Transgenic chickens as bioreactors for protein-based drugs

Sustainable production of multimeric and functional recombinant human adiponectin using genome-edited chickens

BACKGROUND

Adiponectin (ADPN) plays a critical role in endocrine and cardiovascular functions, but traditional production methods, such as Escherichia coli and mammalian systems, have faced challenges in generating sufficiently active middle molecular weight (MMW) and high molecular weight (HMW) forms of recombinant human ADPN (hADPN). In our previous study, we proposed genome-edited chickens as an efficient platform for producing multimeric hADPN. However, the consistency of multimeric hADPN expression in this system across generations had not been further investigated.

RESULTS

In this study, subsequent generations of ovalbumin (OVA) ADPN knock-in chickens showed stable multimeric hADPN production, yielding ~ 26% HMW ADPN (0.59 mg/mL) per hen. Comparative analysis revealed that egg white (EW)-derived hADPN predominantly consisted of hexameric and HMW forms, similar to serum-derived hADPN. In contrast, hADPN obtained from human embryonic kidney (HEK) 293 and High-Five (Hi-5) cells also exhibited the presence of trimers, indicating variability across different production systems. Furthermore, transcriptional expression analysis of ADPN multimerization-associated endoplasmic reticulum chaperone genes (Ero1-Lα, DsbA-L, ERP44, and PDI) indicated upregulation in the oviduct magnum of ADPN KI hens, suggesting the chicken oviduct magnum as the optimal site for HMW ADPN production. Lastly, the functional analysis demonstrated that EW-derived hADPN significantly reduced lipid droplets and downregulated lipid accumulation-related genes (LOX-1, AT1R, FAS, and FABP4) in human umbilical vein endothelial cells (HUVECs).

CONCLUSION

In summary, stable and functional multimeric hADPN can be produced in genome-edited chickens even after generations. This highlights the potential of using chicken bioreactor for producing various high-value proteins.

An ovalbumin fusion strategy to increase recombinant protein secretion in chicken eggs

Maternal secretion of recombinant proteins into chicken eggs may provide a viable approach for pharmaceutical production but remains limited by poor secretion efficiency through the membrane of oviduct cells, despite high expression levels. Here, we used site-specific integration of an EGFP fused to the OVAL gene by a rigid linker, (EAAAK)3, at the endogenous ovalbumin locus in chicken primordial germ cells to generate OVAL-E3-EGFP transgenic chickens, with transgenic chickens expressing CMV immediate enhancer/β-actin-driven EGFP (CAG-EGFP) as a non-secreted control. In OVAL-E3-EGFP chickens, EGFP protein produced in maternal oviducts accumulates to high levels in eggs, but not in eggs of CAG-EGFP chickens. These results indicated that the secretion of foreign proteins can be substantially increased through fusion to the highly secreted endogenous ovalbumin. This study describes a basis for high yield recombinant protein expression in chicken eggs, enabling rapid and scalable production of numerous pharmaceutical proteins or metabolites.

Electroporation-based Easi-CRISPR yields biallelic insertions of EGFP-HiBiT cassette in immortalized chicken oviduct epithelial cells

Laying hens are an excellent experimental oviduct model for studying reproduction biology. Because chicken oviduct epithelial cells (cOECs) have a crucial role in synthesizing and secreting ovalbumin, laying hens have been regarded an ideal bioreactor for producing pharmaceuticals in egg white through transgene or gene editing of the ovalbumin (OVA) gene. However, related studies in cOECs are largely limited because of the lack of immortalized model cells. In addition, the editing efficiency of conventional CRISPR-HDR knock-in in chicken cells is suboptimal (ranging from 1 to 10%) and remains elevated. Here, primary cOECs were isolated from young laying hens, then infected with a retrovirus vector of human telomerase reverse transcriptase (hTERT), and immortalized cOECs were established. Subsequently, an electroporation-based Easi-CRISPR (Efficient additions with ssDNA inserts-CRISPR) method was adopted to integrate an EGFP-HiBiT cassette into the chicken OVA locus (immediately upstream of the stop codon). The immortalized cOECs reflected the self-renewal capability and phenotype of oviduct epithelial cells. This is because these cells not only maintained stable proliferation and normal karyotype and had no potential for malignant transformation, but also expressed oviduct markers and an epithelial marker and had a morphology similar to that of primary cOECs. EGFP expression was detected in the edited cells through microscopy, flow cytometry, and HiBiT/Western blotting. The EGFP-HiBiT knock-in efficiency reached 27.9% after a single round of electroporation, which was determined through genotyping and DNA sequencing. Two single cell clones contained biallelic insertions of EGFP-HiBiT donor cassettes. In conclusion, our established immortalized cOECs could act as an in vitro cell model for gene editing in chicken, and this electroporation-based Easi-CRISPR strategy will contribute to the generation of avian bioreactors and other gene-edited (GE) birds.

Production of recombinant human IgG1 Fc with beneficial N-glycosylation pattern for anti-inflammatory activity using genome-edited chickens

Intravenous immunoglobulin (IVIG) is a plasma-derived polyclonal IgG used for treatment of autoimmune diseases. Studies show that α-2,6 sialylation of the Fc improves anti-inflammatory activity. Also, afucosylation of the Fc efficiently blocks FcγRIIIA by increasing monovalent affinity to this receptor, which can be beneficial for treatment of refractory immune thrombocytopenia (ITP). Here, we generated genome-edited chickens that synthesize human IgG1 Fc in the liver and secrete α-2,6 sialylated and low-fucosylated human IgG1 Fc (rhIgG1 Fc) into serum and egg yolk. Also, rhIgG1 Fc has higher affinity for FcγRIIIA than commercial IVIG. Thus, rhIgG1 Fc efficiently inhibits immune complex-mediated FcγRIIIA crosslinking and subsequent ADCC response. Furthermore, rhIgG1 Fc exerts anti-inflammatory activity in a passive ITP model, demonstrating chicken liver derived rhIgG1 Fc successfully recapitulated efficacy of IVIG. These results show that genome-edited chickens can be used as a production platform for rhIgG1 Fc with beneficial N-glycosylation pattern for anti-inflammatory activities.

