Transgenic pigs produced using in vitro matured oocytes infected with a retroviral vector

Generation of mutant pigs by lipofection-mediated genome editing in embryos

The specificity and efficiency of CRISPR/Cas9 gene-editing systems are determined by several factors, including the mode of delivery, when applied to mammalian embryos. Given the limited time window for delivery, faster and more reliable methods to introduce Cas9-gRNA ribonucleoprotein complexes (RNPs) into target embryos are needed. In pigs, somatic cell nuclear transfer using gene-modified somatic cells and the direct introduction of gene editors into the cytoplasm of zygotes/embryos by microinjection or electroporation have been used to generate gene-edited embryos; however, these strategies require expensive equipment and sophisticated techniques. In this study, we developed a novel lipofection-mediated RNP transfection technique that does not require specialized equipment for the generation of gene-edited pigs and produced no detectable off-target events. In particular, we determined the concentration of lipofection reagent for efficient RNP delivery into embryos and successfully generated MSTN gene-edited pigs (with mutations in 7 of 9 piglets) after blastocyst transfer to a recipient gilt. This newly established lipofection-based technique is still in its early stages and requires improvements, particularly in terms of editing efficiency. Nonetheless, this practical method for rapid and large-scale lipofection-mediated gene editing in pigs has important agricultural and biomedical applications.

CRISPR/Cas9-Mediated Gene Editing in Porcine Models for Medical Research

Pigs have been extensively used as the research models for human disease pathogenesis and gene therapy. They are also the optimal source of cells, tissues, and organs for xenotransplantation due to anatomical and physiological similarities to humans. Several breakthroughs in gene-editing technologies, including the advent of clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated 9 (Cas9), have greatly improved the efficiency of genetic manipulation and significantly broadened the application of gene-edited large animal models. In this review, we have not only outlined the important applications of the CRISPR/Cas9 system in pigs as a means to study human diseases but also discussed the potential challenges of the use of CRISPR/Cas9 in large animals.

Porcine germline genome engineering

Germline editing, the process by which the genome of an individual is edited in such a way that the change is heritable, has been applied to a wide variety of animals [D. A. Sorrell, A. F. Kolb, Biotechnol. Adv. 23, 431-469 (2005); D. Baltimore et al., Science 348, 36-38 (2015)]. Because of its relevancy in agricultural and biomedical research, the pig genome has been extensively modified using a multitude of technologies [K. Lee, K. Farrell, K. Uh, Reprod. Fertil. Dev. 32, 40-49 (2019); C. Proudfoot, S. Lillico, C. Tait-Burkard, Anim. Front. 9, 6-12 (2019)]. In this perspective, we will focus on using pigs as the model system to review the current methodologies, applications, and challenges of mammalian germline genome editing. We will also discuss the broad implications of animal germline editing and its clinical potential.

Breed, Diet, and Interaction Effects on Adipose Tissue Transcriptome in Iberian and Duroc Pigs Fed Different Energy Sources

In this study, we analyzed the effects of breed, diet energy source, and their interaction on adipose tissue transcriptome in growing Iberian and Duroc pigs. The study comprised 29 Iberian and 19 Duroc males, which were kept under identical management conditions except the nutritional treatment. Two isoenergetic diets were used with 6% high oleic sunflower oil (HO) or carbohydrates (CH) as energy sources. All animals were slaughtered after 47 days of treatment at an average live weight of 51.2 kg. Twelve animals from each breed (six fed each diet) were employed for ham subcutaneous adipose tissue RNA-Seq analysis. The data analysis was performed using two different bioinformatic pipelines. We detected 837 and 1456 differentially expressed genes (DEGs) according to breed, depending on the pipeline. Due to the strong effect of breed on transcriptome, the effect of the diet was separately evaluated in the two breeds. We identified 207 and 57 DEGs depending on diet in Iberian and Duroc pigs, respectively. A joint analysis of both effects allowed the detection of some breed–diet interactions on transcriptome, which were inferred from RNA-Seq and quantitative PCR data. The functional analysis showed the enrichment of functions related to growth and tissue development, inflammatory response, immune cell trafficking, and carbohydrate and lipid metabolism, and allowed the identification of potential regulators. The results indicate different effects of diet on adipose tissue gene expression between breeds, affecting relevant biological pathways.

Generation of genetically-engineered animals using engineered endonucleases

The key to successful drug discovery and development is to find the most suitable animal model of human diseases for the preclinical studies. The recent emergence of engineered endonucleases is allowing for efficient and precise genome editing, which can be used to develop potentially useful animal models for human diseases. In particular, zinc finger nucleases, transcription activator-like effector nucleases, and the clustered regularly interspaced short palindromic repeat systems are revolutionizing the generation of diverse genetically-engineered experimental animals including mice, rats, rabbits, dogs, pigs, and even non-human primates that are commonly used for preclinical studies of the drug discovery. Here, we describe recent advances in engineered endonucleases and their application in various laboratory animals. We also discuss the importance of genome editing in animal models for more closely mimicking human diseases.

