High-level expression of single-chain Fv-Fc fusion protein in serum and egg white of genetically manipulated chickens by using a retroviral vector

Current Approaches to and the Application of Intracytoplasmic Sperm Injection (ICSI) for Avian Genome Editing

Poultry are one of the most valuable resources for human society. They are also recognized as a powerful experimental animal for basic research on embryogenesis. Demands for the supply of low-allergen eggs and bioreactors have increased with the development of programmable genome editing technology. The CRISPR/Cas9 system has recently been used to produce transgenic animals and various animals in the agricultural industry and has also been successfully adopted for the modification of chicken and quail genomes. In this review, we describe the successful establishment of genome-edited lines combined with germline chimera production systems mediated by primordial germ cells and by viral infection in poultry. The avian intracytoplasmic sperm injection (ICSI) system that we previously established and recent advances in ICSI for genome editing are also summarized.

Delivery of Fc-fusion Protein by a Recombinant Newcastle Disease Virus Vector

Fc-fusion proteins (FCPs), a new generation biological medicine, have revolutionized the practice of medicines that treat diseases. However, complex manufacturing techniques are required for FCP production, casting the affordability and accessibility issues in low- and middle-income economies (LMIEs). Virus-vectored system may serve as a simple and cost-effective platform for FCP delivery. As a proof-of-concept study, Newcastle disease virus (NDV), a widely-used vector for vaccine generation, was used as a vector to express and deliver a model FCP composed of the hemagglutinin (HA) and IgG Fc. A recombinant NDV expressing the HA-Fc fusion protein was generated using reverse genetics, which had comparable replication and virulence to the parental virus. High levels of expression of soluble HA-Fc were detected in cell culture and embryonated chicken eggs inoculated with the recombinant NDV. In addition, the recombinant NDV replicated in the lung of mouse, delivering the HA-Fc protein to this organ. The HA-Fc expressed by NDV specifically bound to murine FcγRI, which was dependent on the presence of the Fc tag. The recombinant NDV induced high vector-specific antibody response, whereas it failed to elicit H7N9-specific antibody immunity in mice. The absence of HA-specific antibodies may be attributed to deficient incorporation of the HA-Fc protein into NDV virion particles. Our results indicated that NDV may be potentially used as a vector for FCP expression and delivery. This strategy may help to enhance the affordability and equal accessibility of FCP biological medicines, especially in LIMEs.

Genome Editing Mediated by Primordial Germ Cell in Chicken

Genome editing technology has facilitated the studies on exploring specific gene functions in diverse living organisms. The technology has also contributed to creating high-value livestock in industry fields in terms of enhancing productivity or acquiring disease resistance. Particularly, applying genome editing technologies in avian species has been emphasized in both academic and industrial fields due to their unique developmental patterns as well as application possibilities. To accomplish genome editing in avian species, gene integration into chicken primordial germ cell (PGC) genome using a virus or transposition systems has been widely used, and recently developed programmable genome editing technologies including clustered regularly interspaced short palindromic repeat (CRISPR) and CRISPR-associated (Cas9) systems enable to edit the genetic information precisely for maximizing the application potentials of avian species. In these regards, this chapter will cover the methods for producing genome-edited chickens, particularly by CRISPR/Cas9 technologies allowing targeted gene insertion, gene knockout, and gene tagging.

Direct N- or C-Terminal Protein Labeling Via a Sortase-Mediated Swapping Approach

Site-specific protein labeling is important in biomedical research and biotechnology. While many methods allow site-specific protein modification, a straightforward approach for efficient N-terminal protein labeling is not available. We introduce a novel sortase-mediated swapping approach for a one-step site-specific N-terminal labeling with a near-quantitative yield. We show that this method allows rapid and efficient cleavage and simultaneous labeling of the N or C termini of fusion proteins. The method does not require any prior modification beyond the genetic incorporation of the sortase recognition motif. This new approach provides flexibility for protein engineering and site-specific protein modifications.

Genetically engineered birds; pre-CRISPR and CRISPR era

Generating biopharmaceuticals in genetically engineered bioreactors continues to reign supreme. Hence, genetically engineered birds have attracted considerable attention from the biopharmaceutical industry. Fairly recent genome engineering methods have made genome manipulation an easy and affordable task. In this review, we first provide a broad overview of the approaches and main impediments ahead of generating efficient and reliable genetically engineered birds, and various factors that affect the fate of a transgene. This section provides an essential background for the rest of the review, in which we discuss and compare different genome manipulation methods in the pre-CRISPR and CRISPR era in the field of avian genome engineering.