Efficient production of recombinant human adiponectin in egg white using genome edited chickens

The prevalence of obesity-related metabolic diseases caused by insulin resistance is rapidly increasing worldwide. Adiponectin (ADPN), a hormone derived from adipose tissue, is a potential therapeutic agent for insulin resistance. Chickens are considered efficient bioreactors for recombinant protein production because they secrete large amounts of high-concentration proteins from the oviduct. Additionally, chickens express high levels of high-molecular-weight (HMW) ADPN, which is considered the active form in the body. Therefore, in this study, a gene-targeted chicken model was produced in which the gene encoding human ADPN was inserted into Ovalbumin (OVA) using the CRISPR/Cas9 system, and the characteristics of the resulting recombinant ADPN protein were evaluated. As a result, human ADPN was expressed in G₁ hen oviducts and egg whites of OVA ADPN knock-in (KI) chickens. The concentration of ADPN in egg white ranged from 1.47 to 4.59 mg/mL, of which HMW ADPN accounted for ∼29% (0.24-1.49 mg/mL). Importantly, egg white-derived ADPN promoted expression of genes related to fatty acid oxidation and activated the 5'-AMP-activated protein kinase (AMPK) signaling pathway in muscle cells. In summary, the OVA gene-targeted chicken bioreactor proved to be an advantageous model for production of human ADPN, and the resulting protein was of sufficient quantity and efficacy for industrial use.

The science of genetically modified poultry

The exuberant development of targeted genome editing has revolutionized research on the chicken genome, generating chickens with beneficial parameters. The chicken model is a crucial experimental tool that can be utilized for drug manufacture, preclinical research, pathological observation, and other applications. In essence, tweaking the chicken’s genome has enabled the poultry industry to get more done with less, generating genetically modified chickens that lay eggs containing large amounts of lifesaving humanized drugs. The transition of gene editing from concept to practical application has been dramatically hastened by the development of programmable nucleases, bringing scientists closer than ever to the efficient producers of tomorrow’s medicines. Combining the developmental and physiological characteristics of the chicken with cutting-edge genome editing, the chicken furnishes a potent frontier that is foreseen to be actively pursued in the future. Herein we review the current and future prospects of gene editing in chickens and the contributions to the development of humanized pharmaceuticals.

Sequential verification of exogenous protein production in OVA gene-targeted chicken bioreactors

The chicken has potential as an efficient bioreactor system because of its outstanding protein production capacity and low cost. The CRISPR/Cas9-mediated gene-editing system enables production of highly marketable exogenous proteins in transgenic chicken bioreactors. However, because it takes approximately 18 mo to evaluate the recombinant protein productivity of the bioreactor due to the generation interval from G0 founders to G1 egg-laying hens, to verification of the exogenous protein at the early stage is difficult. Here we propose a system for sequential validation of exogenous protein production in chicken bioreactors as in hatching female chicks as well as in egg-laying hens. We generated chicken OVALBUMIN (OVA) EGFP knock-in (KI) chicken (OVA EGFP KI) by CRISPR/Cas9-mediated nonhomologous end joining at the chicken OVA gene locus. Subsequently, the estrogen analog, diethylstilbestrol (DES), was subcutaneously implanted in the abdominal region of 1-wk-old OVA EGFP KI female chicks to artificially increase OVALBUMIN expression. The oviducts of DES-treated OVA EGFP KI female chicks expressed OVA and EGFP at the 3-wk-old stage (10 d after DES treatment). We evaluated the expression of EGFP protein in the oviduct, along with the physical properties of eggs and egg white from OVA EGFP KI hens. The rapid identification and isolation of exogenous protein can be confirmed at a very early stage and high-yield production is possible by targeting the chicken oviduct.

Application of CRISPR/Cas Technology in Spermatogenesis Research and Male Infertility Treatment

As the basis of animal reproductive activity, normal spermatogenesis directly determines the efficiency of livestock production. An in-depth understanding of spermatogenesis will greatly facilitate animal breeding efforts and male infertility treatment. With the continuous development and application of gene editing technologies, they have become valuable tools to study the mechanism of spermatogenesis. Gene editing technologies have provided us with a better understanding of the functions and potential mechanisms of action of factors that regulate spermatogenesis. This review summarizes the applications of gene editing technologies, especially CRISPR/Cas9, in deepening our understanding of the function of spermatogenesis-related genes and disease treatment. The problems of gene editing technologies in the field of spermatogenesis research are also discussed.

Use of Genome Editing Techniques to Produce Transgenic Farm Animals

Recombinant proteins are essential for the treatment and diagnosis of clinical human ailments. The availability and biological activity of recombinant proteins is heavily influenced by production platforms. Conventional production platforms such as yeast, bacteria, and mammalian cells have biological and economical challenges. Transgenic livestock species have been explored as an alternative production platform for recombinant proteins, predominantly through milk secretion; the strategy has been demonstrated to produce large quantities of biologically active proteins. The major limitation of utilizing livestock species as bioreactors has been efforts required to alter the genome of livestock. Advancements in the genome editing field have drastically improved the ability to genetically engineer livestock species. Specifically, genome editing tools such as the CRISPR/Cas9 system have lowered efforts required to generate genetically engineered livestock, thus minimizing restrictions on the type of genetic modification in livestock. In this review, we discuss characteristics of transgenic animal bioreactors and how the use of genome editing systems enhances design and availability of the animal models.