Myogenic potential of mesenchymal stem cells isolated from porcine adipose tissue

Advances in stem cell biology and materials science have provided a basis for developing tissue engineering methods to repair muscle injury. Among stem cell populations with potential to aid muscle repair, adipose-derived mesenchymal stem cells (ASC) hold great promise. To evaluate the possibility of using porcine ASC for muscle regeneration studies, we co-cultured porcine ASC with murine C₂C₁₂ myoblasts. These experiments demonstrated that porcine ASC display significant myogenic potential. Co-culture of ASC expressing green fluorescent protein (GFP) with C₂C₁₂ cells resulted in GFP⁺ myotube formation, indicating fusion of ASC with myoblasts to form myotubes. The presence of porcine lamin A/C positive nuclei in myotubes and RTqPCR analysis of porcine myogenin and desmin expression confirmed that myotube nuclei derived from ASC contribute to muscle gene expression. Co-culturing GFP⁺ASC with porcine satellite cells demonstrated enhanced myogenic capability of ASC, as the percentage of labeled myotubes increased compared to mouse co-cultures. Enhancing myogenic potential of ASC through soluble factor treatment or expansion of ASC with innate myogenic capacity should allow for their therapeutic use to regenerate muscle tissue lost to disease or injury.

Genome editing and genetic engineering in livestock for advancing agricultural and biomedical applications

Genetic modification of livestock has a longstanding and successful history, starting with domestication several thousand years ago. Modern animal breeding strategies predominantly based on marker-assisted and genomic selection, artificial insemination, and embryo transfer have led to significant improvement in the performance of domestic animals, and are the basis for regular supply of high quality animal derived food. However, the current strategy of breeding animals over multiple generations to introduce novel traits is not realistic in responding to the unprecedented challenges such as changing climate, pandemic diseases, and feeding an anticipated 3 billion increase in global population in the next three decades. Consequently, sophisticated genetic modifications that allow for seamless introgression of novel alleles or traits and introduction of precise modifications without affecting the overall genetic merit of the animal are required for addressing these pressing challenges. The requirement for precise modifications is especially important in the context of modeling human diseases for the development of therapeutic interventions. The animal science community envisions the genome editors as essential tools in addressing these critical priorities in agriculture and biomedicine, and for advancing livestock genetic engineering for agriculture, biomedical as well as "dual purpose" applications.

Genome-editing technologies to improve research, reproduction, and production in pigs

The ability to directly manipulate the pig genome through genetic engineering has been available to the research community for over three decades. This technology has progressed from the random insertion of foreign DNA, via a variety of techniques (pronuclear microinjection, sperm mediated gene transfer, and integration of mobile genetic elements), to manipulation of endogenous genes, via homologous recombination in somatic cells followed by somatic cell nuclear transfer. Over the last few years, designer nucleases facilitated the development of techniques that provide efficient ways to introduce foreign DNA or to modify endogenous genes in eggs, zygotes, or somatic cells. Together, these genome-editing technologies have essentially removed the obstacles to gene manipulation in swine. Although the regulatory environment is still unclear for agricultural applications, genetic engineering of pigs will continue to advance biomedicine and biology. In addition, genetic engineering is now sufficiently simple and efficient that agricultural research can now ask basic and applied questions that are not hampered by limited funding.

Genome Editing of Pig

Pigs are important livestock for food and have been used in various biomedical studies, particularly translational research, as experimental animals because of their anatomical and physiological similarity to humans. The recent development of genome editing techniques, such as ZFN, TALEN, and CRISPR/Cas9, has rapidly expanded the use of genome editing tools in a variety of animals, resulting in the relatively easy and efficient generation of gene knock-out pigs. In the past few years, there has been a sustained increase in reports describing the development of genetically modified pigs. This chapter introduces our workflow for establishing the genetically modified cells (nuclear donor cells) necessary to create gene knock-out pigs using somatic cell nuclear transfer and focuses on the actual generation of gene knock-out pigs using a cytoplasmic injection method.

Gene targeting, genome editing: from Dolly to editors

One of the most powerful strategies to investigate biology we have as scientists, is the ability to transfer genetic material in a controlled and deliberate manner between organisms. When applied to livestock, applications worthy of commercial venture can be devised. Although initial methods used to generate transgenic livestock resulted in random transgene insertion, the development of SCNT technology enabled homologous recombination gene targeting strategies to be used in livestock. Much has been accomplished using this approach. However, now we have the ability to change a specific base in the genome without leaving any other DNA mark, with no need for a transgene. With the advent of the genome editors this is now possible and like other significant technological leaps, the result is an even greater diversity of possible applications. Indeed, in merely 5 years, these 'molecular scissors' have enabled the production of more than 300 differently edited pigs, cattle, sheep and goats. The advent of genome editors has brought genetic engineering of livestock to a position where industry, the public and politicians are all eager to see real use of genetically engineered livestock to address societal needs. Since the first transgenic livestock reported just over three decades ago the field of livestock biotechnology has come a long way-but the most exciting period is just starting.