IgY technology: Methods for developing and evaluating avian immunoglobulins for the in vitro detection of biomolecules

The term “IgY technology” was introduced in the literature in the mid 1990s to describe a procedure involving immunization of avian species, mainly laying hens and consequent isolation of the polyclonal IgYs from the “immune” egg yolk (thus avoiding bleeding and animal stress). IgYs have been applied to various fields of medicine and biotechnology. The present article will deal with specific aspects of IgY technology, focusing on the currently reported methods for developing, isolating, evaluating and storing polyclonal IgYs. Other topics such as current information on isolation protocols or evaluation of IgYs from different avian species are also discussed. Specific advantages of IgY technology (e.g., novel antibody specificities that may emerge via the avian immune system) will also be discussed. Recent in vitro applications of polyclonal egg yolk-derived IgYs to the field of disease diagnosis in human and veterinary medicine through in vitro immunodetection of target biomolecules will be presented. Moreover, ethical aspects associated with animal well-being as well as new promising approaches that are relevant to the original IgY technology (e.g., development of monoclonal IgYs and IgY-like antibodies through the phage display technique or in transgenic chickens) and future prospects in the area will also be mentioned.

Improving germline transmission efficiency in chimeric chickens using a multi-stage injection approach

Although different strategies have been developed to generate transgenic poultry, low efficiency of germline transgene transmission has remained a challenge in poultry transgenesis. Herein, we developed an efficient germline transgenesis method using a lentiviral vector system in chickens through multiple injections of transgenes into embryos at different stages of development. The embryo chorioallantoic membrane (CAM) vasculature was successfully used as a novel route of gene transfer into germline tissues. Compared to the other routes of viral vector administration, the embryo’s bloodstream at Hamburger-Hamilton (HH) stages 14–15 achieved the highest rate of germline transmission (GT), 7.7%. Single injection of viral vectors into the CAM vasculature resulted in a GT efficiency of 2.7%, which was significantly higher than the 0.4% obtained by injection into embryos at the blastoderm stage. Double injection of viral vectors into the bloodstream at HH stages 14–15 and through CAM was the most efficient method for producing germline chimeras, giving a GT rate of 13.6%. The authors suggest that the new method described in this study could be efficiently used to produce transgenic poultry in virus-mediated gene transfer systems.

piggyBac Transposition and the Expression of Human Cystatin C in Transgenic Chickens

Simple Summary

The genetic modification of livestock genomes showed the great potential for production of industrial biomaterials as well as improving animal production. Particularly, the transgenic hen’s eggs have been considered for a massive production system of the genetically engineered biomaterials as a bioreactor animal. Virus-mediated transgene transduction is the most powerful strategy to generate the transgenic animals. However, industrial applications were hampered by many obstacles such as relatively low germline transmission and transgene silencing effects, as well as viral safety issues. In this study, a piggyBac transposon which is a non-viral integration technical platform was introduced into chicken primordial germ cells. Finally, we developed transgenic chickens and assayed the bioactivity of human cystatin C in the transgenic chicken’s tissues.

Abstract

A bioreactor can be used for mass production of therapeutic proteins and other bioactive substances. Although various methods have been developed using microorganisms and animal cells, advanced strategies are needed for the efficient production of biofunctional proteins. In microorganisms, post-translational glycosylation and modification are not performed properly, while animal cell systems require more time and expense. To overcome these problems, new methods using products from transgenic animals have been considered, such as genetically modified cow’s milk and hen’s eggs. In this study, based on a non-viral piggyBac transposition system, we generated transgenic bioreactor chickens that produced human cystatin C (hCST3). There were no differences in the phenotype or histochemical structure of the wild-type and hCST3-expressing transgenic chickens. Subsequently, we analyzed the hCST3 expression in transgenic chickens, mainly in muscle and egg white, which could be major deposition warehouses for hCST3 protein. In both muscle and egg white, we detected high hCST3 expression by ELISA and Western blotting. hCST3 proteins were efficiently purified from muscle and egg white of transgenic chickens using a His-tag purification system. These data show that transgenic chickens can be efficiently used as a bioreactor for the mass production of bioactive materials.

LINE-1 vectors mediate recombinant antibody gene transfer by retrotransposition in Chinese hamster ovary cells

Retrotransposons, such as long interspersed element-1 (LINE-1), can copy themselves to other genomic loci via a transposition event (termed retrotransposition). Retrotransposons, therefore, have potential use as an efficient gene delivery tool to integrate multiple copies of a target gene into a host genome. Here, we developed a retrotransposon vector based on LINE-1 that achieves target gene integration of multiple transgene copies. The retrotransposon vector contains a neomycin resistance gene split by an intron as a marker gene, and a gene encoding an antibody single-chain variable fragment (Fv) fused with the constant antibody region (Fc) (scFv-Fc) as a model target gene. G418-resistant Chinese hamster ovary cells were generated using this retrotransposon vector, and scFv-Fc was produced in the culture medium. To regulate retrotransposition, we developed a retrotransposon vector system that separately expressed the two open reading frames (ORF1 and ORF2) of LINE-1. Genomic PCR analysis detected the transgene sequence in almost all tested clones. Compared with clones established using the intact LINE-1 vector, clones generated with the split ORF1 and ORF2 system showed similar specific scFv-Fc productivity and retrotransposition efficiency. This approach of using a retrotransposon-based vector system has the potential to provide a new gene delivery tool for mammalian cells.

