CpG methylation modulates tissue-specific expression of a transgene in chickens

Modification of surface glycan by expression of beta-1,4-N-acetyl-galactosaminyltransferase (B4GALNT2) confers resistance to multiple viruses infection in chicken fibroblast cell

Introduction

Infectious viruses in poultry, such as avian influenza virus (AIV) and Newcastle disease virus (NDV), are one of the most major threats to the poultry industry, resulting in enormous economic losses. AIVs and NDVs preferentially recognize α-2,3-linked sialic acid to bind to target cells. The human beta-1,4-N-acetyl-galactosaminyltransferase 2 (B4GALNT2) modifies α-2,3-linked sialic acid-containing glycan by transferring N-acetylgalactosamine to the sub-terminal galactose of the glycan, thus playing a pivotal role in preventing viruses from binding to cell surfaces. However, chickens lack a homolog of the B4GALNT2 gene.

Methods

Here, we precisely tagged the human B4GALNT2 gene downstream of the chicken GAPDH so that the engineered cells constitutively express the human B4GALNT2. We performed a lectin binding assay to analyze the modification of α-2,3-linked sialic acid-containing glycan by human B4GALNT2. Additionally, we infected the cells with AIV and NDV and compared cell survivability, viral gene transcription, and viral titer using the WST-1 assay, RT-qPCR and TCID50 assay, respectively.

Results

We validated human B4GALNT2 successfully modified α-2,3-linked sialic acid-containing glycan in chicken DF-1 cells. Following viral infection, we showed that human B4GALNT2 reduced infection of two AIV subtypes and NDV at 12-, 24-, and 36-hours post-infection. Moreover, cells expressing human B4GALNT2 showed significantly higher cell survivability compared to wild-type DF-1 cells, and viral gene expression was significantly reduced in the cells expressing human B4GALNT2.

Discussion

Collectively, these results suggest that artificially expressing human B4GALNT2 in chicken is a promising strategy to acquire broad resistance against infectious viruses with a preference for α-2,3-linked sialic acids such as AIV and NDV.

Transcriptional Regulation of the Chicken ASMT Gene- A Preliminary Analysis

1. Melatonin is an indole hormone that, among its myriad biological functions, regulates circadian and seasonal rhythms in animals. The ASMT gene plays an essential role in melatonin synthesis. However, in chickens, little is known about the regulatory elements governing its transcription.2. The following study identified the transcription start site of the chicken ASMT gene by 5'-RACE. Then, the proximal minimal promoter was identified using a series of 5' truncations of the ASMT promoter (e.g., -3502/+17, -2698/+17, -2003/+17, -1378/+17, and -254/+17). Site-directed mutagenesis, overexpression, and electrophoretic mobility shift assay (EMSA) were applied to show that the transcription factor Oct-1 binds to the promoter region of ASMT.3. The translation start site was located 19 bp upstream from the translational start site. The luciferase reporter assay confirmed that the core promoter of chicken ASMT gene was in the -254/+17 region. Using site-directed mutagenesis, overexpression, and EMSA, Oct-1 bound the promoter of ASMT.4. Overall, Oct1 plays an important role in the transcriptional regulation of chicken ASMT gene.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

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.

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.

CRISPR/Cas9 Targets Chicken Embryonic Somatic Cells In Vitro and In Vivo and generates Phenotypic Abnormalities

Chickens are an invaluable model for studying human diseases, physiology and especially development, but have lagged in genetic applications. With the advent of Programmable Engineered Nucleases, genetic manipulation has become efficient, specific and rapid. Here, we show that the CRISPR/Cas9 system can precisely edit the chicken genome. We generated HIRA, TYRP1, DICER, MBD3, EZH2, and 6 other gene knockouts in two chicken cell lines using the CRISPR/Cas9 system, with no off-target effects detected. We also showed that very large deletions (>75 kb) could be achieved. We also achieved targeted modification by homology-directed repair (HDR), producing MEN2A and MEN2B mutations of the RET gene. We also targeted DGCR8 in neural cells of the chicken embryo by in vivo electroporation. After FACS isolation of transfected cells, we observed appropriate sequence changes in DGCR8. Wholemount and frozen section antibody labelling showed reduction of DGCR8 levels in transfected cells. In addition, there was reduced expression levels of DGCR8-associated genes DROSHA, YPEL1 and NGN2. We also observed morphological differences in neural tissue and cardiac-related tissues of transfected embryos. These findings demonstrate that precisely targeted genetic manipulation of the genome using the CRISPR/Cas9 system can be extended to the highly adaptable in vivo chicken embryo model.

