Production of Transgenic Chickens Using Cultured Primordial Germ Cells and Gonocytes

Role of PI3K/AKT signaling pathway involved in self-renewing and maintaining biological properties of chicken primordial germ cells

Avian primordial germ cells (PGCs) are important culture cells for the production of transgenic chickens and preservation of the genetic resources of endangered species; however, culturing these cells in vitro proves challenging. Although the proliferation of chicken PGCs is dependent on insulin, the underlying molecular mechanisms remain unclear. In the present study, we explored the expression of the PI3K/AKT signaling pathway in PGCs, investigated its effects on PGC self-renewal and biological properties, and identified the underlying mechanisms. Our findings indicated that although supplementation with the PI3K/AKT activator IGF-1 failed to promote proliferation under the assessed culture conditions, the PI3K/AKT inhibitor LY294002 resulted in retarded cell proliferation and reduced expression of germ cell-related markers. We further demonstrated that inhibition of PI3K/AKT regulates the cell cycle and promotes apoptosis in PGCs by activating the expression of BAX and inhibiting that of Bcl-2. These findings indicated that the PI3K/AKT pathway is required for cell renewal, apoptosis, and maintenance of the reproductive potential in chicken PGCs. This study aimed to provide a theoretical basis for the optimization and improvement of a culture system for chicken PGCs and provide insights into the self-renewal of vertebrate PGCs as well as potential evolutionary changes in this unique cell population.

Comparison of genes involved in brain development: insights into the organization and evolution of the telencephalic pallium

The mechanisms underlying the organization and evolution of the telencephalic pallium are not yet clear.. To address this issue, we first performed comparative analysis of genes critical for the development of the pallium (Emx1/2 and Pax6) and subpallium (Dlx2 and Nkx1/2) among 500 vertebrate species. We found that these genes have no obvious variations in chromosomal duplication/loss, gene locus synteny or Darwinian selection. However, there is an additional fragment of approximately 20 amino acids in mammalian Emx1 and a poly-(Ala)6-7 in Emx2. Lentiviruses expressing mouse or chick Emx2 (m-Emx2 or c-Emx2 Lv) were injected into the ventricle of the chick telencephalon at embryonic Day 3 (E3), and the embryos were allowed to develop to E12-14 or to posthatchling. After transfection with m-Emx2 Lv, the cells expressing Reelin, Vimentin or GABA increased, and neurogenesis of calbindin cells changed towards the mammalian inside-out pattern in the dorsal pallium and mesopallium. In addition, a behavior test for posthatched chicks indicated that the passive avoidance ratio increased significantly. The study suggests that the acquisition of an additional fragment in mammalian Emx2 is associated with the organization and evolution of the mammalian pallium.

Heavy chain-only antibodies with a stabilized human VH in transgenic chickens for therapeutic antibody discovery

Heavy chain-only antibodies have found many applications where conventional heavy-light heterodimeric antibodies are not favorable. Heavy chain-only antibodies with their single antigen-binding domain offer the advantage of a smaller size and higher stability relative to conventional antibodies, and thus, the potential for novel targeting modalities. Domain antibodies have commonly been sourced from camelids with ex-vivo humanization or transgenic rodents expressing heavy chains without light chains, but these host species are all mammalian, limiting their capacity to elicit robust immune responses to conserved mammalian targets. We have developed transgenic chickens expressing heavy chain-only antibodies with a human variable region to combine the superior target recognition advantages of a divergent, non-mammalian host with the ability to discover single-domain binders. These birds produce robust immune responses, consisting of antigen-specific antibodies targeting diverse epitopes with a range of affinities. Biophysical attributes are favorable, with good developability profiles and low predicted immunogenicity.