Regulatory mechanism of chicken lysozyme gene expression in oviducts examined using transgenic technology

The use of promoters that strongly express target genes in the chicken oviduct is beneficial for the production of proteinaceous materials into egg white by transgenic chickens. To examine the regulatory mechanisms of chicken lysozyme gene expression in vivo, genetically manipulated chickens that express human erythropoietin under the control of a lysozyme promoter-enhancer were established. By using several deletion mutants of the promoter-flanking region, we found that a -1.9 kb DNase I hypersensitive site (DHS) was essential for oviduct-specific expression in genetically manipulated chickens. The concentration of human erythropoietin in egg white was 14-75 μg/ml, suggesting that the chicken lysozyme promoter containing -1.9 kb DHS is sufficient for the production of pharmaceuticals using transgenic chickens.

Calcium carbonate supplementation to chorioallantoic membranes improves hatchability in shell-less chick embryo culture

Developing chick embryos are a classical research tool in developmental biology. The whole embryo culture technique can be applied to various fields, such as embryo manipulation, toxicology, tumorigenesis, and basic research in regenerative medicine. When used for the generation of transgenic chickens, a high hatchability of genetically engineered embryos is essential to support normal embryonic development during culture. In this study, calcium carbonate, which is the main component of eggshells, was added as a calcium source in shell-less chick embryo cultures using a transparent plastic film as a culture vessel. In the absence of a calcium source in the shell-less culture system, embryogenesis ceased during culture, resulting in failed embryonic hatching. We found that the direct addition of calcium carbonate to the chorioallantoic membrane of the developing embryo was effective for the hatching of cultured chick embryos. The amount, timing, and location of calcium carbonate addition were investigated to maximize the hatchability of cultured embryos. Starting from the time of calcium carbonate supplementation, >40% hatchability was obtained with the optimal condition. This established method of shell-less chick embryo culture provides a useful tool in basic and applied fields of chick embryo manipulation.

Primordial germ cell-specific expression of eGFP in transgenic chickens

PR domain zinc finger protein 14 (PRDM14) plays an essential role in the development of primordial germ cells (PGCs) in mice. However, its functions in avian species remain unclear. In the present study, we used CRISPR/Cas9 to edit the PRDM14 locus in chickens in order to demonstrate its importance in development. The eGFP gene was introduced into the PRDM14 locus of cultured chicken PGCs to knockout PRDM14 and label PGCs. Chimeric chickens were established by a direct injection of eGFP knocked-in (gene-trapped) PGCs into the blood vessels of Hamburger-Hamilton stages (HH-stages) 13-16 chicken embryos. Gene-trapped chickens were established by crossing a chimeric chicken with a wild-type hen with very high efficiency. Heterozygous gene-trapped chickens grew normally and SSEA-1-positive cells expressed eGFP during HH-stages 13-30. These results indicated the specific expression of eGFP within circulating PGCs and gonadal PGCs. At the blastodermal stage, the ratio of homozygous gene-trapped embryos obtained by crossing heterozygous gene-trapped roosters and hens was almost normal; however, all embryos died soon afterward, suggesting the important roles of PRDM14 in chicken early development.

Current Approaches and Applications in Avian Genome Editing

Advances in genome-editing technologies and sequencing of animal genomes enable researchers to generate genome-edited (GE) livestock as valuable animal models that benefit biological researches and biomedical and agricultural industries. As birds are an important species in biology and agriculture, their genome editing has gained significant interest and is mainly performed by using a primordial germ cell (PGC)-mediated method because pronuclear injection is not practical in the avian species. In this method, PGCs can be isolated, cultured, genetically edited in vitro, and injected into a recipient embryo to produce GE offspring. Recently, a couple of GE quail have been generated by using the newly developed adenovirus-mediated method. Without technically required in vitro procedures of the PGC-mediated method, direct injection of adenovirus into the avian blastoderm in the freshly laid eggs resulted in the production of germ-line chimera and GE offspring. As more approaches are available in avian genome editing, avian research in various fields will progress rapidly. In this review, we describe the development of avian genome editing and scientific and industrial applications of GE avian species.

