Cloning and assisted reproductive techniques: Influence on early development and adult phenotype

Mitochondrial DNA Depletion in Granulosa Cell Derived Nuclear Transfer Tissues

Somatic cell nuclear transfer (SCNT) is a key technology with broad applications that range from production of cloned farm animals to derivation of patient-matched stem cells or production of humanized animal organs for xenotransplantation. However, effects of aberrant epigenetic reprogramming on gene expression compromise cell and organ phenotype, resulting in low success rate of SCNT. Standard SCNT procedures include enucleation of recipient oocytes before the nuclear donor cell is introduced. Enucleation removes not only the spindle apparatus and chromosomes of the oocyte but also the perinuclear, mitochondria rich, ooplasm. Here, we use a Bos taurus SCNT model with in vitro fertilized (IVF) and in vivo conceived controls to demonstrate a ∼50% reduction in mitochondrial DNA (mtDNA) in the liver and skeletal muscle, but not the brain, of SCNT fetuses at day 80 of gestation. In the muscle, we also observed significantly reduced transcript abundances of mtDNA-encoded subunits of the respiratory chain. Importantly, mtDNA content and mtDNA transcript abundances correlate with hepatomegaly and muscle hypertrophy of SCNT fetuses. Expression of selected nuclear-encoded genes pivotal for mtDNA replication was similar to controls, arguing against an indirect epigenetic nuclear reprogramming effect on mtDNA amount. We conclude that mtDNA depletion is a major signature of perturbations after SCNT. We further propose that mitochondrial perturbation in interaction with incomplete nuclear reprogramming drives abnormal epigenetic features and correlated phenotypes, a concept supported by previously reported effects of mtDNA depletion on the epigenome and the pleiotropic phenotypic effects of mtDNA depletion in humans. This provides a novel perspective on the reprogramming process and opens new avenues to improve SCNT protocols for healthy embryo and tissue development.

Development of bovine embryos derived from reproductive techniques

Assisted reproduction techniques have improved agricultural breeding in the bovine. However, important development steps may differ from the situation in vivo and there is a high mortality rate during the first trimester of gestation. To better understand these events, we investigated the development of embryos and fetal membranes following fixed-time AI (FTAI), IVF and nuclear transfer (NT). The onset of yolk-sac development was not normal in cloned embryos. Later steps differed from conditions in vivo in all three groups; the yolk-sac was yellowish and juxtaposed with the amniotic membrane. Vascularisation of the chorioallantoic membrane was relatively late and low in NT gestations, but normal in the others. The overall development of the embryos was normal, as indicated by morphology and regression analysis of growth rate. However, NT conceptuses were significantly smaller, with the livers in some embryos occupying the abdominal cavity and others exhibiting heart abnormalities. In conclusion, the yolk-sac and the cardiovascular system seem to be vulnerable to morphogenetic alterations. Future studies will focus on gene expression and early vascularisation processes to investigate whether these changes may be responsible for the high incidence of intrauterine mortality, especially in clones.

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.

Demecolcine-assisted enucleation of goat oocytes: protocol optimization, mechanism investigation, and application to improve the developmental potential of cloned embryos

Although demecolcine-assisted enucleation has been performed successfully in porcine and cattle, the mechanism and protocol optimization of chemically assisted enucleation need further investigation. The present study optimized the protocol for goat oocyte enucleation and demonstrated that a 30-min treatment with 0.8 ng/mL demecolcine-induced cytoplasmic protrusions in over 90% of the oocytes. Rates of enucleation, cell fusion, and blastocyst formation were significantly higher after demecolcine-assisted than after blind aspiration enucleation, although differences in rates of live births remain to be unequivocally determined between the two treatments. The ability to form protrusions decreased significantly as spindles became less organized in aged oocytes and the oocytes with a poor cumulus expansion. More than 93% of the demecolcine-induced protrusions persisted for 2 h in the absence of cytochalasin B (CB) but most disappeared within 30 min of CB treatment. The spindle disintegrated, an actin-rich ring formed around the chromosome mass and the MAP kinase activity increased significantly after demecolcine treatment. When oocytes with induced protrusions were treated with CB, however, the contractile ring disappeared, the spindle reintegrated, and both MPF and MAP kinase activities decreased significantly. It is concluded that (1) cytoplasmic protrusions can be induced in goat oocytes with a very low concentration of demecolcine; (2) oocyte selection and enucleation can be achieved simultaneously with demecolcine treatment; and (3) an interactive effect between the MAP kinase, MPF, microfilaments and microtubules might be implicated in the control of cytoplasmic protrusion formation after demecolcine treatment.

