (10°C/min) group. Proliferation occurred in fresh (50%) and cooled (100%) groups, but no cells proliferated after passage 1 (P1). Eland semen yielded cell lines (100%) that were cryopreserved at P1. In samples from GCN and cross-bred milk, cell attachment (83% and 95%, respectively) and proliferation (60% and 37%, respectively) were observed. Immunocytochemical detection of cytokeratin indicated an epithelial origin of semen-derived cells, whereas milk yielded either fibroblasts, epithelial or a mixture of cell types. Deoxyribonucleic acid microsatellite analysis using cattle-derived markers confirmed that eland cells were from the semen donor. Eland epithelial cells were transferred into eland oocytes and 12 (71%), six (35%) and two (12%) embryos cleaved and developed to morulae or blastocyst stages, respectively. In conclusion, we have developed a technique for obtaining somatic cells from semen. We have also demonstrated that semen-derived cells can serve as karyoplast donors for nuclear transfer.

The US FDA and animal cloning: Risk and regulatory approach

The Food and Drug Administration's (FDA's) Center for Veterinary Medicine issued a voluntary request to producers of livestock clones not to introduce food from clones or their progeny into commerce until the agency had assessed whether production of cattle, swine, sheep, or goats by somatic cell nuclear transfer (SCNT) posed any unique risks to the animal(s) involved in the process, humans, or other animals by consuming food from those animals, compared with any other assisted reproductive technology (ART) currently in use. Following a comprehensive review, no anomalies were observed in animals produced by cloning that have not also been observed in animals produced by other ARTs and natural mating. Further systematic review on the health of, and composition of meat and milk from, cattle, swine, and goat clones and the progeny of cattle and sheep did not result in the identification of any food-consumption hazards. The agency therefore concluded that food from cattle, swine, and goat clones was as safe to eat as food from animals of those species derived by conventional means. The agency also concluded that food from the progeny of the clone of any species normally consumed for food is as safe to eat as those animals. The article also describes the methodology used by the agency to analyze data and draw these conclusions, the plans the agency has proposed to manage any identified risks, and the risk communication approaches the agency has used.

Cellular reprogramming for the creation of patient-specific embryonic stem cells

The success of somatic cell nuclear transfer in mammals has opened the possibility to dedifferentiate cells from a patient into embryonic stem cells and in doing so, potentially generate all different cells and tissues of the human body. These cells could be later transplanted to the same patient without immune rejection. Whereas this principle has been demonstrated in laboratory animals, it is yet to be shown to work in primates. Herein we discuss the probability of somatic cell nuclear transfer becoming a real therapeutic alternative as well as the potential emerging dedifferentiation approaches that may eventually replace it.

Differences in gene expression patterns between somatic cell nuclear transfer embryos constructed with either rabbit granulosa cells or their derivatives

Successful production of offspring by somatic cell nuclear transfer (SCNT) is affected by the nature of the donor cells used. The purpose of this study was to determine whether characteristic changes induced in donor cells by culture conditions influenced gene expression patterns in the resultant SCNT embryos. Rabbit granulosa cells (rGC) were cultured under different conditions, either with or without hCG, and the two derivative cell types obtained (named respectively cGC+ and cGC-) were used as donor cells for SCNT. There were characteristic differences between fresh rGC and the two derivative cell types: p450scc expression and progesterone secretion were both higher in cGC+ than in cGC-; expression of bmp4 and fgfr2 was decreased in cGC+ and cGC- compared with rGC; and cGC+ and cGC- cell types gained collagenIV expression. Use of fresh rGC, or cGC+ and cGC- derivative cells, did not alter either the developmental potencies of SCNT oocytes or cell numbers at the blastocyst stage. The expression patterns of four genes (bmp4, fgfr2, gata4, oct3/4) in SCNT embryos and in fertilized embryos were analyzed by quantitative RT-PCR. We found that oct3/4 was expressed in all embryos. The expression patterns of the other three genes showed considerable variation between the different types of embryo: bmp4 was found in most fertilized embryos but only some of rGC and none of cGC+ and cGC- derived SCNT embryos; fgfr2 was present in fertilized embryos but was present in some rGC and cGC- NT embryos and in all cGC+ NT embryos; gata4 was not expressed in fertilized embryos but was present in a few rGC and cGC+ NT embryos and in most cGC- NT embryos. Our results suggest that the gene expression patterns in SCNT embryos derived from granulosa donor cells are affected by characteristic changes to the cells during in vitro culture.