Development and in vitro evaluation of recombinant chicken promoters to efficiently drive transgene expression in chicken oviduct cells

Virus injection into EGK-X embryos is a well-defined approach in avian transgenesis. This system uses a chicken ovalbumin gene promoter to induce transgene expression in the chicken oviduct. Although a reconstructed chicken ovalbumin promoter that links an ovalbumin promoter and estrogen-responsive enhancer element (ERE) is useful, a large viral vector containing the ovalbumin promoter and a target gene restricts viral packaging capacity and produces low-titer virus particles. We newly developed recombinant chicken promoters by linking regulatory regions of ovalbumin and other oviduct-specific genes. Putative enhancer fragments of the genes, such as ovotransferrin (TF), ovomucin alpha subunit (OVOA), and ovalbumin-related protein X (OVALX), were placed at the 5`-flanking region of the 2.8-kb ovalbumin promoter. Basal promoter fragments of the genes, namely, pTF, lysozyme (pLYZ), and ovomucoid (pOVM), were placed at the 3`-flanking region of the 1.6-kb ovalbumin ERE. The recombinant promoters cloned into each reporter vector were evaluated using a dual luciferase assay in human and chicken somatic cells, and LMH/2A cells treated with 0-1,000 nM estrogen, and cultured primary chicken oviduct cells. The recombinant promoters with linking ovalbumin and TF, OVOA, pOVM, and pLYZ regulatory regions had 2.1- to 19.5-fold (P < 0.05) higher luciferase activity than the reconstructed ovalbumin promoter in chicken oviduct cells. Therefore, recombinant promoters may be used to efficiently drive transgene expression in transgenic chickens.

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.

Recent Advances in the Application of CRISPR/Cas9 Gene Editing System in Poultry Species

CRISPR/Cas9 system genome editing is revolutionizing genetics research in a wide spectrum of animal models in the genetic era. Among these animals, is the poultry species. CRISPR technology is the newest and most advanced gene-editing tool that allows researchers to modify and alter gene functions for transcriptional regulation, gene targeting, epigenetic modification, gene therapy, and drug delivery in the animal genome. The applicability of the CRISPR/Cas9 system in gene editing and modification of genomes in the avian species is still emerging. Up to date, substantial progress in using CRISPR/Cas9 technology has been made in only two poultry species (chicken and quail), with chicken taking the lead. There have been major recent advances in the modification of the avian genome through their germ cell lineages. In the poultry industry, breeders and producers can utilize CRISPR-mediated approaches to enhance the many required genetic variations towards the poultry population that are absent in a given poultry flock. Thus, CRISPR allows the benefit of accessing genetic characteristics that cannot otherwise be used for poultry production. Therefore CRISPR/Cas9 becomes a very powerful and robust tool for editing genes that allow for the introduction or regulation of genetic information in poultry genomes. However, the CRISPR/Cas9 technology has several limitations that need to be addressed to enhance its use in the poultry industry. This review evaluates and provides a summary of recent advances in applying CRISPR/Cas9 gene editing technology in poultry research and explores its potential use in advancing poultry breeding and production with a major focus on chicken and quail. This could aid future advancements in the use of CRISPR technology to improve poultry production.

The effect of methyl-beta-cyclodextrin on DNA absorption and quality of posttransfected sperm

Sperm can be selected as a natural vector for the production of transgenic animals. Methyl-beta-cyclodextrin (MBCD) removes cholesterol from the phospholipid membrane of sperm and improves the efficiency of DNA uptake by sperm. In experiment 1, fresh sperm was treated with various concentrations of MBCD. The direct effects of MBCD on sperm parameters were monitored. In experiment 2, different concentrations of MBCD (0, 1, 2, and 4 mmol) were assessed for the transfection of genetically exogenous construction to rooster sperm. Washed semen was divided into 5 equal groups for the incubation and transfection with a pcDNA3.1+/hG-CSF vector (exogenous DNA) as follows; Treatment I-Control (washed semen without DNA); Treatment II-Control (washed semen with DNA); Treatment III-(washed semen incubated with DNA and 1 mmol MBCD); Treatment IV-(washed semen incubated with DNA and 2 mmol MBCD); and Treatment V-(washed semen incubated with DNA and 4 mmol MBCD). We demonstrated that rooster spermatozoa spontaneously can uptake exogenous DNA; this was assessed using exogenous DNA amplification (sperm genomic DNA used as a template for PCR reaction) after DNase I treatment. In addition, total motility (TM), progressive motility (PM), velocity parameters [curvilinear velocity (VCL), straight linear velocity (VSL), sperm track straightness (STR), linearity (LIN)], membrane integrity (MI), and membrane functionality were posttransfectionally evaluated. The concentrations of 1 and 2 mmol MBCD significantly (P < 0.05) improved the motion characteristics and membrane integrity of fresh sperm. The presence of hG-CSF in rooster sperm was detected by PCR and based on sperm analyses MBCD (1 mmol) improved the percentage of motility (98.9 ± 0.81), membrane functionality (64 ± 1.64), and MI (76.2 ± 1.65) after transfection when compared with the other groups (P < 0.05). For the production of transgenic chicken, hens were inseminated (AI) by transfected sperm treated with 1 and 0 mmol MBCD. A PCR analysis of the blood samples and dead embryo tissues of chicks did not reveal the transgene integration.