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.

Cellular dynamics after injection of mesoderm-derived human embryonic kidney 293 cells and fibroblasts into developing chick embryos

This study was conducted to compare localization of transformed or differentiated cells after injection into developing chick embryos. Mesoderm-derived chicken embryonic fibroblasts (CEFs), retrieved from normal tissues and artificially transformed human embryonic kidney (HEK) 293 cells, were injected into the dorsal aorta of stage 17 embryos, incubated for 60 h, and post-injection survival and tissue localization after injection were monitored. Overall survival rates were 43% to 57%, and there was no significant difference between the two cell types (P=0.4453). Migration into various tissues was observed after injection of the HEK 293 cells, and this was greatly reduced after CEF transfer (P<0.0127). Tumorigenic activity was detected in the HEK 293 transferred cells and the major organ colonized was the highly vascularized yolk sac. From these results, we suggest that cell transformation alters post-injected migration activity of cells at organogenesis.

Current genomic editing approaches in avian transgenesis

The chicken was domesticated from Red Jungle Fowl over 8000years ago and became one of the major food sources worldwide. At present, the poultry industry is one of the largest industrial animal stocks in the world, and its economic scale is expanding significantly with increasing consumption. Additionally, since Aristotle used chicken eggs as a model to provide remarkable insights into how life begins, chickens have been used as invaluable and powerful experimental materials for studying embryo development, immune systems, biomedical processes, and hormonal regulation. Combined with advancements in efficient transgenic technology, avian models have become even more important than would have been expected.

Epigenetic changes of lentiviral transgenes in porcine stem cells derived from embryonic origin

Because of the physiological and immunological similarities that exist between pigs and humans, porcine pluripotent cell lines have been identified as important candidates for preliminary studies on human disease as well as a source for generating transgenic animals. Therefore, the establishment and characterization of porcine embryonic stem cells (pESCs), along with the generation of stable transgenic cell lines, is essential. In this study, we attempted to efficiently introduce transgenes into Epiblast stem cell (EpiSC)-like pESCs. Consequently, a pluripotent cell line could be derived from a porcine-hatched blastocyst. Enhanced green fluorescent protein (EGFP) was successfully introduced into the cells via lentiviral vectors under various multiplicities of infection, with pluripotency and differentiation potential unaffected after transfection. However, EGFP expression gradually declined during extended culture. This silencing effect was recovered by in vitro differentiation and treatment with 5-azadeoxycytidine. This phenomenon was related to DNA methylation as determined by bisulfite sequencing. In conclusion, we were able to successfully derive EpiSC-like pESCs and introduce transgenes into these cells using lentiviral vectors. This cell line could potentially be used as a donor cell source for transgenic pigs and may be a useful tool for studies involving EpiSC-like pESCs as well as aid in the understanding of the epigenetic regulation of transgenes.

Biallelic expression of the L-arginine:glycine amidinotransferase gene with different methylation status between male and female primordial germ cells in chickens

The basic functions of DNA methylation include in gene silencing by methylation of specific gene promoters, defense of the host genome from retrovirus, and transcriptional suppression of transgenes. In addition, genomic imprinting, by which certain genes are expressed in a parent-of-origin-specific manner, has been observed in a wide range of plants and animals and has been associated with differential methylation. However, imprinting phenomena of DNA methylation effects have not been revealed in chickens. To analyze whether genomic imprinting occurs in chickens, methyl-DNA immunoprecipitation array analysis was applied across the entire genome of germ cells in early chick embryos. A differentially methylated region (DMR) was detected in the eighth intron of the l-arginine:glycine amidinotransferase (GATM) gene. When the DMR in GATM was analyzed by bisulfite sequencing, the methylation in male primordial germ cells (PGC) of 6-d-old embryos was higher than that in female PGC (57.5 vs. 35.0%). At 8 d, the DMR methylation of GATM in male PGC was 3.7-fold higher than that in female PGC (65.0 vs. 17.5%). Subsequently, to investigate mono- or biallelic expression of the GATM gene during embryo development, we found 2 indel sequences (GTTTAATGC and CAAAAA) within the GATM 3'-untranslated region in Korean Oge (KO) and White Leghorn (WL) chickens. When individual WL and KO chickens were genotyped for indel sequences, 3 allele combinations (homozygous insertion, homozygous deletion, and heterozygotes) were detected in both breeds using a gel shift assay and high-resolution melt assay. The deletion allele was predominant in KO, whereas the insertion allele was predominant in WL. Heterozygous animals were evenly distributed in both breeds (P < 0.01). Despite the different methylation status between male and female PGC, the GATM gene conclusively displayed biallelic expression in PGC as well as somatic embryonic, extraembryonic, and adult chicken tissues.