Meganucleases Revolutionize the Production of Genetically Engineered Pigs for the Study of Human Diseases

Animal models of human diseases are critically necessary for developing an in-depth knowledge of disease development and progression. In addition, animal models are vital to the development of potential treatments or even cures for human diseases. Pigs are exceptional models as their size, physiology, and genetics are closer to that of humans than rodents. In this review, we discuss the use of pigs in human translational research and the evolving technology that has increased the efficiency of genetically engineering pigs. With the emergence of the clustered, regularly interspaced, short palindromic repeat (CRISPR)/CRISPR-associated (Cas) protein 9 system technology, the cost and time it takes to genetically engineer pigs has markedly decreased. We will also discuss the use of another meganuclease, the transcription activator-like effector nucleases , to produce pigs with severe combined immunodeficiency by developing targeted modifications of the recombination activating gene 2 (RAG2).RAG2mutant pigs may become excellent animals to facilitate the development of xenotransplantation, regenerative medicine, and tumor biology. The use of pig biomedical models is vital for furthering the knowledge of, and for treating human, diseases.

The Effects of GH Transgenic Goats on the Microflora of the Intestine, Feces and Surrounding Soil

The development of genetically engineered animals has brought with it increasing concerns about biosafety issues. We therefore evaluated the risks of growth hormone from transgenic goats, including the probability of horizontal gene transfer and the impact on the microbial community of the goats’ gastrointestinal tracts, feces and the surrounding soil. The results showed that neither the GH nor the neoR gene could be detected in the samples. Moreover, there was no significant change in the microbial community of the gastrointestinal tracts, feces and soil, as tested with PCR-denaturing gradient gel electrophoresis and 16S rDNA sequencing. Finally, phylogenetic analysis showed that the intestinal content, feces and soil samples all contained the same dominant group of bacteria. These results demonstrated that expression of goat growth hormone in the mammary of GH transgenic goat does not influence the microflora of the intestine, feces and surrounding soil.

Efficient CRISPR/Cas9-mediated biallelic gene disruption and site-specific knockin after rapid selection of highly active sgRNAs in pigs

Genetic engineering in livestock was greatly enhanced by the emergence of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9), which can be programmed with a single-guide RNA (sgRNA) to generate site-specific DNA breaks. However, the uncertainties caused by wide variations in sgRNA activity impede the utility of this system in generating genetically modified pigs. Here, we described a single blastocyst genotyping system to provide a simple and rapid solution to evaluate and compare the sgRNA efficiency at inducing indel mutations for a given gene locus. Assessment of sgRNA mutagenesis efficiencies can be achieved within 10 days from the design of the sgRNA. The most effective sgRNA selected by this system was successfully used to induce site-specific insertion through homology-directed repair at a frequency exceeding 13%. Additionally, the highly efficient gene deletion via the selected sgRNA was confirmed in pig fibroblast cells, which could serve as donor cells for somatic cell nuclear transfer. We further showed that direct cytoplasmic injection of Cas9 mRNA and the favorable sgRNA into zygotes could generate biallelic knockout piglets with an efficiency of up to 100%. Thus, our method considerably reduces the uncertainties and expands the practical possibilities of CRISPR/Cas9-mediated genome engineering in pigs.

Production of transgenic cloned pigs expressing the far-red fluorescent protein monomeric Plum

Monomeric Plum (Plum), a far-red fluorescent protein with photostability and photopermeability, is potentially suitable for in vivo imaging and detection of fluorescence in body tissues. The aim of this study was to generate transgenic cloned pigs exhibiting systemic expression of Plum using somatic cell nuclear transfer (SCNT) technology. Nuclear donor cells for SCNT were obtained by introducing a Plum-expression vector driven by a combination of the cytomegalovirus early enhancer and chicken beta-actin promoter into porcine fetal fibroblasts (PFFs). The cleavage and blastocyst formation rates of reconstructed SCNT embryos were 81.0% (34/42) and 78.6% (33/42), respectively. At 36-37 days of gestation, three fetuses systemically expressing Plum were obtained from one recipient to which 103 SCNT embryos were transferred (3/103, 2.9%). For generation of offspring expressing Plum, rejuvenated PFFs were established from one cloned fetus and used as nuclear donor cells. Four cloned offspring and one stillborn cloned offspring were produced from one recipient to which 117 SCNT embryos were transferred (5/117, 4.3%). All offspring exhibited high levels of Plum fluorescence in blood cells, such as lymphocytes, monocytes and granulocytes. In addition, the skin, heart, kidney, pancreas, liver and spleen also exhibited Plum expression. These observations demonstrated that transfer of the Plum gene did not interfere with the development of porcine SCNT embryos and resulted in the successful generation of transgenic cloned pigs that systemically expressed Plum. This is the first report of the generation and characterization of transgenic cloned pigs expressing the far-red fluorescent protein Plum.

Role of stem cells in large animal genetic engineering in the TALENs-CRISPR era

The establishment of embryonic stem cells (ESCs) and gene targeting technologies in mice has revolutionised the field of genetics. The relative ease with which genes can be knocked out, and exogenous sequences introduced, has allowed the mouse to become the prime model for deciphering the genetic code. Not surprisingly, the lack of authentic ESCs has hampered the livestock genetics field and has forced animal scientists into adapting alternative technologies for genetic engineering. The recent discovery of the creation of induced pluripotent stem cells (iPSCs) by upregulation of a handful of reprogramming genes has offered renewed enthusiasm to animal geneticists. However, much like ESCs, establishing authentic iPSCs from the domestic animals is still beset with problems, including (but not limited to) the persistent expression of reprogramming genes and the lack of proven potential for differentiation into target cell types both in vitro and in vivo. Site-specific nucleases comprised of zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered regulated interspaced short palindromic repeats (CRISPRs) emerged as powerful genetic tools for precisely editing the genome, usurping the need for ESC-based genetic modifications even in the mouse. In this article, in the aftermath of these powerful genome editing technologies, the role of pluripotent stem cells in livestock genetics is discussed.