Targeted knock-in into the OVA locus of chicken cells using CRISPR/Cas9 system with homology-independent targeted integration

It is anticipated that transgenic avian species will be used as living bioreactors for the production of biopharmaceutical proteins. Precise tissue-specific expression of exogenous genes is a major challenge for the development of avian bioreactors. No robust vector is currently available for highly efficient and specific expression. In recent years, genome-editing techniques such as the CRISPR/Cas9 system have emerged as efficient and user-friendly genetic modification tools. Here, to apply the CRISPR/Cas9 system for the development of transgenic chickens, guide RNA sequences (gRNAs) of the CRISPR/Cas9 system for the ovalbumin (OVA) locus were evaluated for the oviduct-specific expression of exogenous genes. An EGFP gene expression cassette was introduced into the OVA locus of chicken DF-1 and embryonic fibroblasts using the CRISPR/Cas9 system mediated by homology-independent targeted integration. For the knock-in cells, EGFP expression was successfully induced by activation of the endogenous OVA promoter using the dCas9-VPR transactivation system. The combination of gRNAs designed around the OVA TATA box was important to induce endogenous OVA gene expression with high efficiency. These methods provide a useful tool for studies on the creation of transgenic chicken bioreactors and the activation of tissue-specific promoters.

Designing A Transgenic Chicken: Applying New Approaches toward A Promising Bioreactor

Specific developmental characteristics of the chicken make it an attractive model for the generation of transgenic organisms. Chicken possess a strong potential for recombinant protein production and can be used as a powerful bioreactor to produce pharmaceutical and nutritional proteins. Several transgenic chickens have been generated during the last two decades via viral and non-viral transfection. Culturing chicken primordial germ cells (PGCs) and their ability for germline transmission ushered in a new stage in this regard. With the advent of CRISPR/Cas9 system, a new phase of studies for manipulating genomes has begun. It is feasible to integrate a desired gene in a predetermined position of the genome using CRISPR/Cas9 system. In this review, we discuss the new approaches and technologies that can be applied to generate a transgenic chicken with regards to recombinant protein productions.

Efficient production of human interferon beta in the white of eggs from ovalbumin gene-targeted hens

Transgenic chickens could potentially serve as bioreactors for commercial production of recombinant proteins in egg white. Many transgenic chickens have been generated by randomly integrating viral vectors into their genomes, but transgene expression has proved insufficient and/or limited to the initial cohort. Herein, we demonstrate the feasibility of integrating human interferon beta (hIFN-β) into the chicken ovalbumin locus and producing hIFN-β in egg white. We knocked in hIFN-β into primordial germ cells using a CRISPR/Cas9 protocol and then generated germline chimeric roosters by cell transplantation into recipient embryos. Two generation-zero founder roosters produced hIFN-β knock-in offspring, and all knock-in female offspring produced abundant egg-white hIFN-β (~3.5 mg/ml). Although female offspring of the first generation were sterile, their male counterparts were fertile and produced a second generation of knock-in hens, for which egg-white hIFN-β production was comparable with that of the first generation. The hIFN-β bioactivity represented only ~5% of total egg-white hIFN-β, but unfolding and refolding of hIFN-β in the egg white fully recovered the bioactivity. These results suggest that transgene insertion at the chicken ovalbumin locus can result in abundant and stable expression of an exogenous protein deposited into egg white and should be amenable to industrial applications.

Generation of transgenic chickens expressing the human erythropoietin (hEPO) gene in an oviduct-specific manner: Production of transgenic chicken eggs containing human erythropoietin in egg whites

The transgenic chicken has been considered as a prospective bioreactor for large-scale production of costly pharmaceutical proteins. In the present study, we report successful generation of transgenic hens that lay eggs containing a high concentration of human erythropoietin (hEPO) in the ovalbumin. Using a feline immunodeficiency virus (FIV)-based pseudotyped lentivirus vector enveloped with G glycoproteins of the vesicular stomatitis virus, the replication-defective vector virus carrying the hEPO gene under the control of the chicken ovalbumin promoter was microinjected to the subgerminal cavity of freshly laid chicken eggs (stage X). Stable germline transmission of the hEPO transgene to the G1 progeny, which were non-mosaic and hemizygous for the hEPO gene under the ovalbumin promoter, was confirmed by mating of a G0 rooster with non-transgenic hens. Quantitative analysis of hEPO in the egg whites and in the blood samples taken from G1 transgenic chickens showed 4,810 ~ 6,600 IU/ml (40.1 ~ 55.0 μg/ml) and almost no detectable concentration, respectively, indicating tightly regulated oviduct-specific expression of the hEPO transgene. In terms of biological activity, there was no difference between the recombinant hEPO contained in the transgenic egg white and the commercially available counterpart, in vitro. We suggest that these results imply an important step toward efficient production of human cytokines from a transgenic animal bioreactor.

