Identification and Characterization of a 47 Base Pair Activity-Dependent Enhancer of the Rat Nicotinic Acetylcholine Receptor δ-Subunit Promoter

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Abstract
Nicotinic acetylcholine receptor (nAChR) genes are regulated by muscle electrical activity. E-box sequences found in their promoters are necessary for this regulation. However, many muscle genes contain E-boxes, yet are not regulated by muscle depolarization. This suggests that other elements are necessary, perhaps working in conjunction with E-boxes, to confer depolarization-dependent control onto promoter activity. We have used direct DNA injection into muscle as an in vivo assay to identify and characterize these additional elements. Mutagenesis and expression assays identified multiple elements within the first 81 base pairs (bp) of the nAChR δ-subunit promoter that contribute to its regulation by muscle electrical activity. Within this 81 bp sequence, two regions of DNA were identified that were capable of conferring activity-dependent regulation onto a heterologous promoter. The stronger of these two putative enhancers was characterized further. It is a 47 bp sequence that contains an E-box along with sequences similar to the SV40 core enhancer and an SP1 site. Site-directed mutagenesis identified residues within each of these sequences that were necessary for enhancer activity. Furthermore, methylation interference DNA footprinting assays showed increased nuclear protein binding to sequences within both these enhancers after muscle denervation, and this pattern of binding was very similar to that observed with nuclear protein isolated from myotube extracts. These latter results suggest that similar mechanisms may mediate increased nAChR expression during muscle development and after muscle denervation.