The effects of delayed activation and MG132 treatment on nuclear remodeling and preimplantation development of embryos cloned by electrofusion are correlated with the age of recipient cytoplasts

The electrofusion method, used extensively in livestock cloning, cannot be used in mice, because it is believed that the mouse oocytes are more susceptible to detrimental effects of electrical stimulus than oocytes from other species. Reports on whether a delayed activation after electrofusion and a premature chromosome condensation (PCC) is essential for efficient cloning are inconclusive. To address these issues, effects of pulsing on activation and MPF activity of nonenucleated oocytes and effects of delayed activation and MG132 treatment on donor nuclear PCC and preimplantation development of embryos cloned by electrofusion or nuclear injection were compared between different cytoplast ages in mice and goats. The results indicated that the use of oocytes collected early after donor stimulation would make it possible to conduct somatic cell nuclear transfer in mice by electrofusion. Whether a delayed activation is essential was dependent upon the age, or rather, the level, of MPF activity of the cytoplasts at the time of electrofusion, as was the requirement for MG132 treatment. The competence for blastocyst formation of cloned embryos was highly correlated with the level of donor nuclear PCC in recipient cytoplasts. The nuclear injection technique was more adaptable to older cytoplast ages, and hence less dependent on drugs for inhibition of MPF inactivation, compared to electrofusion.

Serial cloning of pigs by somatic cell nuclear transfer: restoration of phenotypic normality during serial cloning

Somatic cell nuclear transfer (scNT) is a useful way to create cloned animals. However, scNT clones exhibit high levels of phenotypic instability. This instability may be due to epigenetic reprogramming and/or genomic damage in the donor cells. To test this, we produced transgenic pig fibroblasts harboring the truncated human thrombopoietin (hTPO) gene and used them as donor cells in scNT to produce first-generation (G1) cloned piglets. In this study, 2,818 scNT embryos were transferred to 11 recipients and five G1 piglets were obtained. Among them, a clone had a dimorphic facial appearance with severe hypertelorism and a broad prominent nasal bridge. The other clones looked normal. Second-generation (G2) scNT piglets were then produced using ear cells from a G1 piglet that had an abnormal nose phenotype. We reasoned that, if the phenotypic abnormality of the G1 clone was not present in the G2 and third-generation (G3) clones, or was absent in the G2 clones but reappeared in the G3 clones, the phenotypic instability of the G1 clone could be attributed to faulty epigenetic reprogramming rather than to inherent/accidental genomic damage to the donor cells. Blastocyst rates, cell numbers in blastocyst, pregnancy rates, term placenta weight and ponderal index, and birth weight between G1 and G2 clones did not differ, but were significantly (P < 0.05) lower than control age- and sex-matched piglets. Next, we analyzed global methylation changes during development of the preimplantation embryos reconstructed by donor cells used for the production of G1 and G2 clones and could not find any significant differences in the methylation patterns between G1 and G2 clones. Indeed, we failed to detect the phenotypic abnormality in the G2 and G3 clones. Thus, the phenotypic abnormality of the G1 clone is likely to be due to epigenetic dysregulation. Additional observations then suggested that expression of the hTPO gene in the transgenic clones did not appear to be the cause of the phenotypic abnormality in the G1 clones and that the abnormality was acquired by only a few of the G1 clone's cells during its gestational development.

Epigenetics in assisted reproductive technology

It has been reported that the rates of epigenetic disorders such as Angelman syndrome (AS) and Beckwith-Wiedemann syndrome (BWS) are high in offspring conceived by assisted reproductive technology (ART). Angelman Syndrome is characterized by intellectual disability and BWS is known as large offspring syndrome (LOS). Weight abnormalities have also been reported in cloned animals. Possible factors underlying these findings include inherent gamete characteristics, influence of in vitro culture and peculiarity of ART methods. It is important to conclusively determine whether such epigenetic abnormalities are present in children conceived by ART, so as to consider the health of next generations.