Production of Recombinant Monoclonal Antibodies in the Egg White of Gene-Targeted Transgenic Chickens

Increased commercial demand for monoclonal antibodies (mAbs) has resulted in the urgent need to establish efficient production systems. We previously developed a transgenic chicken bioreactor system that effectively produced human cytokines in egg whites using genome-edited transgenic chickens. Here, we describe the application of this system to mAb production. The genes encoding the heavy and light chains of humanized anti-HER2 mAb, linked by a 2A peptide sequence, were integrated into the chicken ovalbumin gene locus using a CRISPR/Cas9 protocol. The knock-in hens produced a fully assembled humanized mAb in their eggs. The mAb expression level in the egg white was 1.4–1.9 mg/mL, as determined by ELISA. Furthermore, the antigen binding affinity of the anti-HER2 mAb obtained was estimated to be equal to that of the therapeutic anti-HER2 mAb (trastuzumab). In addition, antigen-specific binding by the egg white mAb was demonstrated by immunofluorescence against HER2-positive and -negative cells. These results indicate that the chicken bioreactor system can efficiently produce mAbs with antigen binding capacity and can serve as an alternative production system for commercial mAbs.

Precise Genome Editing in Poultry and Its Application to Industries

Poultry such as chickens are valuable model animals not only in the food industry, but also in developmental biology and biomedicine. Recently, precise genome-editing technologies mediated by the CRISPR/Cas9 system have developed rapidly, enabling the production of genome-edited poultry models with novel traits that are applicable to basic sciences, agriculture, and biomedical industry. In particular, these techniques have been combined with cultured primordial germ cells (PGCs) and viral vector systems to generate a valuable genome-edited avian model for a variety of purposes. Here, we summarize recent progress in CRISPR/Cas9-based genome-editing technology and its applications to avian species. In addition, we describe further applications of genome-edited poultry in various industries.

Highly elevated base excision repair pathway in primordial germ cells causes low base editing activity in chickens

Base editing technology enables the generation of precisely genome-modified animal models. In this study, we applied base editing to chicken, an important livestock animal in the fields of agriculture, nutrition, and research through primordial germ cell (PGC)-mediated germline transmission. Using this approach, we successfully produced two genome-modified chicken lines harboring mutations in the genes encoding ovotransferrin (TF) and myostatin (MSTN); however, only 55.5% and 35.7% of genome-modified chickens had the desired base substitutions in TF and MSTN, respectively. To explain the low base-editing activity, we performed molecular analysis to compare DNA repair pathways between PGCs and the chicken fibroblast cell line DF-1. The results revealed that base excision repair (BER)-related genes were significantly elevated in PGCs relative to DF-1 cells. Subsequent functional studies confirmed that the editing activity could be regulated by modulating the expression of uracil N-glycosylase (UNG), an upstream gene of the BER pathway. Collectively, our findings indicate that the distinct DNA repair property of chicken PGCs causes low editing activity during genome modification, however, modulation of BER functions could promote the production of genome-modified organisms with the desired genotypes.

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.

HMEJ-mediated efficient site-specific gene integration in chicken cells

BACKGROUND

The production of transgenic chicken cells holds great promise for several diverse areas, including developmental biology and biomedical research. To this end, site-specific gene integration has been an attractive strategy for generating transgenic chicken cell lines and has been successfully adopted for inserting desired genes and regulating specific gene expression patterns. However, optimization of this method is essential for improving the efficiency of genome modification in this species.

RESULTS

Here we compare gene knock-in methods based on homology-independent targeted integration (HITI), homology-directed repair (HDR) and homology mediated end joining (HMEJ) coupled with a clustered regularly interspaced short palindromic repeat associated protein 9 (CRISPR/Cas9) gene editing system in chicken DF-1 cells and primordial germ cells (PGCs). HMEJ was found to be a robust and efficient method for gene knock-in in chicken PGCs. Using this method, we successfully labeled the germ cell specific gene DAZL and the pluripotency-related gene Pou5f3 in chicken PGCs through the insertion of a fluorescent protein in the frame at the 3' end of the gene, allowing us to track cell migration in the embryonic gonad. HMEJ strategy was also successfully used in Ovalbumin, which accounts for more than 60% of proteins in chicken eggs, suggested its good promise for the mass production of protein with pharmaceutical importance using the chicken oviduct system.

CONCLUSIONS

Taken together, these results demonstrate that HMEJ efficiently mediates site-specific gene integration in chicken PGCs, which holds great potential for the biopharmaceutical engineering of chicken cells.

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.

Production of transgenic broilers by non-viral vectors via optimizing egg windowing and screening transgenic roosters

The generation of transgenic chickens is of both biomedical and agricultural significance, and recently chicken transgenesis technology has been greatly advanced. However, major issues still exist in the efficient production of transgenic chickens. This study was designed to optimize the production of enhanced green fluorescence protein (EGFP)-transgenic broilers, including egg windowing at the blunt end (air cell) of egg, and the direct transfection of circulating primordial germ cells by microinjection of the Tol2 plasmid-liposome complex into the early embryonic dorsal aorta. For egg windowing, we discovered that proper manipulation of the inner shell membrane at the blunt end could improve the rate of producing G0 transgenic roosters. From 27 G0 roosters, we successfully collected semen with EGFP-positive sperms from 16 and 19 roosters after direct fluorescence observation and fluorescence-activated cell sorting analyses (13 detected by both methods), respectively. After artificial insemination using the G0 rooster with the highest number of EGFP fluorescent sperm, one G1 EGFP transgenic broiler (1/81, 1.23%) was generated. Our results indicate that appropriate egg windowing and screening of potentially transgene-positive roosters can improve the production of germline-transmitted transgenic birds.

