Germline engineering of the chicken genome using CRISPR/Cas9 by in vivo transfection of PGCs

Development of simple and readily adoptable methods to mediate germline engineering of the chicken genome will have many applications in research, agriculture and industrial biotechnology. We report germline targeting of the endogenous chicken Interferon Alpha and Beta Receptor Subunit 1 (IFNAR1) gene by in vivo transgenic expression of the high-fidelity Cas9 (Cas9-HF1) and guide RNAs (gRNAs) in chickens. First, we developed a Tol2 transposon vector carrying Cas9-HF1, IFNAR1-gRNAs (IF-gRNAs) and green fluorescent protein (GFP) transgenes (pTgRCG) and validated in chicken fibroblast DF1 cells. Next, the pTgRCG plasmid was directly injected into the dorsal aorta of embryonic day (ED) 2.5 chicken embryos targeting the circulating primordial germ cells (PGCs). The resulting chimera roosters generated a fully transgenic generation 1 (G1) hen with constitutive expression of Cas9-HF1 and IF-gRNAs (G1_Tol2-Cas9/IF-gRNA). We detected a spectrum of indels at gRNA-targeted loci in the G1_Tol2-Cas9/IF-gRNA hen and the indels were stably inherited by the G2 progeny. Breeding of the G1_Tol2-Cas9/IF-gRNA hen resulted in up to 10% transgene-free heterozygote IFNAR1 mutants, following null-segregation of the Tol2 insert. The method described here will provide new opportunities for genome editing in chicken and other avian species that lack PGC culture.

Marker counter-selection via CRISPR/Cas9 co-targeting for efficient generation of genome edited avian cell lines and germ cells

Efficient isolation of genetically modified cells that are phenotypically indistinguishable from the unmodified cells remains a major technical barrier for the broader utilization of CRISPR/Cas9. Here, we report a novel enrichment approach to select the genome engineered cells by co-targeting a genomically integrated GFP gene along with the endogenous gene of interest (GOI). Using this co-targeting approach, multiple genomic loci were successfully targeted in chicken (DF1) and quail (CEC-32) fibroblast cell lines by transient transfection of Cas9 and guide RNAs (gRNAs). Clonal isolation of co-targeted DF1 cells showed 75% of cell clones had deletion of GFP and biallelic deletion of the GOI. To assess the utility of this approach to generate genome modified animals, we tested it on chicken primordial germ cells (PGCs) expressing GFP by co-targeting with gRNAs against GFP and endogenous ovomucoid (OVM) gene. PGCs enriched for loss of GFP and confirmed for OVM deletion, derived by co-targeting, were injected into Hamburger and Hamilton stage 14-15 chicken embryos, and their ability to migrate to the genital ridge was confirmed. This simple, efficient enrichment approach could easily be applied to the creation of knock-out or edited cell lines or animals.

Generation of gene edited birds in one generation using sperm transfection assisted gene editing (STAGE)

Generating transgenic and gene edited mammals involves in vitro manipulation of oocytes or single cell embryos. Due to the comparative inaccessibility of avian oocytes and single cell embryos, novel protocols have been developed to produce transgenic and gene edited birds. While these protocols are relatively efficient, they involve two generation intervals before reaching complete somatic and germline expressing transgenic or gene edited birds. Most of this work has been done with chickens, and many protocols require in vitro culturing of primordial germ cells (PGCs). However, for many other bird species no methodology for long term culture of PGCs exists. Developing methodologies to produce germline transgenic or gene edited birds in the first generation would save significant amounts of time and resource. Furthermore, developing protocols that can be readily adapted to a wide variety of avian species would open up new research opportunities. Here we report a method using sperm as a delivery mechanism for gene editing vectors which we call sperm transfection assisted gene editing (STAGE). We have successfully used this method to generate GFP knockout embryos and chickens, as well as generate embryos with mutations in the doublesex and mab-3 related transcription factor 1 (DMRT1) gene using the CRISPR/Cas9 system. The efficiency of the method varies from as low as 0% to as high as 26% with multiple factors such as CRISPR guide efficiency and mRNA stability likely impacting the outcome. This straightforward methodology could simplify gene editing in many bird species including those for which no methodology currently exists.