Activation of oocytes after nuclear transfer

After nuclear transfer, the recipient oocyte must be stimulated to initiate development. This stimulation is achieved by inducing changes in the oocyte cytoplasm that normally are triggered by the sperm during fertilization. In most cases, such changes include a transient increase in the intracellular-free calcium concentration induced by an electrical pulse or alternatively, by chemical agents. Many times, particularly in aged oocytes, this calcium signal is sufficient to stimulate the oocyte developmental program. Other activation protocols were designed to target pathways downstream of the initial calcium signal to affect the activity of regulatory proteins that play central roles in maintaining developmental arrest. This is achieved by the application of protein kinase or protein synthesis inhibitors; combined with a calcium stimulus such inhibitors are widely used for oocyte activation after nuclear transfer and are able to support embryonic development to term.

Examination of a modified cell cycle synchronization method and bovine nuclear transfer using synchronized early G1 phase fibroblast cells

Somatic cell nuclear transfer has a low success rate, due to a high incidence of fetal loss and increased perinatal morbidity/mortality. One factor that may affect the successful development of nuclear transfer embryos is the cell cycle stage of the donor cell. In order to establish a cell cycle synchronization method that can consistently produce cloned embryos and offspring, we examined the effects of different combinations of three cell treatments on the recovery rate of mitotic phase cells using bovine fetal fibroblasts. In the first experiment, we examined the recovery rate of mitotic phase cells by a combination of treatment with a metaphase arrestant (1 microM 2-methoxyestradiol), shaking the plate and selecting cells with a diameter of 20 microns. As a result, 99% of mitotic phase cells were recovered by repeating the combined treatment of metaphase arrestant and shaking, and collection of cells with a specific diameter. In the second experiment, nuclear transfer was carried out using early G1 phase cells by choosing pairs of bridged cells derived from mitotic phase cells recovered by the combined treatment of 1 microM 2-methoxyestradiol and shaking, and collection of cells with a diameter of 20 microns. The reconstructed embryos were transferred to recipient heifers to determine post-implantation development. Development of embryos reconstructed from early G1 phase cells from the >/=6 cells stage on Day 3 to the morula-blastocyst stage on Day 6 was 100%. Ten blastocysts constructed from two cell lines were transferred into 10 recipient heifers. Nine of the 10 recipients delivered single live calves. In conclusion, mitotic phase bovine fibroblast cells were easily recovered by the combined treatments of 1 microM 2-methoxyestradiol, shaking, and selecting cells of the appropriate diameter. Furthermore, nuclear transfer using cells in the early G1 phase as donor cells gave a high rate of offspring production.

Human cloning: can it be made safe?

There are continued claims of attempts to clone humans using nuclear transfer, despite the serious problems that have been encountered in cloning other mammals. It is known that epigenetic and genetic mechanisms are involved in clone failure, but we still do not know exactly how. Human reproductive cloning is unethical, but the production of cells from cloned embryos could offer many potential benefits. So, can human cloning be made safe?

Effect of polyethylene glycol and dimethyl sulfoxide on the fusion of bovine nuclear transfer using mammary gland epithelial cells

The effects of polyethylene glycol and dimethyl sulfoxide (PEG/DMSO) treatment of donor cells on the fusion and subsequent development of bovine nuclear transfer embryos using mammary gland epithelial (MGE) cells before electrofusion (fresh MGE cells) was studied. The same study was conducted on those cells that were frozen and stored in liquid nitrogen, and then thawed (frozen-thawed MGE cells). Experiment 1 showed that the exposure time and pH of PEG/DMSO solution affected the fusion of nuclear transfer, and that a higher fusion rate was obtained when fresh MGE cells were exposed to PEG/DMSO solution at pH 8.0 for 5 min. In Experiment 2, the proportion of fused oocytes with fresh PEG/DMSO-treated cells (70 +/- 6%) was significantly higher than that with non-treated cells (50 +/- 13%, p < 0.05). The same tendency was observed when frozen-thawed cells as donor nuclei were used (48 +/- 6% vs. 34 +/- 12%, p < 0.05). In addition, PEG/DMSO treatment has neither harmful nor beneficial effects on the cleavage and development of the blastocyst stage of reconstructed embryos (p > 0.05). The fusion and cleavage rates of frozen-thawed cells were significantly lower than those of fresh cells (p < 0.05). After 10 blastocysts, derived from fresh PEG/DMSO-treated cells, were transferred to five recipient heifers, one live female calf was obtained. Experiment 3 showed that PEG/DMSO treatment reduced the viability of both fresh and frozen-thawed MGE cells (p < 0.05). We conclude that the PEG/DMSO treatment of fresh MGE cells, as well as the frozen-thawed cells, before electrofusion has a positive effect on the fusion of nuclear transfer without decreasing the in vitro development of reconstructed embryos.

