In vivo enrichment of busulfan-resistant germ cells for efficient production of transgenic avian models

Strategies for the Generation of Gene Modified Avian Models: Advancement in Avian Germline Transmission, Genome Editing, and Applications

Avian models are valuable for studies of development and reproduction and have important implications for food production. Rapid advances in genome-editing technologies have enabled the establishment of avian species as unique agricultural, industrial, disease-resistant, and pharmaceutical models. The direct introduction of genome-editing tools, such as the clustered regularly interspaced short palindromic repeats (CRISPR) system, into early embryos has been achieved in various animal taxa. However, in birds, the introduction of the CRISPR system into primordial germ cells (PGCs), a germline-competent stem cell, is considered a much more reliable approach for the development of genome-edited models. After genome editing, PGCs are transplanted into the embryo to establish germline chimera, which are crossed to produce genome-edited birds. In addition, various methods, including delivery by liposomal and viral vectors, have been employed for gene editing in vivo. Genome-edited birds have wide applications in bio-pharmaceutical production and as models for disease resistance and biological research. In conclusion, the application of the CRISPR system to avian PGCs is an efficient approach for the production of genome-edited birds and transgenic avian models.

Transplantation and enrichment of busulfan-resistant primordial germ cells into adult testes for efficient production of germline chimeras in songbirds†

Zebra finch is a unique model for behavioral, neural, and genomic studies of vocal learning. Several transgenic zebra finches have been produced, although the germline transmission efficiencies are reportedly low. Recently, there have been attempts to produce germline chimeras using primordial germ cells (PGCs). However, this has been hampered by difficulties associated with the manipulation of the small eggs and the fact that the zebra finch is an altricial species that requires parental care after birth, unlike precocial chickens. Consequently, it is difficult to transplant PGCs into embryos and maintain the chimeras. Here, we developed a busulfan-mediated system for transplantation of PGCs into adult testes, to produce germline chimeras with an improved germline transmission capacity. We established microsomal glutathione-S-transferase II (MGSTII)-overexpressing PGCs that are resistant to busulfan, which induces germ cell-specific cytotoxicity, and transplanted them into testes rendered temporarily infertile by busulfan. The recipients were given a second dose of busulfan to deplete endogenous germ cells and enrich the transplanted cells, and donor cell-derived spermatogenesis was accomplished. This method requires fewer recipients due to higher survival rates, and there is no need to wait for maturation of the founders, which is required when transplanting PGCs into embryos. These results are expected to improve transgenic zebra finch production.

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.

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.