High fidelity CRISPR/Cas9 increases precise monoallelic and biallelic editing events in primordial germ cells

Primordial germ cells (PGCs), the embryonic precursors of the sperm and egg, are used for the introduction of genetic modifications into avian genome. Introduction of small defined sequences using genome editing has not been demonstrated in bird species. Here, we compared oligonucleotide-mediated HDR using wild type SpCas9 (SpCas9-WT) and high fidelity SpCas9-HF1 in PGCs and show that many loci in chicken PGCs can be precise edited using donors containing CRISPR/Cas9-blocking mutations positioned in the protospacer adjacent motif (PAM). However, targeting was more efficient using SpCas9-HF1 when mutations were introduced only into the gRNA target sequence. We subsequently employed an eGFP-to-BFP conversion assay, to directly compare HDR mediated by SpCas9-WT and SpCas9-HF1 and discovered that SpCas9-HF1 increases HDR while reducing INDEL formation. Furthermore, SpCas9-HF1 increases the frequency of single allele editing in comparison to SpCas9-WT. We used SpCas9-HF1 to demonstrate the introduction of monoallelic and biallelic point mutations into the FGF20 gene and generate clonal populations of edited PGCs with defined homozygous and heterozygous genotypes. Our results demonstrate the use of oligonucleotide donors and high fidelity CRISPR/Cas9 variants to perform precise genome editing with high efficiency in PGCs.

Applications of Gene Editing in Chickens: A New Era Is on the Horizon

The chicken represents a valuable model for research in the area of immunology, infectious diseases as well as developmental biology. Although it was the first livestock species to have its genome sequenced, there was no reverse genetic technology available to help understanding specific gene functions. Recently, homologous recombination was used to knockout the chicken immunoglobulin genes. Subsequent studies using immunoglobulin knockout birds helped to understand different aspects related to B cell development and antibody production. Furthermore, the latest advances in the field of genome editing including the CRISPR/Cas9 system allowed the introduction of site specific gene modifications in various animal species. Thus, it may provide a powerful tool for the generation of genetically modified chickens carrying resistance for certain pathogens. This was previously demonstrated by targeting the Trp38 region which was shown to be effective in the control of avian leukosis virus in chicken DF-1 cells. Herein we review the current and future prospects of gene editing and how it possibly contributes to the development of resistant chickens against infectious diseases.

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.

Surgical Extraction of Viable Hen ( Gallus gallus domesticus) Follicles for In Vitro Fertilization

Assisted reproduction techniques in birds have been developed for zootechnical purposes and have been adapted for use in conservation of wild bird species. To develop a technique for obtaining follicles in live hens, 5 Rhode Island red hens ( Gallus gallus domesticus) were anesthetized, and abdominal ultrasound was performed to confirm the presence of ovarian follicles. A left celiotomy then was performed to obtain follicles in different stages of maturation for in vitro fertilization. The follicles were located by digital exploration, then extracted by isolating each follicle with the index finger of each hand, holding it by the stigma, and then applying slight traction towards the exterior of the coelomic cavity until the follicle separated from the ovary. In total, 18 of 30 (60%) follicles obtained were suitable for in vitro fertilization, but only 3 (16%) were fertilized successfully. All birds recovered from the procedure and remained in good condition postoperatively. Perfecting assisted reproduction technique holds potential benefits for determining sex of embryos by blastomeres sexing, supporting the conservation efforts of avian species, and benefiting research areas, such as genetic and biopharmaceutical research.

The transgenic chicken derived anti-CD20 monoclonal antibodies exhibits greater anti-cancer therapeutic potential with enhanced Fc effector functions

Modern genetic techniques, enable the use of animal bioreactor systems for the production and functional enhancement of anti-cancer antibodies. Chicken is the most efficient animal bioreactor for the production of anti-cancer antibodies because of its relatively short generation time, plentiful reproductive capacity, and daily deposition in the egg white. Although several studies have focused on the production of anti-cancer antibodies in egg white, in-depth studies of the biological activity and physiological characteristics of transgenic chicken-derived anti-cancer antibodies have not been fully carried out. Here, we report the production of an anti-cancer monoclonal antibody against the CD20 protein from egg whites of transgenic hens, and validated the bio-functional activity of the protein in B-lymphoma and B-lymphoblast cells. Quantitative analysis showed that deposition of the chickenised CD20 monoclonal antibody (cCD20 mAb) from transgenic chickens increased in successive generations and with increasing transgene copy number. Ultra-performance liquid chromatography (UPLC) tandem mass spectrometry (LC/MS/MS) analysis showed that the cCD20 mAb exhibited 14 N-glycan patterns with high-mannose, afucosylation and terminal galactosylation. The cCD20 mAb did not exhibit significantly improved Fab-binding affinity, but showed markedly enhanced Fc-related functions, including complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC) compared to commercial rituximab, a chimeric mAb against CD20. Our results suggest that the transgenic chicken bioreactor is an efficient system for producing anti-cancer therapeutic antibodies with enhanced Fc effector functions.