Suboptimal in vitro culture conditions: an epigenetic origin of long-term health effects

The foetal origins of adult diseases or Barker hypothesis suggests that there can be adverse in uterus effects on the foetus that can lead to certain diseases in adults. Extending this hypothesis to the early stages of embryo development, in particular, to preimplantation stages, it was recently demonstrated that, long-term programming of postnatal development, growth and physiology can be irreversibly affected during this period of embryo development by suboptimal in vitro culture (IVC). As an example, it was found in two recent studies that, mice derived from embryos cultured in suboptimal conditions can suffer from obesity, increased anxiety, and deficiencies on their implicit memory system. In addition, it was observed that suboptimal IVC can cause disease in mature animals by promoting alterations in their genetic imprinting during preimplantation development. Imprinting and other epigenetic mechanisms control the establishment and maintenance of gene expression patterns in the embryo, placenta and foetus. The previously described observations, suggest that the loss of epigenetic regulation during preimplantation development may lead to severe long-term effects. Although mostly tested in rodents, the hypothesis that underlies these studies can also fit assisted reproductive technology (ART) procedures in other species, including humans. The lack of information on how epigenetic controls are lost during IVC, and on the long-term consequences of ART, underscore the necessity for sustained epigenetic analysis of embryos produced in vitro and long-term tracking of the health of the human beings conceived using these procedures.

Comparative proteomic analysis associated with term placental insufficiency in cloned pig

Somatic cell-derived nuclear transfer (scNT) is a method of animal cloning in which the oocyte reprograms a somatic cell nucleus to divide and execute developmental programs. Despite many successes in this field, cloning by scNT remains very inefficient. Unlike other cloned animals, pigs derived by scNT have placentas with severe villous hypoplasia. To obtain a better understanding of the protein networks involved in this phenomenon, we assessed global protein expression profiles in term placentas from scNT-derived and control animals. Proteomic analysis of term placentas from scNT-derived animals identified 43 proteins that were differentially expressed compared to control animals. Among them, 14-3-3 proteins and Annexin V, which are closely involved in the apoptotic signaling pathway, were significantly down- and up-regulated, respectively. Western blot analysis and immunohistochemistry indicated that down-regulation of 14-3-3 proteins in scNT-derived placentas induced apoptosis of cytotrophoblast cells via mitochondria-mediated apoptosis. Taken together, our results suggest that placental insufficiency in scNT-derived placentas may be due to apoptosis, induced in part by the down-regulation of 14-3-3 proteins and up-regulation of Annexin V. They also indicate that proteomic maps represent an important tool for future studies of placental insufficiency and pathology.

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.

Developmental programming of obesity in mammals

Converging lines of evidence from epidemiological studies and animal models now indicate that the origins of obesity and related metabolic disorders lie not only in the interaction between genes and traditional adult risk factors, such as unbalanced diet and physical inactivity, but also in the interplay between genes and the embryonic, fetal and early postnatal environment. Whilst studies in man initially focused on the relationship between low birth weight and risk of adult obesity and metabolic syndrome, evidence is also growing to suggest that increased birth weight and/or adiposity at birth can also lead to increased risk for childhood and adult obesity. Hence, there appears to be increased risk of obesity at both ends of the birth weight spectrum. Animal models, including both under- and overnutrition in pregnancy and lactation lend increasing support to the developmental origins of obesity. This review focuses upon the influence of the maternal nutritional and hormonal environment in pregnancy in permanently programming appetite and energy expenditure and the hormonal, neuronal and autocrine mechanisms that contribute to the maintenance of energy balance in the offspring. We discuss the potential maternal programming 'vectors' and the molecular mechanisms that may lead to persistent pathophysiological changes resulting in subsequent disease. The perinatal environment, which appears to programme subsequent obesity, provides a potential therapeutic target, and work in this field will readily translate into improved interventional strategies to stem the growing epidemic of obesity, a disease which, once manifest, has proven particularly resistant to treatment.

Increased Th1/Th2 (IFN-gamma/IL-4) Cytokine mRNA ratio of rat embryos in the pregnant mouse uterus

Somatic cell nuclei can be dedifferentiated in ooplasm from another species, and interspecies cloned embryos can be implanted into the uteri of surrogates. However, no full pregnancies have been achieved through interspecific mammalian cloning. Rat blastocysts transferred into mouse uteri provide a unique model for studying the causes of interspecific pregnancy failure. In this study, intraspecific pregnancy (mouse-mouse) and interspecific pregnancy (rat-mouse) models were established. On Day 9 of pregnancy, the fetoplacental units were separated from the uterine implantation sites and the expression of messenger (m)RNA was quantitated by real-time PCR. We compared the mRNA expression levels of type-1 T helper (Th1) and type-2 T helper (Th2) cytokines, interferon-gamma (IFN-gamma), and interleukin-4 (IL-4) in fetoplacental units between intraspecific and interspecific pregnancy groups. The mRNA expression of IFN-gamma in the fetoplacental units of the interspecific pregnancy group was significantly higher than that of the intraspecific pregnancy group (P<0.05). The mRNA expression of IL-4 in the interspecific pregnancy group was significantly lower than that in the intraspecific pregnancy group (P<0.05). We also analyzed the ratio of IFN-gamma/IL-4 mRNA, and an increased IFN-gamma/IL-4 mRNA ratio was observed in the interspecific pregnancy compared with that in the intraspecific pregnancy group. The IFN-gamma and IL-4 mRNA expressions indicate that there is a Th1/Th2 imbalance in the feto-maternal interface of interspecific pregnancies. Bias of Th1 cytokine dominance may be a barrier to reproductive success between species.

