COUP-TF plays a dual role in the regulation of the ovalbumin gene

Study of the regulatory elements of the Ovalbumin gene promoter using CRISPR technology in chicken cells

BACKGROUND

Hormone-dependent promoters are very efficient in transgene expression. Plasmid-based reporter assays have identified regulatory sequences of the Ovalbumin promoter that are involved in response to estrogen and have shown that the deletion of the steroid-dependent regulatory element (SDRE) and negative regulatory element (NRE) leads to a steroid-independent expression of a reporter. However, the functional roles of these regulatory elements within the native genomic context of the Ovalbumin promoter have not been evaluated.

RESULTS

In this study, we show that the negative effects of the NRE element on the Ovalbumin gene can be counteracted by CRISPR interference. We also show that the CRISPR-mediated deletion of SDRE and NRE promoter elements in a non-oviduct cell can lead to the significant expression of the Ovalbumin gene. In addition, the targeted knock-in of a transgene reporter in the Ovalbumin coding region and its expression confirms that the truncated promoter of the Ovalbumin gene can be efficiently used for an estrogen-independent expression of a foreign gene.

CONCLUSIONS

The methodology applied in this paper allowed the study of promoter regulatory sequences in their native nuclear organization.

Oviduct-specific expression of human neutrophil defensin 4 in lentivirally generated transgenic chickens

The expression of oviduct-specific recombinant proteins in transgenic chickens is a promising technology for the production of therapeutic biologics in eggs. In this study, we constructed a lentiviral vector encoding an expression cassette for human neutrophil defensin 4 (HNP4), a compound that displays high activity against Escherichia coli, and produced transgenic chickens that expressed the recombinant HNP4 protein in egg whites. After the antimicrobial activity of the recombinant HNP4 protein was tested at the cellular level, a 2.8-kb ovalbumin promoter was used to drive HNP4 expression specifically in oviduct tissues. From 669 injected eggs, 218 chickens were successfully hatched. Ten G₀ roosters, with semens identified as positive for the transgene, were mated with wild-type hens to generate G₁ chickens. From 1,274 total offspring, fifteen G₁ transgenic chickens were positive for the transgene, which was confirmed by PCR and Southern blotting. The results of the Southern blotting and genome walking indicated that a single copy of the HNP4 gene was integrated into chromosomes 1, 2, 3, 4, 6 and 24 of the chickens. As expected, HNP4 expression was restricted to the oviduct tissues, and the levels of both transcriptional and translational HNP4 expression varied greatly in transgenic chickens with different transgene insertion sites. The amount of HNP4 protein expressed in the eggs of G₁ and G₂ heterozygous transgenic chickens ranged from 1.65 μg/ml to 10.18 μg/ml. These results indicated that the production of transgenic chickens that expressed HNP4 protein in egg whites was successful.

An oviduct-specific and enhancer-like element resides at about -3000 in the chicken ovalbumin gene

The chicken ovalbumin (Ov) gene is one of the best models to study tissue-specific gene regulation because it is only expressed in the oviduct. In this paper, a tissue-specific element was characterized by 5'-flanking region deletion in combination with in vivo gene electroporation (EP). Plasmids with varying lengths of truncated Ov 5'-flanking region fused to the Renilla luciferase reporter gene were transfected in vivo into oviduct, muscle, and pancreas. A chicken oviduct-specific and enhancer-like region (designated COSE) was implicated between -3100 and -2800. The COSE showed up-regulation of gene expression in oviduct, but not in muscle or in pancreas. The COSE region was further characterized by gel mobility shift assays using nuclear extracts from oviduct, pancreas and liver. With the region from -2900 to -2851, designated T2, there were two distinct protein-DNA complexes: one found only in oviduct extract and the other detected only in pancreas and liver. These data suggest a model where the regulation of Ov gene expression in the oviduct and non-oviduct tissues is exerted at least in part by the presence of protein modulators that bind to the COSE element.

Transgenic chickens as bioreactors for protein-based drugs

The potential of using transgenic animals for the synthesis of therapeutic proteins was suggested over twenty years ago. Considerable progress has been made in developing methods for the production of transgenic animals and specifically in the expression of therapeutic proteins in the mammary glands of cows, sheep and goats. Development of transgenic hens for protein production in eggs has lagged behind these systems. The positive features associated with the use of the chicken in terms of cost, speed of development of a production flock and potentially appropriate glycosylation of target proteins have led to significant advances in transgenic chicken models in the past few years.

Differences in transactivation between rat CYP3A1 and human CYP3A4 genes by human pregnane X receptor

In an assay system using a human CYP3A4 reporter constructed with the promoter (+11 nt to -362 nt) and enhancer (-7.2 knt to -7.8 knt) regions including everted repeat separated by six nucleotides (ER-6) and direct repeat separated by three nucleotides (DR-3) motifs, the CYP3A4 transactivation was detected without overexpression of any nuclear receptors in rifampicin-treated HepG2 cells. Overexpression of human pregnane X receptor (hPXR) enhanced the transactivation. Rat CYP3A1 reporter constructed with the promoter region (+31 nt to -171 nt) including both DR-3 and ER-6 motifs was, however, not transactivated in rifampicin-treated cells, even after overexpression of hPXR. Although overexpression of retinoid X receptor alpha (RXRalpha) had no clear effect for both CYP3A reporters, co-expression of apolipoprotein AI regulatory protein-1 (ARP-1) with hPXR resulted in the rifampicin-induced transactivation of the CYP3A1 reporter. A truncated CYP3A4 reporter retaining the both motifs showed the rifampicin-induced transactivation by overexpression of hPXR and ARP-1, while the transactivation in hPXR-overexpressed cells was not observed. These results support the idea that a nuclear receptor other than RXRalpha may play a role in the CYP3A transactivation together with hPXR. The present study also suggests the involvement of a novel cis-element in the hPXR-mediated CYP3A4 transactivation.

The COUP-adjacent repressor (CAR) element participates in the tissue-specific expression of the ovalbumin gene

The ovalbumin (Ov) gene is an excellent model for the study of tissue-specific gene regulation as it is only active in the estrogen-stimulated oviduct. Previous studies have demonstrated that the negative regulatory element (NRE) in the Ov gene 5'-flanking region is responsible for silencing the gene in oviduct in the absence of steroids. Linker scanning analysis defined an element within the NRE designated the COUP-adjacent repressor (CAR) element as a repressor of Ov gene expression. However, the role of the CAR element in non-oviduct tissues has not been addressed. Using transient transfection analysis of various Ov 5'-flanking region constructs into the estrogen-responsive chicken hepatocyte cell line LMH/2A, we demonstrate that Ov gene expression is not induced by estrogen and that an active repressor element exists in the NRE. Deletion analysis indicates that the region from -134 to -87, which includes the CAR element, mediates this repression. Mutation of the CAR element relieves repression, leading to high levels of gene expression. These data support a model where the inhibition of Ov gene expression in non-oviduct cells is a combination of the lack of essential positive factors and the presence of an active repressor, which binds to the CAR element.