Primordial germ cell-mediated transgenesis and genome editing in birds

Transgenesis and genome editing in birds are based on a unique germline transmission system using primordial germ cells (PGCs), which is quite different from the mammalian transgenic and genome editing system. PGCs are progenitor cells of gametes that can deliver genetic information to the next generation. Since avian PGCs were first discovered in nineteenth century, there have been numerous efforts to reveal their origin, specification, and unique migration pattern, and to improve germline transmission efficiency. Recent advances in the isolation and in vitro culture of avian PGCs with genetic manipulation and genome editing tools enable the development of valuable avian models that were unavailable before. However, many challenges remain in the production of transgenic and genome-edited birds, including the precise control of germline transmission, introduction of exogenous genes, and genome editing in PGCs. Therefore, establishing reliable germline-competent PGCs and applying precise genome editing systems are critical current issues in the production of avian models. Here, we introduce a historical overview of avian PGCs and their application, including improved techniques and methodologies in the production of transgenic and genome-edited birds, and we discuss the future potential applications of transgenic and genome-edited birds to provide opportunities and benefits for humans.

Gene editing in birds takes flight

The application of gene editing (GE) technology to create precise changes to the genome of bird species will provide new and exciting opportunities for the biomedical, agricultural and biotechnology industries, as well as providing new approaches for producing research models. Recent advances in modifying both the somatic and germ cell lineages in chicken indicate that this species, and conceivably soon other avian species, has joined a growing number of model organisms in the gene editing revolution.

The chorioallantoic membrane (CAM) assay for the study of human bone regeneration: a refinement animal model for tissue engineering

Biomaterial development for tissue engineering applications is rapidly increasing but necessitates efficacy and safety testing prior to clinical application. Current in vitro and in vivo models hold a number of limitations, including expense, lack of correlation between animal models and human outcomes and the need to perform invasive procedures on animals; hence requiring new predictive screening methods. In the present study we tested the hypothesis that the chick embryo chorioallantoic membrane (CAM) can be used as a bioreactor to culture and study the regeneration of human living bone. We extracted bone cylinders from human femoral heads, simulated an injury using a drill-hole defect, and implanted the bone on CAM or in vitro control-culture. Micro-computed tomography (μCT) was used to quantify the magnitude and location of bone volume changes followed by histological analyses to assess bone repair. CAM blood vessels were observed to infiltrate the human bone cylinder and maintain human cell viability. Histological evaluation revealed extensive extracellular matrix deposition in proximity to endochondral condensations (Sox9+) on the CAM-implanted bone cylinders, correlating with a significant increase in bone volume by μCT analysis (p < 0.01). This human-avian system offers a simple refinement model for animal research and a step towards a humanized in vivo model for tissue engineering.

Comparison of Different Electroporation Parameters on Transfection Efficiency of Sheep Testicular Cells

OBJECTIVE

Electroporation can be a highly efficient method for introducing the foreign genetic materials into the targeted cells for transient and/or permanent genetic modification. Considering the application of this technique as a very efficient method for drug, oligonucleotide, antibody and plasmid delivery for clinical applications and production of transgenic animals, the present study aimed to optimize the transfection efficiency of sheep testicular cells including spermatogonial stem cells (SSCs) via electroporation.

MATERIALS AND METHODS

This study is an experimental research conducted in Biotechnology Research Center (Avicenna Research Institute, Tehran, Iran) from September 2013 to March 2014. Following isolation and propagation of one-month lamb testicular cells (SSCs and somatic testicular cells including; Sertoli, Leydig, and myoid cells), the effect of different electroporation parameters including total voltages (280, 320, and 350 V), burst durations (10, 8, and 5 milliseconds), burst modes (single or double) and addition of dimethyl sulfoxide (DMSO) were evaluated on transfection efficiency, viability rate and mean fluorescent intensity (MFI) of sheep testicular cells.

RESULTS

The most transfection efficiency was obtained in 320 V/8 milliseconds/single burst group in transduction medium with and without DMSO. There was a significantly inverse correlation between transfection efficiency with application of both following parameters: addition of DMSO and double burst. After transfection, the highest and lowest viability rates of testicular cells were demonstrated in 320 V/8 milliseconds with transduction medium without DMSO and 350 V/5 milliseconds in medium containing DMSO. Ad- dition of DMSO to transduction medium in all groups significantly decreased the viability rate. The comparison of gene expression indicated that Sertoli and SSCs had the most fluorescence intensity in 320 V/double burst/DMSO positive. However, myoid and Leydig cells showed the maximum expression in 320 V/single burst and/or 350 V/double burst/ DMSO positive.

CONCLUSION

We optimized the electroporation method for transfection of sheep testicular cells and recommended the application of 320 V/8 milliseconds/single pulse/DMSO negative for transduction of plasmid vector into these cells. Among testicular cells, the most external gene expression was demonstrated in SSC population.

Advances in genetic engineering of the avian genome: "Realising the promise"

This review provides an historic perspective of the key steps from those reported at the 1st Transgenic Animal Research Conference in 1997 through to the very latest developments in avian transgenesis. Eighteen years later, on the occasion of the 10th conference in this series, we have seen breakthrough advances in the use of viral vectors and transposons to transform the germline via the direct manipulation of the chicken embryo, through to the establishment of PGC cultures allowing in vitro modification, expansion into populations to analyse the genetic modifications and then injection of these cells into embryos to create germline chimeras. We have now reached an unprecedented time in the history of chicken transgenic research where we have the technology to introduce precise, targeted modifications into the chicken genome, ranging from; new transgenes that provide improved phenotypes such as increased resilience to economically important diseases; the targeted disruption of immunoglobulin genes and replacement with human sequences to generate transgenic chickens that express "humanised" antibodies for biopharming; and the deletion of specific nucleotides to generate targeted gene knockout chickens for functional genomics. The impact of these advances is set to be realised through applications in chickens, and other bird species as models in scientific research, for novel biotechnology and to protect and improve agricultural productivity.

