The incredible, edible, and therapeutic egg

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.

Efficient knock-in at the chicken ovalbumin locus using adenovirus as a CRISPR/Cas9 delivery system

In this study, efficient knock-in (KI) of human epidermal growth factor (hEGF) cDNA at the ovalbumin (OV) locus in cultured chicken cells was achieved using adenovirus as a delivery for CRISPR/Cas9 elements and optimizing donor vector construction. The strategy of recruiting donor DNA to the insertion site further improved the KI efficiency. The inserted hEGF cDNA can expressed in primary oviduct cells and secreted hEGF promoted proliferation of Hela cells. Moreover, we achieved efficient KI in blastoderm cells without altering their induction in vitro and obtained germline chimeric KI chicken embryos by transplanting KI blastoderm cells as well as injecting adenovirus directly, in vivo. Our results provided an efficient KI method for chicken cells and embryos, and lay the foundation for more convenient production of KI chicken at the OV locus, which will promote the development of oviduct-specific bioreactor.

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.

Targeted mutagenesis in chicken using CRISPR/Cas9 system

The CRISPR/Cas9 system is a simple and powerful tool for genome editing in various organisms including livestock animals. However, the system has not been applied to poultry because of the difficulty in accessing their zygotes. Here we report the implementation of CRISPR/Cas9-mediated gene targeting in chickens. Two egg white genes, ovalbumin and ovomucoid, were efficiently (>90%) mutagenized in cultured chicken primordial germ cells (PGCs) by transfection of circular plasmids encoding Cas9, a single guide RNA, and a gene encoding drug resistance, followed by transient antibiotic selection. We transplanted CRISPR-induced mutant-ovomucoid PGCs into recipient chicken embryos and established three germline chimeric roosters (G0). All of the roosters had donor-derived mutant-ovomucoid spermatozoa, and the two with a high transmission rate of donor-derived gametes produced heterozygous mutant ovomucoid chickens as about half of their donor-derived offspring in the next generation (G1). Furthermore, we generated ovomucoid homozygous mutant offspring (G2) by crossing the G1 mutant chickens. Taken together, these results demonstrate that the CRISPR/Cas9 system is a simple and effective gene-targeting method in chickens.

Long term-cultured and cryopreserved primordial germ cells from various chicken breeds retain high proliferative potential and gonadal colonisation competency

When derived from chicken embryos, avian primordial germ cells (PGCs) have been reported to keep their germline-specific properties and proliferative potential even after long-term culture and genetic modifications. Few teams to date have reported such long-term expansion and engineering without differentiation of primary avian PGCs’ cultures. We have developed original and robust methods that allow more than 1 year culture, expansion and cryobanking of primary cultures of PGCs without obvious effects on their biological properties, including their ability to colonise the genital ridges. Overall, 38% of embryonic samples gave rise to PGCs lines derived from three commercial layers and two Belgian endangered breeds. The lines kept their proliferative potential and their characteristic PGCs phenotype after 20 months in culture, whether or not interrupted by a cryopreservation step. All the resulting lines appeared devoid of female cells, although initially pooled from male and female embryos. Labelled PGCs from 12 long-term cultured lines colonised the genital ridges of recipient embryos. Thus, this procedure allows derivation, long-term expansion and cryobanking of primary cultures of PGCs without obvious changes to their original characteristics, providing an alternative access to applications in avian biotechnology and preservation of genetic resources.

Improvement of transfection efficiency in cultured chicken primordial germ cells by percoll density gradient centrifugation

Chicken primordial germ cells (PGCs) differentiate into germ cells in gonads. Because PGCs can be cloned and cultured maintaining germline competency, they are a good means of modifying the chicken genome, but the efficiency of plasmid transfection into PGCs is very low. In this study, I attempted to improve the efficiency of PGC transfection. Cultured PGCs were purified by Percoll density gradient centrifugation, and were then transfected with plasmid DNA. For transient transfection, the transfection efficiency increased more than 7-fold by the Percoll method. The efficiency of stable transfection of PGCs also increased significantly. The stable transfectants that were isolated by this method accumulated in the developing gonads after microinjection into bloodstream of chick embryos, indicating that gene transfection by Percoll purification did not alter the function of PGCs in vivo.

Retroviral gene transduction into chicken embryo gonads through blood circulation

A retroviral vector was injected into the heart of developing chicken embryos to determine the best timing for effective gene transduction into the gonads. Transduction efficiency and transgene expression were both highest when the viral solution was injected into embryos at stages 14-15 (Hamburger-Hamilton stages), in which primordial germ cells (PGCs) can be found in the blood circulation. This study shows that PGCs in the blood are important targets for gene transfer into the gonads using retroviral vectors.

Production of pharmaceutical proteins by transgenic animals

Proteins started being used as pharmaceuticals in the 1920s with insulin extracted from pig pancreas. In the early 1980s, human insulin was prepared in recombinant bacteria and it is now used by all patients suffering from diabetes. Several other proteins and particularly human growth hormone are also prepared from bacteria. This success was limited by the fact that bacteria cannot synthesize complex proteins such as monoclonal antibodies or coagulation blood factors which must be matured by post-translational modifications to be active or stable in vivo. These modifications include mainly folding, cleavage, subunit association, γ-carboxylation and glycosylation. They can be fully achieved only in mammalian cells which can be cultured in fermentors at an industrial scale or used in living animals. Several transgenic animal species can produce recombinant proteins but presently two systems started being implemented. The first is milk from farm transgenic mammals which has been studied for 20 years and which allowed a protein, human antithrombin III, to receive the agreement from EMEA (European Agency for the Evaluation of Medicinal Products) to be put on the market in 2006. The second system is chicken egg white which recently became more attractive after essential improvement of the methods used to generate transgenic birds. Two monoclonal antibodies and human interferon-β1a could be recovered from chicken egg white. A broad variety of recombinant proteins were produced experimentally by these systems and a few others. This includes monoclonal antibodies, vaccines, blood factors, hormones, growth factors, cytokines, enzymes, milk proteins, collagen, fibrinogen and others. Although these tools have not yet been optimized and are still being improved, a new era in the production of recombinant pharmaceutical proteins was initiated in 1987 and became a reality in 2006. In the present review, the efficiency of the different animal systems to produce pharmaceutical proteins are described and compared to others including plants and micro-organisms.