Targeted knock-in of an scFv-Fc antibody gene into the hprt locus of Chinese hamster ovary cells using CRISPR/Cas9 and CRIS-PITCh systems

Chinese hamster ovary (CHO) cells have been used as host cells for the production of pharmaceutical proteins. For the high and stable production of target proteins, the transgene should be integrated into a suitable genomic locus of host cells. Here, we generated knock-in CHO cells, in which transgene cassettes without a vector backbone sequence were integrated into the hprt locus of the CHO genome using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 and CRISPR-mediated precise integration into target chromosome (CRIS-PITCh) systems. We investigated the efficiency of targeted knock-in of transgenes using these systems. As a practical example, we generated knock-in CHO cells producing an scFv-Fc antibody using the CRIS-PITCh system mediated by microhomology sequences for targeting. We found that the CRIS-PITCh system can facilitate targeted knock-in for CHO cell engineering.

Genome Modification Technologies and Their Applications in Avian Species

The rapid development of genome modification technology has provided many great benefits in diverse areas of research and industry. Genome modification technologies have also been actively used in a variety of research areas and fields of industry in avian species. Transgenic technologies such as lentiviral systems and piggyBac transposition have been used to produce transgenic birds for diverse purposes. In recent years, newly developed programmable genome editing tools such as transcription activator-like effector nuclease (TALEN) and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) have also been successfully adopted in avian systems with primordial germ cell (PGC)-mediated genome modification. These genome modification technologies are expected to be applied to practical uses beyond system development itself. The technologies could be used to enhance economic traits in poultry such as acquiring a disease resistance or producing functional proteins in eggs. Furthermore, novel avian models of human diseases or embryonic development could also be established for research purposes. In this review, we discuss diverse genome modification technologies used in avian species, and future applications of avian biotechnology.

Avian Biotechnology

Primordial germ cells (PGCs) generate new individuals through differentiation, maturation and fertilization. This means that the manipulation of PGCs is directly linked to the manipulation of individuals, making PGCs attractive target cells in the animal biotechnology field. A unique biological property of avian PGCs is that they circulate temporarily in the vasculature during early development, and this allows us to access and manipulate avian germ lines. Following the development of a technique for transplantation, PGCs have become central to avian biotechnology, in contrast to the use of embryo manipulation and subsequent transfer to foster mothers, as in mammalian biotechnology. Today, avian PGC transplantation combined with recent advanced manipulation techniques, including cell purification, cryopreservation, depletion, and long-term culture in vitro, have enabled the establishment of genetically modified poultry lines and ex-situ conservation of poultry genetic resources. This chapter introduces the principles, history, and procedures of producing avian germline chimeras by transplantation of PGCs, and the current status of avian germline modification as well as germplasm cryopreservation. Other fundamental avian reproductive technologies are described, including artificial insemination and embryo culture, and perspectives of industrial applications in agriculture and pharmacy are considered, including poultry productivity improvement, egg modification, disease resistance impairment and poultry gene "pharming" as well as gene banking.

Expression of interferon-inducible transmembrane proteins in the chicken and possible role in prevention of viral infections

In mammals, interferon-inducible transmembrane proteins (IFITMs) prevent infections by various enveloped viruses. The expression of IFITMs in chicken was herein examined in the adult and embryonic organs using a quantitative reverse-transcription-polymerase chain reaction. The results obtained revealed that IFITM3 was expressed at a higher level than IFITM1, 2 and 5, in both embryonic and adult organs. However, the expression levels of IFITMs in embryonic organs were less than 5 % of those in adult lungs. Among the embryonic tissues examined, primordial germ cells (PGCs) at day 2.5 expressed relatively higher levels of IFITM3. IFITM3 expression levels were 1.5-fold higher in the chicken cell line DF-1 than in PGCs. The knockdown of IFITM3 in DF-1 cells by siRNA increased the infectivity of a vesicular stomatitis virus G protein-pseudotyped lentiviral vector, suggesting that lower levels of IFITM3 are still sufficient to restrict this viral vector.

Analyses of chicken sialyltransferases related to O-glycosylation

The chicken β-galactoside α2,3-sialyltransferase 1, 2, and 5 (ST3Gal1, 2, and 5) genes were cloned, and their enzymes were expressed in 293FT cells. ST3Gal1 and 2 exhibited enzymatic activities toward galactose-β1,3-N-acetylgalactosamine and galactose-β1,3-N-acetylglucosamine. ST3Gal5 only exhibited activity toward lactosylceramide. ST3Gal1 and 2 and previously cloned ST3Gal3 and 6 transferred CMP-sialic acid to asialofetuin. Reverse-transcription-quantitative PCR indicated that ST3Gal1 was expressed at higher levels in the trachea, lung, spleen, and magnum, and the strong expression of ST3Gal5 was observed in the spleen, magnum, and small and large intestines. ST3Gal1, 5, and 6 were also expressed in the tubular gland cells of the magnum, which secretes egg-white proteins. ST3Gal1, 5, and 6 were expressed in the egg chorioallantoic membrane, in which influenza viruses are propagated for the production of vaccines.

