A dual function activity-dependent, muscle-specific enhancer from rat nicotinic acetylcholine receptor delta-subunit...

Nicotinic acetylcholine receptors (nAChR) mediate communication between nerve and muscle. The expression of these receptors increases dramatically during muscle development when myoblasts are fusing into multinucleated myotubes. The molecular mechanisms mediating this muscle developmental stage specific expression are not well understood. We report here the identification of nAChR delta-subunit promoter DNA sequences that differentially interact with nuclear proteins isolated from myoblasts, myotubes, and nonmuscle cells. The functional role these sequences play in mediating muscle-specific expression was explored using mutagenesis and enhancer assays. These studies resulted in the identification of a 47-bp muscle-specific enhancer that mediates increased expression of the nAChR delta-subunit gene during myotube formation. This enhancer contains an E-box and an element with similarity to the SV40 core enhancer (SVCE). Point mutations throughout this 47-bp enhancer showed that the E-box and the SVCE sequence are both necessary for conferring muscle-specific expression onto a heterologous promoter. Interestingly, this same DNA sequence also functions as an activity-dependent enhancer.