The association of mitochondrial potential and copy number with pig oocyte maturation and developmental potential

ATP is critical for oocyte maturation, fertilization, and subsequent embryo development. Both mitochondrial membrane potential and copy number expand during oocyte maturation. In order to differentiate the roles of mitochondrial metabolic activity and mtDNA copy number during oocyte maturation, we used two inhibitors, FCCP (carbonyl cyanide p-(tri-fluromethoxy)phenyl-hydrazone) and ddC (2’3-dideoxycytidine), to deplete the mitochondrial membrane potential (Δφ_m) and mitochondrial copy number, respectively. FCCP (2000 nM) reduced ATP production by affecting mitochondrial Δφ_m, decreased the mRNA expression of Bmp15 (bone morphogenetic protein 15), and shortened the poly(A) tails of Bmp15, Gdf9 (growth differentiation factor 9), and Cyclin B1 transcripts. FCCP (200 and 2000 nM) also affected p34^cdc2 kinase activity. By contrast, ddC did not alter ATP production. Instead, ddC significantly decreased mtDNA copy number (P < 0.05). FCCP (200 and 2000 nM) also decreased extrusion of the first polar body, whereas ddC at all concentrations did not affect the ability of immature oocytes to reach metaphase II. Both FCCP (200 and 2000 nM) and ddC (200 and 2000 µM) reduced parthenogenetic blastocyst formation compared with untreated oocytes. However, these inhibitors did not affect total cell number and apoptosis. These findings suggest that mitochondrial metabolic activity is critical for oocyte maturation and that both mitochondrial metabolic activity and replication contribute to the developmental competence of porcine oocytes.

Generation of transgenic Wuzhishan miniature pigs expressing monomeric red fluorescent protein by somatic cell nuclear transfer

Red fluorescent protein and its variants enable researchers to study gene expression, localization, and protein-protein interactions in vitro in real-time. Fluorophores with higher wavelengths are usually preferred since they efficiently penetrate tissues and produce less toxic emissions. A recently developed fluorescent protein marker, monomeric red fluorescent protein (mRFP1), is particularly useful because of its rapid maturation and minimal interference with green fluorescent protein (GFP) and GFP-derived markers. We generated a pCX-mRFP1-pgk-neoR construct and evaluated the ability of mRFP1 to function as a fluorescent marker in transgenic Wuzhishan miniature pigs. Transgenic embryos were generated by somatic cell nuclear transfer (SCNT) of nuclei isolated from ear fibroblasts expressing mRFP1. Embryos generated by SCNT developed into blastocysts in vitro (11.65%; 31/266). Thereafter, a total of 685 transgenic embryos were transferred into the oviducts of three recipients, two of which became pregnant. Of these, one recipient had six aborted fetuses, whereas the other recipient gave birth to four offspring. All offspring expressed the pCX-mRFP1-pgk-neoR gene as shown by PCR and fluorescence in situ hybridization analysis. The transgenic pigs expressed mRFP1 in all organs and tissues at high levels. These results demonstrate that Wuzhishan miniature pigs can express mRFP1. To conclude, this transgenic animal represents an excellent model with widespread applications in medicine and agriculture.

Nutritional composition analysis of meat from human lactoferrin transgenic bulls

Transgenic technology has many potential advantages in food production. However, the transgenic technology process may influence the composition of food products derived from genetically engineered (GE) animals, which may be adverse to human health. Therefore, it is very important to research the compositions of GE animal products. Here, we analyzed the compositions of meat from the offspring of human lactoferrin (hLF) transgenic cows, which can express human lactoferrin proteins in their mammary gland. Six hLF transgenic bulls and three wide-type (WT) bulls, 10 months of age, were slaughtered for meat composition analysis. To determine the comparative health of hLF bulls for meat analysis, hematological analyses, organ/body weight analyses and pathology analyses were conducted. Results of the meat analysis show that there were no significant differences in the hematological parameters, organ/body weight ratios of hLF and WT bulls (P>0.05), and histopathological examination of the main organs of hLF bulls revealed no abnormalities. Nutrient parameters of meat compositions of hLF and WT bulls did not show any significant differences (P>0.05). All of these results suggest that the hLF transgene did not have an impact on the meat nutrient compositions of hLF bulls.

Genetically engineered pig models for human diseases

Although pigs are used widely as models of human disease, their utility as models has been enhanced by genetic engineering. Initially, transgenes were added randomly to the genome, but with the application of homologous recombination, zinc finger nucleases, and transcription activator-like effector nuclease (TALEN) technologies, now most any genetic change that can be envisioned can be completed. To date these genetic modifications have resulted in animals that have the potential to provide new insights into human diseases for which a good animal model did not exist previously. These new animal models should provide the preclinical data for treatments that are developed for diseases such as Alzheimer's disease, cystic fibrosis, retinitis pigmentosa, spinal muscular atrophy, diabetes, and organ failure. These new models will help to uncover aspects and treatments of these diseases that were otherwise unattainable. The focus of this review is to describe genetically engineered pigs that have resulted in models of human diseases.