Homologous Recombination-Independent Large Gene Cassette Knock-in in CHO Cells Using TALEN and MMEJ-Directed Donor Plasmids

Gene knock-in techniques have rapidly evolved in recent years, along with the development and maturation of genome editing technology using programmable nucleases. We recently reported a novel strategy for microhomology-mediated end-joining-dependent integration of donor DNA by using TALEN or CRISPR/Cas9 and optimized targeting vectors, named PITCh (Precise Integration into Target Chromosome) vectors. Here we describe TALEN and PITCh vector-mediated integration of long gene cassettes, including a single-chain Fv-Fc (scFv-Fc) gene, in Chinese hamster ovary (CHO) cells, with comparison of targeting and cloning efficiency among several donor design and culture conditions. We achieved 9.6-kb whole plasmid integration and 7.6-kb backbone-free integration into a defined genomic locus in CHO cells. Furthermore, we confirmed the reasonable productivity of recombinant scFv-Fc protein of the knock-in cells. Using our protocol, the knock-in cell clones could be obtained by a single transfection and a single limiting dilution using a 96-well plate, without constructing targeting vectors containing long homology arms. Thus, the study described herein provides a highly practical strategy for gene knock-in of large DNA in CHO cells, which accelerates high-throughput generation of cell lines stably producing any desired biopharmaceuticals, including huge antibody proteins.

Site-specific recombination in the chicken genome using Flipase recombinase-mediated cassette exchange

Targeted genome recombination has been applied in diverse research fields and has a wide range of possible applications. In particular, the discovery of specific loci in the genome that support robust and ubiquitous expression of integrated genes and the development of genome-editing technology have facilitated rapid advances in various scientific areas. In this study, we produced transgenic (TG) chickens that can induce recombinase-mediated gene cassette exchange (RMCE), one of the site-specific recombination technologies, and confirmed RMCE in TG chicken-derived cells. As a result, we established TG chicken lines that have, Flipase (Flp) recognition target (FRT) pairs in the chicken genome, mediated by piggyBac transposition. The transgene integration patterns were diverse in each TG chicken line, and the integration diversity resulted in diverse levels of expression of exogenous genes in each tissue of the TG chickens. In addition, the replaced gene cassette was expressed successfully and maintained by RMCE in the FRT predominant loci of TG chicken-derived cells. These results indicate that targeted genome recombination technology with RMCE could be adaptable to TG chicken models and that the technology would be applicable to specific gene regulation by cis-element insertion and customized expression of functional proteins at predicted levels without epigenetic influence.

Germline Modification and Engineering in Avian Species

Production of genome-edited animals using germline-competent cells and genetic modification tools has provided opportunities for investigation of biological mechanisms in various organisms. The recently reported programmed genome editing technology that can induce gene modification at a target locus in an efficient and precise manner facilitates establishment of animal models. In this regard, the demand for genome-edited avian species, which are some of the most suitable model animals due to their unique embryonic development, has also increased. Furthermore, germline chimera production through long-term culture of chicken primordial germ cells (PGCs) has facilitated research on production of genome-edited chickens. Thus, use of avian germline modification is promising for development of novel avian models for research of disease control and various biological mechanisms. Here, we discuss recent progress in genome modification technology in avian species and its applications and future strategies.

Expression of recombinant human lysozyme in egg whites of transgenic hens

Chicken egg lysozyme (cLY) is an enzyme with 129 amino acid (AA) residue enzyme. This enzyme is present not only in chicken egg white but also in mucosal secretions such as saliva and tears. The antibacterial properties of egg white can be attributed to the presence of lysozyme, which is used as an anti-cancer drug and for the treatment of human immunodeficiency virus (HIV) infection. In this study, we constructed a lentiviral vector containing a synthetic cLY signal peptide and a 447 bp synthetic human lysozyme (hLY) cDNA sequence driven by an oviduct-specific ovalbumin promoter, and microinjected into the subgerminal cavity of stage X chick embryos to generate transgenic chicken. The transgene inserted in the chicken chromosomes directs the synthesis and secretion of hLY which has three times higher specific activity than cLY. Three G₁ transgenic chickens were identified, the only female of which expressed recombinant human lysozyme (rhLY) at 57.66 ± 4.10 μg/ml in the egg white and the G₂ transgenic hens of the G₁ transgenic cock A011 expressed rhLY at 48.72 ± 1.54 μg/ml. This experiment demonstrated that transgenic hens with stable oviduct-specific expression of recombinant human lysozyme proteins can be created by microinjection of lentiviral vectors. The results of this research could be contribute to the technological development using transgenic hens as a cost-effective alternative to other mammalian systems, such as cow, sheep and goats, for the production of therapeutic proteins and other applications.