Overexpression of Anti-Müllerian Hormone Disrupts Gonadal Sex Differentiation, Blocks Sex Hormone Synthesis, and Supports Cell Autonomous Sex Development in the Chicken

The primary role of Anti-Müllerian hormone (AMH) during mammalian development is the regression of Müllerian ducts in males. This highly conserved function is retained in birds and is supported by the high levels of AMH expression in developing testes. Mammalian AMH expression is regulated by a combination of transcription factors, the most important being Sry-type high-mobility-group box transcription factor-9 (SOX9). In the chicken embryo, however, AMH mRNA expression precedes that of SOX9, leading to the view that AMH may play a more central role in avian testicular development. To define its role in chicken gonadal development, AMH was overexpressed using the RCASBP viral vector. AMH caused the gonads of both sexes to develop as small and undeveloped structures at both embryonic and adult stages. Molecular analysis revealed that although female gonads developed testis-like cords, gonads lacked Sertoli cells and were incapable of steroidogenesis. A similar gonadal phenotype was also observed in males, with a complete loss of both Sertoli cells, disrupted SOX9 expression and gonadal steroidogenesis. At sexual maturity both sexes showed a female external phenotype but retained sexually dimorphic body weights that matched their genetic sexes. These data suggest that AMH does not operate as an early testis activator in the chicken but can affect downstream events, such as sex steroid hormone production. In addition, this study provides a unique opportunity to assess chicken sexual development in an environment of sex hormone deficiency, demonstrating the importance of both hormonal signaling and direct cell autonomous factors for somatic sex identity in birds.

Transgenic Chickens Overexpressing Aromatase Have High Estrogen Levels but Maintain a Predominantly Male Phenotype

Estrogens play a key role in sexual differentiation of both the gonads and external traits in birds. The production of estrogen occurs via a well-characterized steroidogenic pathway, which is a multistep process involving several enzymes, including cytochrome P450 aromatase. In chicken embryos, the aromatase gene (CYP19A1) is expressed female-specifically from the time of gonadal sex differentiation. Ectopic overexpression of aromatase in male chicken embryos induces gonadal sex reversal, and male embryos treated with estradiol become feminized; however, this is not permanent. To test whether a continuous supply of estrogen in adult chickens could induce stable male to female sex reversal, 2 transgenic male chickens overexpressing aromatase were generated using the Tol2/transposase system. These birds had robust ectopic aromatase expression, which resulted in the production of high serum levels of estradiol. Transgenic males had female-like wattle and comb growth and feathering, but they retained male weights, displayed leg spurs, and developed testes. Despite the small sample size, this data strongly suggests that high levels of circulating estrogen are insufficient to maintain a female gonadal phenotype in adult birds. Previous observations of gynandromorph birds and embryos with mixed sex chimeric gonads have highlighted the role of cell autonomous sex identity in chickens. This might imply that in the study described here, direct genetic effects of the male chromosomes largely prevailed over the hormonal profile of the aromatase transgenic birds. This data therefore support the emerging view of at least partial cell autonomous sex development in birds. However, a larger study will confirm this intriguing observation.

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.

Toll-like receptor 7 ligands inhibit influenza A infection in chickens

Avian influenza virus is endemic in many regions around the world and remains a pandemic threat, a scenario tied closely to outbreaks of the virus in poultry. The innate immune system, in particular the nucleic acid-sensing toll-like receptors (TLRs) -3, -7, -8, and -9, play a major role in coordinating antiviral immune responses. In this study we have investigated the use of TLR ligands as antivirals against influenza A in chickens. The TLR7 ligand poly-C inhibited low-path influenza A growth in the chicken macrophage cell line HD-11 more effectively than poly(I:C), which acts via TLR3. The TLR7 ligand 7-allyl-8-oxoguanosine (loxoribine) inhibited influenza A replication in vitro and in ovo in a dose-dependent manner. Treatment of primary chicken splenocytes with loxoribine resulted in the induction of interferons-α, -β, and -λ, and interferon-stimulated genes PKR and Mx. These results demonstrate that nucleic acid-sensing TLR ligands show considerable potential as antivirals in chickens and could be incorporated into antiviral strategies.