Completion of the swine genome will simplify the production of swine as a large animal biomedical model

Background

Anatomic and physiological similarities to the human make swine an excellent large animal model for human health and disease.

Methods

Cloning from a modified somatic cell, which can be determined in cells prior to making the animal, is the only method available for the production of targeted modifications in swine.

Results

Since some strains of swine are similar in size to humans, technologies that have been developed for swine can be readily adapted to humans and vice versa. Here the importance of swine as a biomedical model, current technologies to produce genetically enhanced swine, current biomedical models, and how the completion of the swine genome will promote swine as a biomedical model are discussed.

Conclusions

The completion of the swine genome will enhance the continued use and development of swine as models of human health, syndromes and conditions.

Impacts of human lysozyme transgene on the microflora of pig feces and the surrounding soil

The rapid development of genetic engineering and extensive applications of genetically engineered (GE) animals have provided many research benefits, but concerns have been raised over the potential environmental impact of transgenic animals. We investigated the effects of human lysozyme (hLZ) transgenic pigs which can express hLZ in their mammary glands on the surrounding environment from the angle of the changes of pig feces and the surrounding soil, including the probability of horizontal gene transfer (HGT), the impact on microbial communities in pig gastrointestinal (GI) tracts and soil, and the influence on the total nitrogen (TN) and total phosphorus (TP) content of pig excrement and surrounding soil. Results showed that hLZ gene was not detected by polymerase chain reaction (PCR) or quantitative real-time PCR (Q-PCR) in gut microbial DNA extracts of manure or microbial DNA extracts of topsoil. PCR-Denaturing Gradient Gel Electrophoresis (PCR-DGGE) analysis and 16S rDNA sequence analysis showed that hLZ gene had no impact on the microflora structure of pig guts or soil. Finally, TN and TP contents were not significantly different in pig manure or soils taken at different distances from the pig site (P>0.25).

Human germline genetic modification: scientific and bioethical perspectives

The latest mammalian genetic modification technology offers efficient and reliable targeting of genomic sequences, in the guise of designer genetic recombination tools. These and other improvements in genetic engineering technology suggest that human germline genetic modification (HGGM) will become a safe and effective prospect in the relatively near future. Several substantive ethical objections have been raised against HGGM including claims of unacceptably high levels of risk, damage to the status of future persons, and violations of justice and autonomy. This paper critically reviews the latest GM science and discusses the key ethical objections to HGGM. We conclude that major benefits are likely to accrue through the use of safe and effective HGGM and that it would thus be unethical to take a precautionary stance against HGGM.

Precision editing of large animal genomes

Transgenic animals are an important source of protein and nutrition for most humans and will play key roles in satisfying the increasing demand for food in an ever-increasing world population. The past decade has experienced a revolution in the development of methods that permit the introduction of specific alterations to complex genomes. This precision will enhance genome-based improvement of farm animals for food production. Precision genetics also will enhance the development of therapeutic biomaterials and models of human disease as resources for the development of advanced patient therapies.

Welfare assessment in transgenic pigs expressing green fluorescent protein (GFP)

Since large animal transgenesis has been successfully attempted for the first time about 25 years ago, the technology has been applied in various lines of transgenic pigs. Nevertheless one of the concerns with the technology--animal welfare--has not been approached through systematic assessment and statements regarding the welfare of transgenic pigs have been based on anecdotal observations during early stages of transgenic programs. The main aim of the present study was therefore to perform an extensive welfare assessment comparing heterozygous transgenic animals expressing GFP with wildtype animals along various stages of post natal development. The protocol used covered reproductory performance and behaviour in GFP and wildtype sows and general health and development, social behaviour, exploratory behaviour and emotionality in GFP and wildtype littermates from birth until an age of roughly 4 months. The absence of significant differences between GFP and wildtype animals in the parameters observed suggests that the transgenic animals in question are unlikely to suffer from deleterious effects of transgene expression on their welfare and thus support existing anecdotal observations of pigs expressing GFP as healthy. Although the results are not surprising in the light of previous experience, they give a more solid fundament to the evaluation of GFP expression as being relatively non-invasive in pigs. The present study may furthermore serve as starting point for researchers aiming at a systematic characterization of welfare relevant effects in the line of transgenic pigs they are working with.

Genetic modifications of pigs for medicine and agriculture

Genetically modified swine hold great promise in the fields of agriculture and medicine. Currently, these swine are being used to optimize production of quality meat, to improve our understanding of the biology of disease resistance, and to reduced waste. In the field of biomedicine, swine are anatomically and physiologically analogous to humans. Alterations of key swine genes in disease pathways provide model animals to improve our understanding of the causes and potential treatments of many human genetic disorders. The completed sequencing of the swine genome will significantly enhance the specificity of genetic modifications, and allow for more accurate representations of human disease based on syntenic genes between the two species. Improvements in both methods of gene alteration and efficiency of model animal production are key to enabling routine use of these swine models in medicine and agriculture.

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.

