Quantitative analysis of tetracycline-inducible expression of the green fluorescent protein gene in transgenic chickens.

The use of transgenic farm animals as "bioreactors" to address the growing demand for biopharmaceuticals, both in terms of increased quantity and greater number, represents a key development in the advancement of medical science. However, the potential for detrimental side-effects as a result of uncontrolled constitutive expression of foreign genes in transgenic animals is a well-recognized limitation of such systems. Previously, using a tetracycline-inducible expression system, we demonstrated the induction of expression of a transgene encoding green fluorescent protein (GFP) in transgenic chickens by feeding with doxycycline, a tetracycline derivative; expression of GFP reverted to pre-induction levels when the inducer was removed from the diet. As a proof of principle study, however, quantitative assessment of expression was not possible, as only one G0 and one G1 transgenic chicken was obtained. In the current study, a sufficient number of G2 and G3 transgenic chickens were obtained, and quantification analysis demonstrated up to a 20-fold induction of expression by doxycycline. In addition, stable transmission of the transgene without any apparent genetic modifications was observed through several generations. The use of an inducible expression system that can be regulated by dietary supplementation could help mitigate the physiological disruption that can occur in transgenic animals as a result of uncontrolled constitutive expression of a transgene. Importantly, these results also support the use of the retroviral system for generating transgenic animals with minimal risk in terms of biosafety.

Tetracycline-inducible gene expression in nuclear transfer embryos derived from porcine fetal fibroblasts transformed with retrovirus vectors.

A critical problem of transgenic livestock production is uncontrollable constitutive expression of the foreign gene, which usually results in serious physiological disturbances in transgenic animals. One of the best solutions for this problem may be use of controllable gene expression system. In this study, using retrovirus vectors designed to express the enhanced green fluorescent protein (EGFP) gene under the control of the tetracycline-inducible promoter, we examined whether the expression of the transgene could be controllable in fibroblast cells and nuclear transfer (NT) embryos of porcine. Transformed fibroblast cells were cultured in medium supplemented with or without doxycycline (a tetracycline analog) for 48 hr, and the induction efficiency was measured by comparing EGFP gene expression using epifluorescence microscopy and Western and Northern blot analyses. After the addition of doxycycline, EGFP expression increased up to 17-fold. The nuclei of transformed fibroblast cells were transferred into enucleated oocytes. Fluorescence emission data revealed strong EGFP gene expression in embryos cultured with doxycycline, but little or no expression in the absence of the antibiotic. Our results demonstrate the successful regulation of transgene expression in porcine nuclear transfer embryos, and support the application of an inducible expression system in transgenic pig production to solve the inherent problems of side-effects due to constitutive expression of the transgene.