Analyses of chicken sialyltransferases related to N-glycosylation

Proteins exogenously expressed and deposited in the egg whites of transgenic chickens did not contain terminal sialic acid in their N-glycan. Since this sugar is important for the biological stability of therapeutic proteins, we examined chicken sialyltransferases (STs). Based on homologies in DNA sequences, we cloned and expressed several chicken STs, which appeared to be involved in N-glycosylation in mammals, in 293FT cells. Enzymatic activity was detected with ST3Gal3, ST3Gal6 and ST6Gal1 using galactose-β1,4-N-acetylglucosamine (Galβ1,4GlcNAc) as an acceptor. Using Golgi fractions from the cell-free extracts of chicken organs, α2,3- and/or α2,6-ST activities were detected in the liver and kidney, but were absent in the oviduct cells in which egg-white proteins were produced. This result suggested that the lack of ST activities in oviduct cells mainly caused the lack of sialic acid in the N-glycan of proteins exogenously expressed and deposited in egg white.

Current and future reproductive technologies for avian species

The global demand for poultry meat and eggs is expected to increase exponentially in the next several decades. Increasing global poultry production in the future would require significant improvements in genetics, nutrition, and managerial practices including reproduction. This chapter summarizes some of the recent developments in ameliorating reproductive dysfunction in broiler breeder chickens, cryopreservation of avian spermatozoa, sex selection, and avian transgenesis.

Transgenesis and imaging in birds, and available transgenic reporter lines

Avian embryos are important model organism to study higher vertebrate development. Easy accessibility to developing avian embryos enables a variety of experimental applications to understand specific functions of molecules, tissue-tissue interactions, and cell lineages. The whole-mount ex ovo culture technique for avian embryos permits time-lapse imaging analysis for a better understanding of cell behaviors underlying tissue morphogenesis in physiological conditions. To study mechanisms of blood vessel formation and remodeling in developing embryos by using a time-lapse imaging approach, a transgenic quail model, Tg(tie1:H2B-eYFP), was generated. From a cell behavior perspective, Tg(tie1:H2B-eYFP) quail embryos are a suitable model to shed light on how the structure and pattern of blood vessels are established in higher vertebrates. In this manuscript, we give an overview on the biological and technological background of the transgenic quail model and describe procedures for the ex ovo culture of quail embryos and time-lapse imaging analysis.

Heat-inducible transgene expression with transcriptional amplification mediated by a transactivator

PURPOSE

Control of therapeutic gene expression in tumours is a major goal of gene therapy research, as it can restrict cytotoxic gene expression in cancer cells. In addition, the combination of hyperthermia with gene therapy through the application of heat-inducible vectors can result in considerable improvements in therapeutic efficiency. In this study, to combine heat-inducibility with high-level transgene expression, we developed a heat-inducible transgene expression system with transcriptional amplification mediated by a tetracycline-responsive transactivator.

MATERIALS AND METHODS

A hybrid promoter was generated by placing the heat shock protein (HSP) 70B' promoter under the tetracycline-repressor responsive element sequence, and a reporter/therapeutic gene expression plasmid was constructed by placing a reporter/therapeutic gene under the control of this hybrid promoter.

RESULTS

When the transactivator expression plasmid harbouring an expression cassette of the tetracycline-responsive transactivator gene was co-transfected with a reporter gene expression plasmid, the reporter gene expression was controlled by heat treatment. With this system, high levels of heat-induced transgene expression were observed compared to that from the HSP promoter alone without the transactivator. Evaluation of in vitro therapeutic effects using cancer cell lines revealed that therapeutic gene expression effectively caused cell death in a greater percentage of the cells.

CONCLUSION

These findings indicate that this strategy improves the efficacy of cancer gene therapy.

Transgenic quail production by microinjection of lentiviral vector into the early embryo blood vessels

Several strategies have been used to generate transgenic birds. The most successful method so far has been the injection of lentiviral vectors into the subgerminal cavity of a newly laid egg. We report here a new, easy and effective way to produce transgenic quails through direct injection of a lentiviral vector, containing an enhanced-green fluorescent protein (eGFP) transgene, into the blood vessels of quail embryos at Hamburger-Hamilton stage 13-15 (HH13-15). A total of 80 embryos were injected and 48 G0 chimeras (60%) were hatched. Most injected embryo organs and tissues of hatched quails were positive for eGFP. In five out of 21 mature G0 male quails, the semen was eGFP-positive, as detected by polymerase chain reaction (PCR), indicating transgenic germ line chimeras. Testcross and genetic analyses revealed that the G0 quail produced transgenic G1 offspring; of 46 G1 hatchlings, 6 were transgenic (6/46, 13.0%). We also compared this new method with the conventional transgenesis using stage X subgerminal cavity injection. Total 240 quail embryos were injected by subgerminal cavity injection, of which 34 (14.1%) were hatched, significantly lower than the new method. From these hatched quails semen samples were collected from 19 sexually matured males and tested for the transgene by PCR. The transgene was present in three G0 male quails and only 4/236 G1 offspring (1.7%) were transgenic. In conclusion, we developed a novel bird transgenic method by injection of lentiviral vector into embryonic blood vessel at HH 13-15 stage, which result in significant higher transgenic efficiency than the conventional subgerminal cavity injection.