Pig transgenesis by Sleeping Beauty DNA transposition

Modelling of human disease in genetically engineered pigs provides unique possibilities in biomedical research and in studies of disease intervention. Establishment of methodologies that allow efficient gene insertion by non-viral gene carriers is an important step towards development of new disease models. In this report, we present transgenic pigs created by Sleeping Beauty DNA transposition in primary porcine fibroblasts in combination with somatic cell nuclear transfer by handmade cloning. Göttingen minipigs expressing green fluorescent protein are produced by transgenesis with DNA transposon vectors carrying the transgene driven by the human ubiquitin C promoter. These animals carry multiple copies (from 8 to 13) of the transgene and show systemic transgene expression. Transgene-expressing pigs carry both transposase-catalyzed insertions and at least one copy of randomly inserted plasmid DNA. Our findings illustrate critical issues related to DNA transposon-directed transgenesis, including coincidental plasmid insertion and relatively low Sleeping Beauty transposition activity in porcine fibroblasts, but also provide a platform for future development of porcine disease models using the Sleeping Beauty gene insertion technology.

Production of genetically modified porcine blastocysts by somatic cell nuclear transfer: preliminary results toward production of xenograft-competent miniature pigs

Galα1-3Gal (α-Gal epitope) is the major xenoantigenic epitope responsible for hyperacute rejection upon pig-to-human xenotransplantation. Endo-β-galactosidase C (EndoGalC) from Clostridium perfringens can digest the α-Gal epitope. In this study, gene-engineered primary cultured porcine embryonic fibroblasts (PEF) expressing EndoGalC were obtained and subjected to somatic cell nuclear transfer (SCNT) to test whether xenograft-competent pigs can be created. The EndoGalC-expressing PEF clones exhibited highly reduced expression of α-Gal epitope, as revealed by cytochemical staining with BS-I-B(4) isolectin, a lectin that specifically binds to α-Gal epitope, and FACS analysis. The pattern of low level of α-Gal epitope expression continued for at least 6 months (more than 10 generations) after isolation. SCNT of nuclei from these cells resulted in the generation of blastocysts that displayed nearly complete loss of α-Gal epitope from their cell surface. This is the first study to demonstrate that SCNT using EndoGalC-expressing PEFs as donors would be useful for production of genetically modified cloned pigs suitable for xenotransplantation.

Production of transgenic piglets using ICSI-sperm-mediated gene transfer in combination with recombinase RecA

Sperm-mediated gene transfer (SMGT) is a method for the production of transgenic animals based on the intrinsic ability of sperm cells to bind and internalize exogenous DNA molecules and to transfer them into the oocyte at fertilization. Recombinase-A (RecA) protein-coated exogenous DNA has been used previously in pronuclear injection systems increasing integration into goat and pig genomes. However, there are no data regarding transgene expression after ICSI. Here, we set out to investigate whether the expression of transgenic DNA in porcine embryos is improved by recombinase-mediated DNA transfer and if it is possible to generate transgenic animals using this methodology. Different factors which could affect the performance of this transgenic methodology were analyzed by studying 1) the effect of the presence of exogenous DNA and RecA protein on boar sperm functionality; 2) the effect of recombinase RecA on in vitro enhanced green fluorescent protein (EGFP)-expressing embryos produced by ICSI or IVF; and 3) the efficiency of generation of transgenic piglets by RecA-mediated ICSI. Our results suggested that 1) the presence of exogenous DNA and RecA-DNA complexes at 5 microg/ml did not affect sperm functionality in terms of motility, viability, membrane lipid disorder, or reactive oxygen species generation; 2) EGFP-expressing embryos were obtained with a high efficiency using the SMGT-ICSI technique in combination with recombinase; however, the use of IVF system did not result in any fluorescent embryos; and 3) transgenic piglets were produced by this methodology. To our knowledge, this is the first time that transgenic pigs have been produced by ICSI-SGMT and a recombinase.

Application of genetically modified and cloned pigs in translational research

Pigs are increasingly being recognized as good large-animal models for translational research, linking basic science to clinical applications in order to establish novel therapeutics. This article reviews the current status and future prospects of genetically modified and cloned pigs in translational studies. It also highlights pigs specially designed as disease models, for xenotransplantation or to carry cell marker genes. Finally, use of porcine somatic stem and progenitor cells in preclinical studies of cell transplantation therapy is also discussed.

Lentiviral integration preferences in transgenic mice

Lentiviral gene transfer has a significant impact on the development of biomedical research. One of the most important features of lentiviruses is the capability to infect both dividing and nondividing cells. However, little is known whether integration preference exists, specifically in early embryos. An in-depth genome analysis on 112 independent lentiviral integration sites from 43 transgenic founder mice was performed to determine if there are preferable sites for lentiviral integration in early embryonic genome. Our results demonstrated that lentiviruses were biased in integrating within intragenic regions, especially in the introns. However, no integration preference was found associated with specific chromosomes, repetitive elements, or CpG islands, nor was there any preference for integrating at close proximity to transcription start sites. Our findings suggested that lentiviruses were biased to integrate into the intragenic regions of early embryonic genome of mouse.