Native microRNA loop sequences can improve short hairpin RNA processing for virus gene silencing in animal cells

Introduction of small interfering RNAs (siRNAs) into cells results in transitory silencing of target genes with complementary sequence. Incorporating siRNAs into short-hairpin RNAs (shRNAs) or microRNA-adapted shRNAs (shRNAmir) is a popular tool for targeted gene silencing. shRNAmirs mimicking endogenous pre-microRNAs (unprocessed hairpin microRNAs) are more difficult to design and result in longer RNA molecules. The use of microRNA (miRNA) loop sequences in shRNAs as an alternative to an entire pre-microRNA structure on silencing efficiency has not been studied extensively. This report shows that loop sequences derived from native miRNAs improves the efficiency of silencing due to the processing of the shRNAs into mature siRNAs.

Characterization and comparison of chicken U6 promoters for the expression of short hairpin RNAs

RNA interference (RNAi) is a powerful method of sequence-specific gene knockdown that can be mediated by DNA-based expression of short hairpin RNA (shRNA) molecules. A number of vectors for expression of shRNA have been developed with promoters for a small group of RNA polymerase III (pol III) transcripts of either mouse or human origin. To advance the use of RNAi as a tool for functional genomic research and future development of specific therapeutics in the chicken species, we have developed shRNA expression vectors featuring chicken U6 small nuclear RNA (snRNA) promoters. These sequences were identified based on the presence of promoter element sequence motifs upstream of matching snRNA sequences that are characteristic of these types of pol III promoters. To develop suitable shRNA expression vectors specifically for chicken functional genomic RNAi applications, we compared the efficiency of each of these promoters to express shRNA molecules. Promoter activity was measured in the context of RNAi by targeting and silencing the reporter gene encoding the enhanced green fluorescent protein (EGFP). Plasmids containing one of four identified chicken U6 promoters gave a similar degree of knockdown in DF-1 cells (chicken); although, there was some variability in Vero cells (monkey). Because the chicken promoters were not stronger than the benchmark mouse U6 promoter, we suggest that the promoter sequence and structure is more important in determining efficiency in vitro rather than its species origin.

Comparison of chicken 7SK and U6 RNA polymerase III promoters for short hairpin RNA expression

BACKGROUND

RNA polymerase III (pol III) type 3 promoters such as U6 or 7SK are commonly used to express short-hairpin RNA (shRNA) effectors for RNA interference (RNAi). To extend the use of RNAi for studies of development using the chicken as a model system, we have developed a system for expressing shRNAs using the chicken 7SK (ch7SK) promoter.

RESULTS

We identified and characterised the ch7SK promoter sequence upstream of the full-length 7SK small nuclear RNA (snRNA) sequence in the chicken genome and used this to construct vectors to express shRNAs targeting enhanced green fluorescent protein (EGFP). We transfected chicken DF-1 cells with these constructs and found that anti-EGFP-shRNAs (shEGFP) expressed from the ch7SK promoter could induce efficient knockdown of EGFP expression. We further compared the efficiency of ch7SK-directed knockdown to that of chicken U6 (cU6) promoters and found that the efficiency of the ch7SK promoter was not greater than, but comparable to the efficiency of cU6 promoters.

CONCLUSION

In this study we have demonstrated that the ch7SK promoter can express shRNAs capable of mediating efficient RNAi in a chicken cell line. However, our finding that RNAi driven by the ch7SK promoter is not more efficient than cU6 promoters contrasts previous comparisons of mammalian U6 and 7SK promoters. Since the ch7SK promoter is the first non-mammalian vertebrate 7SK promoter to be characterised, this finding may be helpful in understanding the divergence of pol III promoter activities between mammalian and non-mammalian vertebrates. This aside, our results clearly indicate that the ch7SK promoter is an efficient alternative to U6-based shRNA expression systems for inducing efficient RNAi activity in chicken cells.