Cloning adult farm animals: a review of the possibilities and problems associated with somatic cell nuclear transfer

In 1997, Wilmut et al. announced the birth of Dolly, the first ever clone of an adult animal. To date, adult sheep, goats, cattle, mice, pigs, cats and rabbits have been cloned using somatic cell nuclear transfer. The ultimate challenge of cloning procedures is to reprogram the somatic cell nucleus for development of the early embryo. The cell type of choice for reprogramming the somatic nucleus is an enucleated oocyte. Given that somatic cells are easily obtained from adult animals, cultured in the laboratory and then genetically modified, cloning procedures are ideal for introducing specific genetic modifications in farm animals. Genetic modification of farm animals provides a means of studying genes involved in a variety of biological systems and disease processes. Moreover, genetically modified farm animals have created a new form of 'pharming' whereby farm animals serve as bioreactors for production of pharmaceuticals or organ donors. A major limitation of cloning procedures is the extreme inefficiency for producing live offspring. Dolly was the only live offspring produced after 277 attempts. Similar inefficiencies for cloning adult animals of other species have been described by others. Many factors related to cloning procedures and culture environment contribute to the death of clones, both in the embryonic and fetal periods as well as during neonatal life. Extreme inefficiencies of this magnitude, along with the fact that death of the surrogate may occur, continue to raise great concerns with cloning humans.

Epigenetic reprogramming in mammalian nuclear transfer

With the exception of lymphocytes, the various cell types in a higher multicellular organism have basically an identical genotype but are functionally and morphologically different. This is due to tissue-specific, temporal, and spatial gene expression patterns which are controlled by genetic and epigenetic mechanisms. Successful cloning of mammals by transfer of nuclei from differentiated tissues into enucleated oocytes demonstrates that these genetic and epigenetic programs can be largely reversed and that cellular totipotency can be restored. Although these experiments indicate an enormous plasticity of nuclei from differentiated tissues, somatic cloning is a rather inefficient and unpredictable process, and a plethora of anomalies have been described in cloned embryos, fetuses, and offspring. Accumulating evidence indicates that incomplete or inappropriate epigenetic reprogramming of donor nuclei is likely to be the primary cause of failures in nuclear transfer. In this review, we discuss the roles of various epigenetic mechanisms, including DNA methylation, chromatin remodeling, imprinting, X chromosome inactivation, telomere maintenance, and epigenetic inheritance in normal embryonic development and in the observed abnormalities in clones from different species. Nuclear transfer represents an invaluable tool to experimentally address fundamental questions related to epigenetic reprogramming. Understanding the dynamics and mechanisms underlying epigenetic control will help us solve problems inherent in nuclear transfer technology and enable many applications, including the modulation of cellular plasticity for human cell therapies.

Donor cells for nuclear cloning: many are called, but few are chosen

The few viable clones obtained at the end of a typical cloning experiment are genetic copies of the donor cell genome of a non-reproductive (somatic) or embryonic cell used for nuclear transfer. Nuclear totipotency has to be reestablished by erasing epigenetic constraints imposed on the donor genome during differentiation in a process which involves active chromatin remodeling. Various donor cell types and cell cycle combinations have proven to be capable of generating cloned offspring. However, an ideal nuclear donor may have not yet been found. This review summarizes current theoretical aspects of donor cell selection. It focuses on the impact of genetic and epigenetic differences between donor cell types on successful mammalian cloning.

Producing proteins in transgenic plants and animals

The requirement for large quantities of therapeutic proteins has fueled interest in the production of recombinant proteins in plants and animals. The first commercial products to be made in this way have experienced much success, and it is predicted that in the future a plethora of protein products will be made using these 'natural' bioreactors.