Oral immunotherapy for pollen allergy using T-cell epitope-containing egg white derived from genetically manipulated chickens

Peptide immunotherapy using T-cell epitopes is expected to be an effective treatment for allergic diseases such as Japanese cedar (Cryptomeria japonica; Cj) pollinosis. To develop a treatment for pollen allergy by inducing oral tolerance, we generated genetically manipulated (GM) chickens by retroviral gene transduction, to produce a fusion protein of chicken egg white lysozyme and a peptide derived from seven dominant human T-cell epitopes of Japanese cedar pollen allergens (cLys-7crp). The transgene sequence was detected in all chickens transduced with the retroviral vector. Transduction efficiency in blood cells correlated to transgene expression. Western blot analysis revealed that cLys-7crp was expressed in the egg white of GM hens. Mice induced to develop allergic rhinitis by Cj pollinosis were fed with cLys-7crp-containing egg white produced by GM chickens. Total and Cj allergen (Cry j 1)-specific IgE levels were significantly decreased in allergic mice fed with cLys-7crp-containing egg white compared with allergic mice fed with normal egg white. These results suggest that oral administration of T-cell epitope-containing egg white derived from GM chickens is effective for the induction of immune tolerance as an allergy therapy.

piggyBac transposition into primordial germ cells is an efficient tool for transgenesis in chickens

Transgenic birds embody one of the most potent and exciting research tools in biotechnology for agriculture, medicine, and model animals. To date, retrovirus- or lentivirus-mediated transgenesis has been established in chickens and quail. However, despite having a valid technique for viral transduction to achieve transgenic birds, many obstacles exist for practical applications because of relatively low and variable rates of germ-line transmission and transgenic offspring showing transgene silencing, as well as safety issues related to viral vector use. Thus, the generation of transgenic poultry by nonviral integration is a prerequisite for the introduction of biotechnology to practical applications. Herein, we show that a germ-line-competent chicken primordial germ-cell (PGC) line was established with high efficiency of transmission to offspring and that piggyBac transposition into PGCs improved the efficiency of transgenic chicken production and led to high-level transgene expression. GFP transgene-expressing donor PGC-transferred recipient chickens produced donor-derived progenies, and the germ-line transmission efficiency of donor PGCs was 95.2% on average. Subsequently, half of the donor-derived offspring (52.2%) were transgenic chicks because GFP-expressing donor PGCs, in which a transgene was inserted into one chromosome 20, were heterozygous. In all of the transgenic chickens, GFP expression was constant and strong, regardless of age. Our results demonstrate that piggyBac transposition into the chicken PGC line could be the surest way to generate transgenic chickens safely for practical applications.

Transgenic chimera quail production by microinjecting lentiviral vector into the blood vessel of the early embryo

In the past, several strategies have been used to generate transgenic birds. The most successful method has proven to be injection of lentiviral vector into the subgerminal cavity of the newly laid egg. In this study, we directly injected lentiviral vector into the blood vessel of HH13-15 quail embryos to produce transgenic chimeras. In the manipulated, hatched birds, the green fluorescent protein (GFP) gene driven by a cytomegalovirus (CMV) promoter was extensively expressed. All tissues analyzed were GFP-positive, and gonad cells from some of the manipulated embryos expressed GFP. The semen genome of 21.4% of mature male birds was determined to be GFP-positive by PCR, indicating these male birds were transgenic chimeras.

Avian biomodels for use as pharmaceutical bioreactors and for studying human diseases

Animal-based biotechnologies involve the use of domestic animals for the production of pharmaceuticals and various proteins in milk and eggs, as disease models, as tools for stem cell research and animal cloning, and as sources of organs for xenotransplantation into humans. Avian species offer several advantages over mammalian models, and they have been used historically to advance the fields of embryology, immunology, oncology, virology, and vaccine development. In addition, avian species can be used for studying the etiology of human ovarian cancer and other human diseases such as disorders based on the abnormal metabolism of lipids and as unique mechanisms for the biosynthesis and transport of cholesterol. This review integrates recent progress and insight into the molecular and physiologic mechanisms associated with transgenic birds and gives an overview of the use of avian models as pharmaceutical bioreactors and as tools for studying human diseases.