Production of cloned goats by nuclear transfer of cumulus cells and long-term cultured fetal fibroblast cells into abattoir-derived oocytes

Dairy goats are ideal for the transgenic production of therapeutic recombinant proteins. The use of recombinant somatic cell lines for nuclear transfer (NT) allows the introduction of genes by transfection, increases the efficiency of transgenic animal production to 100%, and overcomes the problem of founder mosaicism. Although viable animals have been cloned via NT from somatic cells of 11 species, the efficiency has been extremely low. Both blastomere and somatic cell NT increased fetal loss and perinatal morbidity/mortality in cattle and sheep, but fetal loss and perinatal mortality appear to be relatively low in goats. In this study, we produced cloned goats by NT from cumulus cells and long-term cultured fetal fibroblast cells (FFCs) to abattoir-derived oocytes. NT embryos were constructed from electrofusion of cumulus cells (CCs), FFCs, or skin fibroblast cells (SFCs) with cytoplasts prepared from abattoir-derived ovaries. The NT embryos were activated with an optimized activating protocol (1 min exposure to 2.5 microM ionomycin followed by 2 hr incubation in 2mM 6-DMAP). Two viable cloned kids from CCs and one from long-term cultured FFCs (at passage 20-25) were born. Microsatellite analysis of 10 markers confirmed that all cloned offspring were derived from corresponding donor cells. To our knowledge, the production of cloned goat offspring using abattoir-derived oocytes receiving nuclei from CCs and long-term cultured FFCs has not been reported. The production of viable cloned animals after activation with reduced intensity of ionomycin and 6-DMAP treatment has also not been reported. Loss of cloned embryos was obvious after 45 and 90 days of pregnancy, and a lack of cotyledons, heart defects, and improperly closed abdominal wall were observed in the aborted fetuses and one cloned kid. The fusibility and in vitro developmental potential of embryos reconstructed from FFCs at passage 20-25 were significantly lower than those of embryos reconstructed from FFCs at passage 3-5, and the cloning efficiency of the long-term cultured cells was low (0.5%).

Placental function in development and disease

The placenta is an organ that clinicians and embryologists would all agree is important for pregnancy success. Unfortunately, however, they too often ignore it when they are exploring causes for embryonic, fetal and perinatal complications. The core function of the placenta is to mediate the transport of nutrients between the maternal and fetal circulation, but it also has critical endocrine functions that alter different maternal physiological systems in order to sustain pregnancy. Both its development and ongoing functions can be dynamically regulated by environmental factors, including nutrient status and tissue oxygenation. In recent years, mainstream attention has begun to shift onto the placenta and it is now becoming clear that placental pathology is associated with several complications in human and animal pregnancies, including embryonic lethality, fetal growth restriction, pre-eclampsia and the high rates of fetal deaths observed after nuclear transfer (cloning).

Epigenetic modification is central to genome reprogramming in somatic cell nuclear transfer

The recent high-profile reports of the derivation of human embryonic stem cells (ESCs) from human blastocysts produced by somatic cell nuclear transfer (SCNT) have highlighted the possibility of making autologous cell lines specific to individual patients. Cell replacement therapies have much potential for the treatment of diverse conditions, and differentiation of ESCs is highly desirable as a means of producing the ranges of cell types required. However, given the range of immunophenotypes of ESC lines currently available, rejection of the differentiated cells by the host is a potentially serious problem. SCNT offers a means of circumventing this by producing ESCs of the same genotype as the donor. However, this technique is not without problems because it requires resetting of the gene expression program of a somatic cell to a state consistent with embryonic development. Some remodeling of parental DNA does occur within the fertilized oocyte, but the somatic genome presented in a radically different format to those of the gametes. Hence, it is perhaps unsurprising that many genes are expressed aberrantly within "cloned" embryos and the ESCs derived from them. Epigenetic modification of the genome through DNA methylation and covalent modification of the histones that form the nucleosome is the key to the maintenance of the differentiated state of the cell, and it is this that must be reset during SCNT. This review focuses on the mechanisms by which this is achieved and how this may account for its partial failure in the "cloning" process. We also highlight the potential dangers this may introduce into ESCs produced by this technology.