Transgenic-cloned pigs systemically expressing red fluorescent protein, Kusabira-Orange

Genetically engineered pigs with cell markers such as fluorescent proteins are highly useful in lines of research that include the tracking of transplanted cells or tissues. In this study, we produced transgenic-cloned pigs carrying a gene for the newly developed red fluorescent protein, humanized Kusabira-Orange (huKO), which was cloned from the coral stone Fungia concinna. The nuclear transfer embryos, reconstructed with fetal fibroblast cells that had been transduced with huKO cDNA using retroviral vector D Delta Nsap, developed efficiently in vitro into blastocysts (28.0%, 37/132). Nearly all (94.6%, 35/37) of the cloned blastocysts derived from the transduced cells exhibited clear huKO gene expression. A total of 429 nuclear transfer embryos were transferred to four recipients, all of which became pregnant and gave birth to 18 transgenic-cloned offspring in total. All of the pigs highly expressed huKO fluorescence in all of the 23 organs and tissues analyzed, including the brain, eyes, intestinal and reproductive organs, skeletal muscle, bone, skin, and hoof. Furthermore, such expression was also confirmed by histological analyses of various tissues such as pancreatic islets, renal corpuscles, neuronal and glial cells, the retina, chondrocytes, and hematopoietic cells. These data demonstrate that transgenic-cloned pigs exhibiting systemic red fluorescence expression can be efficiently produced by nuclear transfer of somatic cells retrovirally transduced with huKO gene.

Nuclear remodeling and nuclear reprogramming for making transgenic pigs by nuclear transfer

A better understanding of the cellular and molecular events that occur when a nucleus is transferred to the cytoplasm of an oocyte will permit the development of improved procedures for performing nuclear transfer and cloning. In some cases it appears that the gene(s) are reprogrammed, while in other cases there appears to be little effect on gene expression. Not only does the pattern of gene expression need to be reprogrammed, but other structures within the nucleus also need to be remodeled. While nuclear transfer works and transgenic and knockout animals can be created, it still is an inefficient process. However, even with the current low efficiencies this technique has proved very valuable for the production of animals that might be useful for tissue or organ transplantation to humans.

Development and calcium level changes in pre-implantation porcine nuclear transfer embryos activated with 6-DMAP after fusion

This study investigated the effect of treatment with 6-dimethylaminopurine (6-DMAP) following fusion on in vitro development of porcine nuclear transfer (NT) embryos. Frozen thawed ear skin cells were transferred into the perivitelline space of enucleated oocytes. Reconstructed oocytes were fused and activated with electric pulse in 0.3 M mannitol supplemented with either 0.1 or 1.0 mM CaCl(2). In each calcium concentration, activated oocytes were divided into three groups. Two groups of them were exposed to either ionomycin (I + 6-DMAP or 6-DMAP alone. In experiment 2, fused NT embryos in 0.3 M mannitol containing 1.0 mM CaCl(2) were exposed to 6-DMAP either immediately or 20 min after fusion/activation. For 0.1 mM CaCl(2), oocytes activated with either I + 6-DMAP or 6-DMAP alone showed a higher (P < 0.05) developmental rate to the blastocyst stage than those activated with an electric pulse alone (26.7 and 22.5 vs. 12.5%). For 1.0 mM CaCl(2), oocytes activated with either I + 6-DMAP or 6-DMAP alone showed significantly higher (P < 0.05) developmental rate to the blastocyst stage (35.6 and 28.3 vs. 19.8%). Developmental rate to the blastocyst stage was (P < 0.05) increased in NT embryos activated with 6-DMAP 20 min after fusion. 6-DMAP made a higher and wider Ca(2+) transient compared to that induced by electric pulses (Fig. 3). The fluctuation lasted during the time that oocytes were cultured in 6-DMAP. Regardless of Ca(2+) concentration in fusion medium, activation with 6-DMAP following electric pulses supported more development of porcine NT embryos. Activation of NT embryos with 6-DMAP after fusion in the presence of 1.0 mM CaCl(2) could support better developmental rate to the blastocyst stage.

Lentiviral vectors: are they the future of animal transgenesis?

Lentiviral vectors have become a promising new tool for the establishment of transgenic animals and the manipulation of the mammalian genome. While conventional microinjection-based methods for transgenesis have been successful in generating small and large transgenic animals, their relatively low transgenic efficiency has opened the door for alternative approaches, including lentiviral vectors. Lentiviral vectors are an appealing tool for transgenesis in part because of their ability to incorporate into genomic DNA with high efficiency, especially in cells that are not actively dividing. Lentiviral vector-mediated transgene expression can also be maintained for long periods of time. Recent studies have documented high efficiencies for lentiviral transgenesis, even in animal species and strains, such as NOD/ scid and C57Bl/6 mouse, that are very difficult to manipulate using the standard transgenic techniques. These advantages of the lentiviral vector system have broadened its use as a gene therapy vector to additional applications that include transgenesis and knockdown functional genetics. This review will address the components of the lentiviral vector system and recent successes in lentiviral transgenesis using both male- and female-derived pluripotent cells. The advantages and disadvantages of lentiviral transgenesis vs. other approaches to produce transgenic animals will be compared with regard to efficiency, the ability to promote persistent transgene expression, and the time necessary to generate a sufficient number of animals for phenotyping.