Purification and Characterization of Recombinant Human Lysozyme from Eggs of Transgenic Chickens

Transgenic chickens as bioreactors have several advantages, such as the simple establishment procedure, correct glycosylation profile of expressed proteins, etc. Lysozyme is widely used in food industry, livestock farming, and medical field as a replacement of antibiotics because of its antibacterial and complement system-modulating activity. In this study, we used RT-PCR, Western blot, and immunofluorescence to detect the expression of recombinant human lysozyme (rhLY) in the transgenic chicken. We demonstrated that the transgene of rhLY was genetically stable across different generations. We next optimized the purification procedure of rhLY from the transgenic eggs by utilizing two steps of cation-exchange chromatography and one gel-filtration chromatography. About 6 mg rhLY with the purity exceeding 90% was obtained from ten eggs, and the purification efficiency was about 75%. The purified rhLY had similar physicochemical and biological properties in molecular mass and antibacterial activity compared to the commercial human lysozyme. Additionally, both of them exhibited thermal stability at 60°C and tolerated an extensive pH range of 2 to 11. In conclusion, our study proved that the transgenic chickens we have previously generated were genetically stable and suitable for the production of active rhLY. We also provided a pipeline for purifying the recombinant proteins from transgenic eggs, which could be useful for other studies.

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.

Oviduct-specific expression of human neutrophil defensin 4 in lentivirally generated transgenic chickens

The expression of oviduct-specific recombinant proteins in transgenic chickens is a promising technology for the production of therapeutic biologics in eggs. In this study, we constructed a lentiviral vector encoding an expression cassette for human neutrophil defensin 4 (HNP4), a compound that displays high activity against Escherichia coli, and produced transgenic chickens that expressed the recombinant HNP4 protein in egg whites. After the antimicrobial activity of the recombinant HNP4 protein was tested at the cellular level, a 2.8-kb ovalbumin promoter was used to drive HNP4 expression specifically in oviduct tissues. From 669 injected eggs, 218 chickens were successfully hatched. Ten G₀ roosters, with semens identified as positive for the transgene, were mated with wild-type hens to generate G₁ chickens. From 1,274 total offspring, fifteen G₁ transgenic chickens were positive for the transgene, which was confirmed by PCR and Southern blotting. The results of the Southern blotting and genome walking indicated that a single copy of the HNP4 gene was integrated into chromosomes 1, 2, 3, 4, 6 and 24 of the chickens. As expected, HNP4 expression was restricted to the oviduct tissues, and the levels of both transcriptional and translational HNP4 expression varied greatly in transgenic chickens with different transgene insertion sites. The amount of HNP4 protein expressed in the eggs of G₁ and G₂ heterozygous transgenic chickens ranged from 1.65 μg/ml to 10.18 μg/ml. These results indicated that the production of transgenic chickens that expressed HNP4 protein in egg whites was successful.

Passive Immunization

Whereas active immunity refers to the process of exposing the individual to an antigen to generate an adaptive immune response, passive immunity refers to the transfer of antibodies from one individual to another. Passive immunity provides immediate but short-lived protection, lasting several weeks up to 3 or 4 months. Passive immunity can occur naturally, when maternal antibodies are transferred to the fetus through the placenta or from breast milk to the gut of the infant. It can also be produced artificially, when antibody preparations derived from sera or secretions of immunized donors or, more recently, different antibody producing platforms are transferred via systemic or mucosal route to nonimmune individuals. Passive immunization has recently become an attractive approach because of the emergence of new and drug-resistant microorganisms, diseases that are unresponsive to drug therapy and individuals with an impaired immune system who are unable to respond to conventional vaccines. This chapter addresses the contributions of natural and artificial acquired passive immunity in understanding the concept of passive immunization. We will mainly focus on administration of antibodies for protection against various infectious agents entering through mucosal surfaces.

Nonviral minicircle generation of induced pluripotent stem cells compatible with production of chimeric chickens

Chickens are vitally important in numerous countries as a primary food source and a major component of economic development. Efforts have been made to produce transgenic birds through pluripotent stem cell [primordial germ cells and embryonic stem cells (ESCs)] approaches to create animals with improved traits, such as meat and egg production or even disease resistance. However, these cell types have significant limitations because they are hard to culture long term while maintaining developmental plasticity. Induced pluripotent stem cells (iPSCs) are a novel class of stem cells that have proven to be robust, leading to the successful development of transgenic mice, rats, quail, and pigs and may potentially overcome the limitations of previous pluripotent stem cell systems in chickens. In this study we generated chicken (c) iPSCs from fibroblast cells for the first time using a nonviral minicircle reprogramming approach. ciPSCs demonstrated stem cell morphology and expressed key stem cell markers, including alkaline phosphatase, POU5F1, SOX2, NANOG, and SSEA-1. These cells were capable of rapid growth and expressed high levels of telomerase. Late-passage ciPSCs transplanted into stage X embryos were successfully incorporated into tissues of all three germ layers, and the gonads demonstrated significant cellular plasticity. These cells provide an exciting new tool to create transgenic chickens with broad implications for agricultural and transgenic animal fields at large.