Comparison of bovine RNA polymerase III promoters for short hairpin RNA expression

RNA interference (RNAi) mediated by DNA-based expression of short hairpin RNA (shRNA) is a powerful method of sequence-specific gene knockdown. A number of vectors for expression of shRNA have been developed that feature promoters from RNA polymerase III (pol III)-transcribed genes of mouse or human origin. To advance the use of RNAi as a tool for functional genomic research and for future development of specific therapeutics in the bovine species, we have developed shRNA expression vectors that feature novel bovine RNA pol III promoters. We characterized two bovine U6 small nuclear RNA (snRNA) promoters (bU6-2 and bU6-3) and a bovine 7SK snRNA promoter (b7SK). We compared the efficiency of each of these promoters to express shRNA molecules. Promoter activity was measured in the context of RNAi by targeting and suppressing the reporter gene encoding enhanced green fluorescent protein. Results show that the b7SK promoter induced the greatest level of suppression in a range of cell lines. The comparison of these bovine promoters in shRNA expression is an important component for the future development of bovine-specific RNAi-based research.

First identification of a ranavirus from green pythons (Chondropython viridis)

Ten juvenile green pythons (Chondropython viridis) died or were euthanized shortly after having been illegally imported into Australia from Indonesia in 1998. Histologic examination of two of the three snakes that died revealed moderately severe chronic ulceration of the nasal mucosa and focal or periacinar degeneration and necrosis of the liver. In addition there was severe necrotizing inflammation of the pharyngeal submucosa accompanied by numerous macrophages, heterophils, and edema. An iridovirus was isolated in culture from several tissues and characterized by immunohistochemistry, electron microscopy, enzyme-linked immunosorbent Assay, polyacrylamide gel electrophoresis, polymerase chain reaction and sequence analysis, restriction endonuclease digestion, and DNA hybridization. This is the first report of a systemic ranavirus infection in any species of snake and is a new member of the genus, Ranavirus.

Ultrastructure of Hendra virus and Nipah virus within cultured cells and host animals

The ultrastructure of Hendra and Nipah viruses is described in cultured cells, pigs, horses and humans. Differences in ultrastructure between the viruses are evident within infected cell cultures and lungs from infected amplifier hosts. These differences are important in viral identification and differentiation and understanding the pathogenesis of disease.

Expression of the potyvirus coat protein mediated by recombinant vaccinia virus and assembly of potyvirus-like particles in mammalian cells

The coat protein of the potyvirus, Johnsongrass mosaic virus (JGMV), was expressed using a recombinant vaccinia virus (VV) system. Ultra-thin section electron microscopy demonstrated that the coat protein assembled into potyvirus-like particles (PVLPs) in recombinant VV infected cells. Infection of cells with two additional VV recombinants expressing coat protein plus N-terminal and N- and C-terminal extensions also resulted in the formation of PVLPs. These results suggest that the ability of VV to express the potyvirus coat protein at sufficient levels to allow PVLP formation in vitro, could make VV a suitable vector for the delivery of PVLPs displaying vaccine antigens in vivo without the need for particle purification and/or inclusion of adjuvant. Use of such a vaccine strategy would also benefit from the proven advantages of poxviruses as vaccines such as stability in a freeze dried form, resistance to environmental factors and the potential for oral administration.

Latent cytomegalovirus infection of BALB/c mouse spleens detected by an explant culture technique

Latent murine cytomegalovirus (MCMV) infection of BALB/c mouse spleens was studied using several methods including an explant tissue culture technique, co-cultivation on allogeneic and syngeneic cell cultures and nucleic acid hybridization. BALB/c mice experience latent infection which persists for at least 6 months and involves only a small fraction of spleen cells. The explant culture technique proved to be much more sensitive than other methods for detecting latent infection of lymphoid tissues.