Enhanced transgenesis by intracytoplasmic injection of envelope-free lentivirus

We demonstrate enhanced transgenesis in mice by intracytoplasmic injection of envelope-free lentivirus. Envelope-free lentivirus carrying the green fluorescent protein (GFP) gene under the control of the ubiquitin promoter (LVU-GFP) was microinjected into the cytoplasm of mouse zygotes prior to embryo transfer. Ninety-seven percent (31/32) of the adult mice were confirmed transgenic by PCR and Southern blot analysis; all founder mice express GFP when tail snips were examined by fluorescent microscopy prior to genomic DNA extraction. Transgene insertion numbers ranging from 1 to 32 were revealed by Southern blot analysis. Germline transmission was confirmed by the presence of transgene in F1 offspring. As expected, a lower transgenic rate (2.2%; 1/46) resulted when envelope-free LVU-GFP was microinjected into the perivitelline space (PVS) because cell recognition followed by membrane fusion between the viral envelope and the target cell is prerequisite for successful infection by envelope viruses. Here we demonstrate the competence of envelope-free lentivirus in establishing stable gene integration by germline transgenesis in mice at high efficiency, by intracytoplasmic viral injection (INVI) of envelope-free lentivirus into mouse zygotes.

Activation of ribosomal RNA genes in porcine embryos produced in vitro or by somatic cell nuclear transfer

The onset of ribosomal RNA (rRNA) synthesis occurs during the second half of the third cell cycle, that is, at the four-cell stage, in porcine embryos developed in vivo. In the present study the onset of rRNA synthesis was investigated in porcine embryos produced in vitro (IVP) or by somatic cell nuclear transfer (SCNT) using fluorescence in situ hybridization (FISH) with an rDNA probe and subsequent visualization of the nucleolar proteins by silver staining. In the 205 IVP embryos investigated, all two-cell embryos (n = 34) were categorized as transcriptionally inactive. At the late four-cell stage (n = 45), 38% of the embryos contained 1-3 nuclei with signs of rRNA transcription, indicating an asynchronous transcription initiation. This pattern continued in the following stages, as 78% (n = 47), 47% (n = 42) and 83% (n = 37) of the embryos revealed a mixture of transcriptionally inactive and active cells at the eight-cell, 16-cell and blastocyst stage, respectively. In the 143 SCNT embryos investigated, all two-cell embryos (n = 34) and early four-cell embryos (n = 12) were also transcriptionally inactive. At the late four-cell stage (n = 33) and at the eight-cell stage (n = 24) there were equal proportions of transcriptionally active and inactive embryos and essentially all embryos that developed to the 16-cell stage (n = 21) and further to the blastocyst stage (n = 19) contained only transcriptionally active cells. In conclusion, porcine embryos produced in vitro had an asynchronous pattern of rRNA transcription initiation when compared to SCNT and in vivo developed porcine embryos.

Multipotent Adult Progenitor Cell Lines Originating from the Peripheral Blood of Green Fluorescent Protein Transgenic Swine

Multipotent self-renewing stem cell lines have been established using peripheral blood mononuclear cells from adult green fluorescent protein transgenic swine. These cells proliferate as nonadherent spheroids in primordial-specific culture media and readily differentiate into angiogenic, osteogenic, adipogenic, and neurogenic phenotypes when cultured under the appropriate conditions. These cells are designated peripheral blood-derived multipotent adult progenitor cells (PBD-MAPCs). When differentiated in endothelial-specific media, these cells exhibit a cobblestone morphology and express von Willebrand factor (vWF), take up 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarboxyanine-labeled acetylated low-density lipoprotein DiI-Ac-LDL, and form tubes with lumens when grown on pads of Matrigel. Under different culture conditions, the cells appear whorl-like in appearance and express alpha-actin, indicative of smooth muscle phenotype. In the presence of dexamethasone and ascorbic acid, PBD-MAPCs differentiate into cells that produce Alizarin Red-staining extracellular mineral, consistent with an osteogenic potential. Under different conditions the cells produce Oil Red O-staining lipid vacuoles, suggestive of an adipocyte phenotype. We have also developed conditions that induce PBDMAPCs to differentiate into neural cells, confirmed by the expression of specific neuron- and glial-specific markers. Upon transplantation into rat brain, the neurogenic cells survive and migrate throughout the striatum and corpus callosum. The cells remain brightly fluorescent throughout their time in culture, during in vitro differentiation, and after in vivo transplantation. PBD-MAPCs have been maintained in primordial cell media for more than 100 doublings, yet can be induced to differentiate rapidly and efficiently into distinct cell types. PBD-MAPCs are ideal tools to study the mechanisms of differentiation and may be superior to embryonic stem cells as cellular therapeutics.

Production of a transgenic piglet by a sperm injection technique in which no chemical or physical treatments were used for oocytes or sperm

As a method of producing transgenic animals, spermatozoa have been used to fertilize mammalian oocytes through natural copulation, artificial insemination (AI), and in vitro fertilization (IVF). Our objective was to produce live piglets expressing the enhanced green fluorescent protein (eGFP) by the modified ICSI procedure based on Yong et al. (2003) (Hum. Reprod. 18:2390) where this procedure resulted in an improvement in development in vitro as compared to conventional ICSI and IVF. After injecting frozen-thawed sperm, recovered from the descendant of a transgenic boar derived by oocyte transduction, into in vitro matured oocytes the injected oocytes were surgically transferred into the oviduct of six surrogate gilts. Two gilts (33%) became pregnant. One gave birth to a healthy male piglet. Expression of the eGFP was easily observed in the nose and hooves by direct epifluorescent examination in the newborn piglet. These results show the production of the first viable transgenic piglet by in vitro maturation and our new sperm injection method.