Human extracellular superoxide dismutase (EC-SOD) expression in transgenic chicken

Extracellular superoxide dismutase (EC-SOD) is a metalloprotein and functions as an antioxidant enzyme. In this study, we used lentiviral vectors to generate transgenic chickens that express the human EC-SOD gene. The recombinant lentiviruses were injected into the subgerminal cavity of freshly laid eggs. Subsequently, the embryos were incubated to hatch using phases II and III of the surrogate shell ex vivo culture system. Of 158 injected embryos, 16 chicks (G0) hatched and were screened for the hEC-SOD by PCR. Only 1 chick was identified as a transgenic bird containing the transgene in its germline. This founder (G0) bird was mated with wild-type hens to produce transgenic progeny, and 2 transgenic chicks (G1) were produced. In the generated transgenic hens (G2), the hEC-SOD protein was expressed in the egg white and showed antioxidant activity. These results highlight the potential of the chicken for production of biologically active proteins in egg white. [BMB Reports 2013; 46(8): 404-409]

A novel piggyBac transposon inducible expression system identifies a role for AKT signalling in primordial germ cell migration

In this work, we describe a single piggyBac transposon system containing both a tet-activator and a doxycycline-inducible expression cassette. We demonstrate that a gene product can be conditionally expressed from the integrated transposon and a second gene can be simultaneously targeted by a short hairpin RNA contained within the transposon, both in vivo and in mammalian and avian cell lines. We applied this system to stably modify chicken primordial germ cell (PGC) lines in vitro and induce a reporter gene at specific developmental stages after injection of the transposon-modified germ cells into chicken embryos. We used this vector to express a constitutively-active AKT molecule during PGC migration to the forming gonad. We found that PGC migration was retarded and cells could not colonise the forming gonad. Correct levels of AKT activation are thus essential for germ cell migration during early embryonic development.

Chicken 7SK promoter drives efficient shRNA transcription with species specificity

To extend the use of RNAi in chicken, we have developed a RNA interference (RNAi) system using a shortened chicken 7SK (ch7SK) promoter. The results stated that the cloned ch7SK promoter includes multiple Oct-1 motifs, SPH domain, PSE and TATA box, without CACCC box. All RNAi groups driven by ch7SK promoter showed significant mean fluorescence intensity (MFI) reduction. In the pch7SK-shEGFP transfected DF-EGFP cell culture, the MFI reduction ratio was smaller than the pmU6-shEGFP did. In the pmU6-shEGFP transfected Vero-EGFP cell culture, the MFI was reduced significantly than the pch7SK-shEGFP did. In summary, the essential part of ch7SK promoter was capable of efficiently expressing shRNAs with relatively different interfering degrees in avian and mammalian cells, respectively. Our results suggest that ch7SK promoter is an efficient alternative to commercially mouse U6 promoter in shRNA expression with chicken cells, and provide references for furthering functional genome analysis and disease resistant breeding in chicken.

Chick stem cells: current progress and future prospects

Chick embryonic stem cells (cESCs) can be derived from cells obtained from stage X embryos (blastoderm stage); these have the ability to contribute to all somatic lineages in chimaeras, but not to the germ line. However, lines of stem cells that are able to contribute to the germ line can be established from chick primordial germ cells (cPGCs) and embryonic germ cells (cEGCs). This review provides information on avian stem cells, emphasizing different sources of cells and current methods for derivation and culture of pluripotent cells from chick embryos. We also review technologies for isolation and derivation of chicken germ cells and the production of transgenic birds.

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Chick embryonic stem cells (cESCs) can be derived from a variety of sources.

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cESCs can contribute to all somatic cell types but not to the germ line.

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germ cells can be isolated from early embryos, embryonic blood and gonads.

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germ cells can establish self-renewing lines and contribute to the germline.

A new method for producing transgenic birds via direct in vivo transfection of primordial germ cells

Traditional methods of avian transgenesis involve complex manipulations involving either retroviral infection of blastoderms or the ex vivo manipulation of primordial germ cells (PGCs) followed by injection of the cells back into a recipient embryo. Unlike in mammalian systems, avian embryonic PGCs undergo a migration through the vasculature on their path to the gonad where they become the sperm or ova producing cells. In a development which simplifies the procedure of creating transgenic chickens we have shown that PGCs are directly transfectable in vivo using commonly available transfection reagents. We used Lipofectamine 2000 complexed with Tol2 transposon and transposase plasmids to stably transform PGCs in vivo generating transgenic offspring that express a reporter gene carried in the transposon. The process has been shown to be highly effective and as robust as the other methods used to create germ-line transgenic chickens while substantially reducing time, infrastructure and reagents required. The method described here defines a simple direct approach for transgenic chicken production, allowing researchers without extensive PGC culturing facilities or skills with retroviruses to produce transgenic chickens for wide-ranging applications in research, biotechnology and agriculture.

Quantitative analysis of tetracycline-inducible expression of the green fluorescent protein gene in transgenic chickens

The use of transgenic farm animals as "bioreactors" to address the growing demand for biopharmaceuticals, both in terms of increased quantity and greater number, represents a key development in the advancement of medical science. However, the potential for detrimental side-effects as a result of uncontrolled constitutive expression of foreign genes in transgenic animals is a well-recognized limitation of such systems. Previously, using a tetracycline-inducible expression system, we demonstrated the induction of expression of a transgene encoding green fluorescent protein (GFP) in transgenic chickens by feeding with doxycycline, a tetracycline derivative; expression of GFP reverted to pre-induction levels when the inducer was removed from the diet. As a proof of principle study, however, quantitative assessment of expression was not possible, as only one G0 and one G1 transgenic chicken was obtained. In the current study, a sufficient number of G2 and G3 transgenic chickens were obtained, and quantification analysis demonstrated up to a 20-fold induction of expression by doxycycline. In addition, stable transmission of the transgene without any apparent genetic modifications was observed through several generations. The use of an inducible expression system that can be regulated by dietary supplementation could help mitigate the physiological disruption that can occur in transgenic animals as a result of uncontrolled constitutive expression of a transgene. Importantly, these results also support the use of the retroviral system for generating transgenic animals with minimal risk in terms of biosafety.