Serologic responses after two sequential doses of influenza A/New Jersey/76 virus vaccine in normal young adults

The serologic responses after two sequential nonreactive doses of either chemically disrupted or whole-virus influenza A/New Jersey/76 virus vaccine were evaluated in 112 normal young adults. In general, levels of hemagglutination-inhibiting (HAI) antibody were low after the first dose of vaccine and increased significantly (P less than 0.05) in response to a second dose. Whereas one dose of the preparation from Merck Sharp and Dohme (West Point, Pa.) effectively vaccinated this population, two doses of the vaccines prepared by Parke, Davis and Company (Detroit, Mich.) and Merrell-National Laboratories (Cincinnati, Ohio) were required to produce a similar serologic response. The preparation from Wyeth Laboratories (Philadelphia, Pa.) produced low levels of HAI antibodies even after two doses. These different serologic responses correlated with the viral hemagglutinin content of each vaccine.

Immunogenicity and reactogenicity of influenza A/New Jersey/76 virus vaccines in normal adults

Inactivated influenza A/New Jersey/76 virus vaccines were administered intramuscularly to 199 normal adults, aged 19-59, in doses of 200, 400, or 800 chick cell-agglutinating units in a double-blind, placebo-controlled trial. Systemic reactions (including fever) were uncommon, were mild, lasted less than 24 hr, and were more frequently associated with the largest dose. Local reactions were common but mild. A single, rapidly reversible, allergic reaction was noted in a volunteer 2 hr after vaccination. There was a trend toward fewer systemic reactions in vaccines who had preexisting hemagglutination-inhibiting (HAI) antibodies to the vaccine virus in their sera as compared with seronegative vaccines. All vaccine preparations at all three dosages evoked serum HAI titers of greater than or equal to 20 to greater than or equal to 40 in a high proportion of seronegative recipients, with significantly greater geometric mean titers at the highest dosage. Vaccines between the ages of 19 and 23 years manifested significantly lower serologic responses than did vaccinees over the age of 23. Thus, normal adults over the age of 23 can be immunized with a single, well-tolerated dose of A/New Jersey/76 vaccines.

Serologic responses and systemic reactions in adults after vaccination with bivalent A/Victoria/75-A/New Jersey/76 and monovalent B/Hong Kong/72 influenza vaccines

Bivalent A/Victoria/75-A/New Jersey/76 and monovalent B/Hong Kong/72 influenza vaccines were given alone or together to adutls, and systemic reactions and antibody responses were determined. The rates of systemic reactivity observed varied among vaccine groups. Disrupted vaccines and whole-virus vaccines containing type B antigen only did not cause significant reactivity. Systemic reactions were observed after administration of the bivalent A whole-virus vaccines, and this reactivity was increased if the B vaccine was also administered. Reactions to the more reactive vaccines were less frequent in older subjects or in younger individuals with evidence of previous exposure to influenza antigens in the vaccine. Antibody responses in this study indicated that individuals older than 25 years responded better to A/New Jersey antigens than did younger subjects. The A/Victoria antigen produced lower antibody levels in older individuals than in younger subjects. The B/Hong Kong antibody responses were similar in all vaccine groups.

Serological responses to whole and split A/New Jersey vaccines in humans and mice following priming infection with influenza A viruses

Experiments were performed in mice to investigate the role of previous infection on responses to A/NJ/76 vaccines. Results from human studies have demonstrated that the serological responses to A/NJ/76 vaccines varied according to the age of the vaccinee and appeared to be related to their previous exposure to the different strains of influenza A virus. Mice were infected with influenza A viruses representative of the major strains (Hsw1N1, HON1, H2N2, H3N2) and later inoculated with varying doses of whole or subunit A/NJ/ML virus vaccines. Results from these experiments demonstrated a low antigenicity in non-primed mice of the subunit vaccine compared to whole-virus vaccine, but that the antigenicity of both vaccines was enhanced in mice primed by previous infection with earlier H0 and H1 viruses. The responses of heterotypically primed mice were qualitatively similar to those of primed humans following A/New Jersey vaccination.