A Novel, Efficient Method to Isolate Chicken Primordial Germ Cells from Embryonic Blood Using Cell Culture Inserts

Primordial germ cells (PGCs) play a crucial role in preserving poultry genetic resources and conducting transgenic research. A system for the rapid isolation of PGCs from single chicken embryonic blood was established in this paper. We found that PGCs can migrate to the lower layer of chicken embryonic fibroblasts (CEFs) through pores smaller than their diameter, while blood cells cannot, when co-cultured with CEFs of passages two to three. Based on the characteristics of PGCs, we developed a new PGC isolation method (cell culture insert/CEF adhesion method) that utilizes a 3 μm cell culture insert and CEFs of passages two to three. Using this method, approximately 700 PGCs can be isolated from the blood of a single chicken embryo at Hamburger and Hamilton (H&H) stage 17 of development. The separation rate achieved was 87.5%, with a separation purity of 95%. The separation rate of this method was 41.4% higher than the common Percoll density gradient centrifugation method and 33.6% higher than lysis with ACK buffer. PGCs isolated from embryonic blood could proliferate 37-fold within 2 weeks when cultured in a feeder-free culture system. They also continued to express the SSEA-1 and DAZL proteins and retained the ability to migrate in vivo. Overall, PGCs separated using cell culture inserts/CEF adhesion method retain their stem cell characteristics and migration ability. PGCs also exhibit good proliferation efficiency, making them suitable for subsequent transgenic experiments or genetic resource preservation.

Enhanced cultivation of chicken primordial germ cells

The cultivation and expansion of chicken primordial germ cells (cPGCs) are of critical importance for both biotechnological applications and the management of poultry genetic biodiversity. The feeder-free culture system has become the most popular approach for the cultivation and expansion of cPGCs. However, despite some success in the cultivation of cPGCs, the reproducibility of culture conditions across different laboratories remains a challenge. This study aimed to compare two defined and enriched media for the growth of cPGCs originating from the Hubbard JA57 broiler. To this end, cPGCs were isolated from the embryonic blood of Hamburger-Hamilton (HH) stages 14-16 and cultured at various time points. The Growth properties and characteristics of these cells were evaluated in two different culture conditions (the defined or enriched medium) and their migratory properties were assessed after genetic engineering and injection into the vasculature of 2.5-day-old chicken embryos. The main finding of this study was that the use of an enriched medium (the defined medium with Knock-Out Serum Replacement; KOSR) resulted in improved growth properties of cPGCs originating from the Hubbard JA57 broiler compared to a defined medium. The ability to cultivate and expand cPGCs is crucial for the generation of both genetically engineered birds and breeds of interest from local or commercial origins. Therefore, these results highlight the importance of choosing an appropriate culture medium for cPGCs growth and expansion.

Quantitative analysis of CRISPR/Cas9-mediated provirus deletion in blue egg layer chicken PGCs by digital PCR

Primordial germ cells (PGCs), the precursors of sperm and oocytes, pass on the genetic material to the next generation. The previously established culture system of chicken PGCs holds many possibilities for functional genomics studies and the rapid introduction of desired traits. Here, we established a CRISPR/Cas9-mediated genome editing protocol for the genetic modification of PGCs derived from chickens with blue eggshell color. The sequence targeted in the present report is a provirus (EAV-HP) insertion in the 5'-flanking region of the SLCO1B3 gene on chromosome 1 in Araucana chickens, which is supposedly responsible for the blue eggshell color. We designed pairs of guide RNAs (gRNAs) targeting the entire 4.2 kb provirus region. Following transfection of PGCs with the gRNA, genomic DNA was isolated and analyzed by mismatch cleavage assay (T7EI). For absolute quantification of the targeting efficiencies in homozygous blue-allele bearing PGCs a digital PCR was established, which revealed deletion efficiencies of 29% when the wildtype Cas9 was used, and 69% when a high-fidelity Cas9 variant was employed. Subsequent single cell dilutions of edited PGCs yielded 14 cell clones with homozygous deletion of the provirus. A digital PCR assay proved the complete absence of this provirus in cell clones. Thus, we demonstrated the high efficiency of the CRISPR/Cas9 system in introducing a large provirus deletion in chicken PGCs. Our presented workflow is a cost-effective and rapid solution for screening the editing success in transfected PGCs.