Genetic modification of chicken germ cells

Over the past two decades numerous reports have demonstrated that the genetic modification of poultry genomes has great potential for improving poultry production; moreover, it may be used as a powerful tool for the production of industrial proteins. To date, transgenic techniques have been established for generating transgenic birds that express recombinant human proteins in hen eggs, as well as tissue-specific genes as an animal model. The production of transgenic birds is a promising approach that could have practical applications in agriculture and biopharmacology, in addition to advancing our understanding of avian biology. Finally, germ cell-mediated transgenesis could provide a more efficient strategy for creating gene-targeted insertions and deletions in avian species.

Rediscovering the chick embryo as a model to study retinal development

The embryonic chick occupies a privileged place among animal models used in developmental studies. Its rapid development and accessibility for visualization and experimental manipulation are just some of the characteristics that have made it a vertebrate model of choice for more than two millennia. Until a few years ago, the inability to perform genetic manipulations constituted a major drawback of this system. However, the completion of the chicken genome project and the development of techniques to manipulate gene expression have allowed this classic animal model to enter the molecular age. Such techniques, combined with the embryological manipulations that this system is well known for, provide a unique toolkit to study the genetic basis of neural development. A major advantage of these approaches is that they permit targeted gene misexpression with extremely high spatiotemporal resolution and over a large range of developmental stages, allowing functional analysis at a level, speed and ease that is difficult to achieve in other systems. This article provides a general overview of the chick as a developmental model focusing more specifically on its application to the study of eye development. Special emphasis is given to the state of the art of the techniques that have made gene gain- and loss-of-function studies in this model a reality. In addition, we discuss some methodological considerations derived from our own experience that we believe will be beneficial to researchers working with this system.

Expression of biologically active human interferon alpha 2b in the milk of transgenic mice

Interferon alpha 2b (IFNα-2b) is an important immune regulator widely used in clinic, for the treatment of chronic hepatitis, hairy cell leukemia, chronic myelogenous leukemia and multiple myeloma, etc. The clinically used IFNα-2b is generally produced by E.Coli, which lacks the post-translational O-glycosylation presents on naturally synthesized protein, and has a short serum half-life. In this study, a transgenic cassette pBCN-IFN-pA-CMV-EGFP was constructed, with a 5.2 kb beta-casein regulation fragment from Jersey cow and a 6×His tagged human Interferon alpha 2b (hIFNα-2b) gene fragment. By using pronuclear microinjection technique, transgenic mice were generated and the expression of IFNα-2b in the milk was assayed. The hIFNα-2b was correctly translated in milk of transgenic mice according to Western blot analysis. The expression level of hIFNα-2b was varied among the transgenic mice, and the highest one was about 29.71 μg/L. The recombinant protein exhibited biological activity in vitro by increasing the luminescence value and the MxA gene expression in established WISH cells, and the specific activity is approximately 2.8 × 10(7 )IU/mg. The expression of recombinant hIFNα-2b in mammary glands of transgenic mice constitutes an important step towards low-cost and full biological activity production of this protein drug in mammary gland bioreactor.

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.

In ovo eye electroporation

Electroporation has been used successfully to introduce macromolecules such as DNA into the chick embryo for at least 15 years. Purified plasmid DNA is microinjected into embryo and then a series of low voltage electrical pulses are applied to the embryo which allows naked DNA to enter cells. Following entrance into the cytoplasm, the DNA is transported to the nucleus where it is transiently expressed. This powerful technique is useful for studies involving overexpression, misexpression, and knockdown of genes of interest at a variety of developmental timepoints.

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.