Enhancement of cell‐specific transgene expression from a Tet‐Off regulatory system using a transcriptional amplification strategy in the rat brain

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Abstract
Background: The Tet‐Off system uses a tetracycline‐controlled transactivator protein (tTA) and a tetracycline‐responsive promoter element (TRE) to regulate expression of a target gene. This system can be used to achieve regulatable transgene expression in specific cell types by employing a cell‐specific promoter to drive tTA expression. Wide applications of this attractive approach are, however, hindered by relatively weak transcriptional activity of most cell‐specific promoters. We report here the feasibility of using a transcriptional amplification strategy to overcome the problem.Methods and results: In the developed cell‐type‐specific, Tet‐inducible lentiviral system, two distinct cellular promoters were tested, a human synapsin‐1 promoter for neurons and a compact glial fibrillary acidic protein promoter for astroglial cells. Lentiviral vectors were constructed that contained two copies of one or the other of these two promoters. One copy was used to drive the expression of a chimeric transactivator consisting of a part of the transcriptional activation domain of the NF‐κB p65 protein fused to the DNA‐binding domain of the yeast GAL4 protein. The second copy of the cell‐specific promoter was modified by introduction of the GAL4 binding sequences at its 5′ end. This copy was used to drive expression of tTA. A gene encoding a red fluorescent protein was cloned into another lentiviral vector under transcriptional control of TRE. Co‐transduction with the two types of viral vectors provided doxycycline‐regulated transgene expression in a neuron‐ or astrocyte‐specific manner. Compared to control viruses without transcriptional amplification, our enhanced systems were approximately 8‐fold more potent in cultured neurons and astroglial cells and at least 8‐ to 12‐fold more potent in the rat brain in vivo.Conclusions: Our results demonstrate the effectiveness of the transcriptional amplification strategy in developing viral gene delivery systems that combine the advantages of specific cell type targeting and Tet‐inducible expression. Copyright © 2008 John Wiley & Sons, Ltd.