CMV enhancer/human PDGF-β promoter for neuron-specific transgene expression

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Abstract
Using cell-type-specific promoters to restrict expression of therapeutic genes to particular cells is an attractive approach for gene therapy, but often hindered by inefficient transcriptional activities of the promoters. Knowing the enhancer for the human cytomegalovirus (CMV) immediate-early gene improves activities of several cell-type- or tissue-type-specific promoters, we set out to investigate whether it improves neuronal transgene expression driven by a neuron-specific promoter, the platelet-derived growth factor B-chain (PDGF-β) promoter. A hybrid promoter was constructed by appending a 380-bp fragment of the CMV enhancer 5′ to the PDGF-β promoter. The plasmid containing the promoter was complexed with polyethylenimine for in vitro and in vivo gene transfer. In cultured cells, the plasmid with the hybrid promoter significantly augmented expression of a luciferase reporter gene, providing expression levels 8- to 90-fold and 7- to 178-fold higher than those from two baseline constructs containing the PDGF-β promoter alone and the CMV enhancer alone, respectively. In particular, the activities of the hybrid promoter in two neural cell lines were close to or higher than that of the CMV immediate-early gene enhancer/promoter, a transcriptional control element that has been considered to be the most robust one identified thus far. After stereotaxic injection into the hippocampus and striatum in rats, the hybrid promoter displayed a neuronal specificity, driving gene expression almost exclusively in neurons. Transgene expression in the brain driven by the hybrid promoter was detectable 24 h after injection, being 10-fold higher than that driven by the PDGF-β promoter alone. The expression peaked around 5 days at 1.5 × 105 relative light units per brain and lasted for at least 4 weeks. This differed strikingly from the expression driven by the PDGF-β promoter, which was no longer detectable on day 3. The new gene regulatory construct reported in this study will be useful to improve neuronal transgene expression required for gene therapy of neurological disorders and functional studies of the nervous system.