Cultivation and characterization of primordial germ cells from blue layer hybrids (Araucana crossbreeds) and generation of germline chimeric chickens

The chicken (Gallus gallus) is one of the most common and widespread domestic species, with an estimated total population of 25 billion birds worldwide. The vast majority of chickens in agriculture originate from hybrid breeding programs and is concentrated on few commercially used high performance lines, whereas numerous local and indigenous breeds are at risk to become extinct. To preserve the genomic resources of rare and endangered chicken breeds innovative methods are necessary. Here, we established a solid workflow for the derivation and biobanking of chicken primordial germ cells (PGCs) from blue layer hybrids. To achieve this, embryos of a cross of heterozygous blue egg layers were sampled to obtain blood derived and gonadal male as well as female PGCs of different genotypes (homozygous, heterozygous and nullizygous blue-allele bearing). The total efficiency of established PGC lines was 45% (47/104) within an average of 49 days until they reached sufficient numbers of cells for cryopreservation. The stem-cell character of the cultivated PGCs was confirmed by SSEA-1 immunostaining, and RT-PCR amplification of the pluripotency- and PGC-specific genes cPOUV, cNANOG, cDAZL and CVH. The Sleeping Beauty transposon system allowed to generate a stable integration of a Venus fluorophore reporter into the chicken genome. Finally, we demonstrated that, after re-transfer into chicken embryos, Venus-positive PGCs migrated and colonized the forming gonads. Semen samples of 13 raised cell chimeric roosters were analyzed by flow cytometry for the efficiency of germline colonization by the transferred PGCs carrying the Venus reporter and their proper differentiation into vital spermatids. Thus, we provide a proof-of-concept study for the potential use of PGCs for the cryobanking of rare breeds or rare alleles.

Common light chain chickens produce human antibodies of high affinity and broad epitope coverage for the engineering of bispecifics

Bispecific antibodies are an important and growing segment in antibody therapeutics, particularly in the immuno-oncology space. Manufacturing of a bispecific antibody with two different heavy chains is greatly simplified if the light chains can be the same for both arms of the antibody. Here, we introduce a strain of common light chain chickens, called OmniClic®, that produces antibody repertoires largely devoid of light chain diversity. The antibody repertoire in these chickens is composed of diverse human heavy chain variable regions capable of high-affinity antigen-specific binding and broad epitope diversity when paired with the germline human kappa light chain. OmniClic birds can be used in immunization campaigns for discovery of human heavy chains to different targets. Subsequent pairing of the heavy chain with a germline human kappa light chain serves to facilitate bispecific antibody production by increasing the efficiency of correct pairing.

Abbreviations: AID: activation-induced cytidine deaminase; bsAb: bispecific antibody; CDR: complementarity-determining region; CL: light chain constant region; CmLC: common light chain; D: diversity region; ELISA: enzyme-linked immunosorbent assay; FACS: fluorescence-activated cell sorting; Fc: fragment crystallizable; FcRn: neonatal Fc receptor; FR: framework region; GEM: gel-encapsulated microenvironment; Ig: immunoglobulin; IMGT: the international ImMunoGeneTics information system®; J: joining region; KO: knockout; mAb: monoclonal antibody; NGS: next-generation sequencing; PBS: phosphate-buffered saline; PCR: polymerase chain reaction; PGC: primordial germ cell; PGRN: progranulin; TCR: T cell receptor; V: variable region; VK: kappa light chain variable region; VL: light chain variable region; VH: heavy chain variable region

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

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

Dynamics of the transcriptome during chicken embryo development based on primordial germ cells

Objective

Regulation of gene expression during embryo development on the basis of migration of primordial germ cells (PGCs) in vivo has been rarely studied due to limited cell number and the necessity to isolate PGCs from a large number of embryos. Moreover, little is known about the comprehensive dynamics of the transcriptome in chicken PGCs during early developmental stages. The current study investigated transcriptome dynamics of chicken PGCs at key developmental stages: 4.5, 8 and 12 days of embryo incubation. PGCs were collected, and RNA was isolated using a commercial kit for single cells. The isolated RNA was subjected to microarray analysis (Agilent Technologies).

Results

Between 8 and 12 days of incubation, the highest number of genes was regulated. These data indicate that the most intense biological activity occurs between 8 and 12 days of embryo development. Heat map showed a significant decrease in gene expression on day 8, while it increased on day 12. The development of a precise method to isolate bird PGCs as well as the method to isolate RNA from single cells isolated from one embryo allows for early molecular analysis and detection of transcriptome changes during embryonic development.

Targeting lentiviral vectors to primordial germ cells (PGCs): An efficient strategy for generating transgenic chickens

Recent advances in avian transgenic studies highlight the possibility of utilizing lentiviral vectors as tools to generate transgenic chickens. However, low rates of gonadal chimerism and germ line transmission efficiency still limit the broad usage of this method in creating transgenic chickens. In this study, we implemented a simple strategy using modified lentiviral vectors targeted to chicken primordial germ cells (PGCs) to generate transgenic chickens. The lentiviral vectors were pseudotyped with a modified Sindbis virus envelope protein (termed M168) and conjugated with an antibody specific to PGC membrane proteins. We demonstrated that these optimized M168-pseudotyped lentiviral vectors conjugated with SSEA4 antibodies successfully targeted transduction of PGCs in vitro and in vivo. Compared with the control, 50.0%-66.7% of chicken embryos expressed green fluorescent protein (GFP) in gonads transduced by the M168-pseudotyped lentivirus. This improved the targeted transduction efficiency by 30.0%-46.7%. Efficient chimerism of exogenous genes was also observed. This targeting technology could improve the efficiency of germ line transmission and provide greater opportunities for transgenic poultry studies.

Advances in Isolation and Culture of Chicken Embryonic Stem Cells In Vitro

Chicken embryonic stem cells (cESCs) isolated from the egg at the stage X hold great promise for cell therapy, tissue engineering, pharmaceutical, and biotechnological applications. They are considered to be pluripotent cells with the capacity to self-renewal and differentiate into specialized cells. However, long-term maintenance of cESCs cannot be realized now, which impedes the establishment of cESC line and limits their applications. Therefore, the separation locations, isolation methods, and culture conditions especially the supplements and action mechanisms of cytokines, including leukemia inhibitory factor, fibroblast growth factor, transforming growth factor beta, bone morphogenic protein, and activin for cESCs in vitro, have been reviewed here. These defined strategies will contribute to identify the key mechanism on the self-renewal of cESCs, facilitate to optimize system that supports the derivation and longtime maintenance of cESCs, establish the cESC line, and develop the biobank of genetic resources in chicken.

Precise CRISPR/Cas9 editing of the NHE1 gene renders chickens resistant to the J subgroup of avian leukosis virus

Avian leukosis virus subgroup J (ALV-J) is an important concern for the poultry industry. Replication of ALV-J depends on a functional cellular receptor, the chicken Na⁺/H⁺ exchanger type 1 (chNHE1). Tryptophan residue number 38 of chNHE1 (W38) in the extracellular portion of this molecule is a critical amino acid for virus entry. We describe a CRISPR/Cas9-mediated deletion of W38 in chicken primordial germ cells and the successful production of the gene-edited birds. The resistance to ALV-J was examined both in vitro and in vivo, and the ΔW38 homozygous chickens tested ALV-J-resistant, in contrast to ΔW38 heterozygotes and wild-type birds, which were ALV-J-susceptible. Deletion of W38 did not manifest any visible side effect. Our data clearly demonstrate the antiviral resistance conferred by precise CRISPR/Cas9 gene editing in the chicken. Furthermore, our highly efficient CRISPR/Cas9 gene editing in primordial germ cells represents a substantial addition to genotechnology in the chicken, an important food source and research model.