Neural regulation of muscle acetylcholine receptor ε- and α-subunit gene promoters in transgenic mice

Abstract

The effects of denervation were investigated in mice with transgenes containing promoter elements from the muscle acetylcholine receptor ε- and α-subunit genes. The promoter sequences were coupled to a nuclear localization signal-β-galactosidase fusion gene (nlacZ) as a reporter. While many postsynaptic specializations form in the embryo, expression of the ε subunit is induced during the first two postnatal weeks. When muscles were denervated at birth, before the onset of ε expression, εnlacZ still appeared at the former synaptic sites on schedule. This result suggests that the nerve leaves a localized "trace" in the muscle that can continue to regulate transcription. An additional finding was that εnlacZ expression was much stronger in denervated than in intact muscles. This suggests that the ε promoter is similar to the other subunits in containing elements that are activated on cessation of neural activity. However, even after denervation, εnlacZ expression was always confined to the synaptic region whereas αnlacZ expression increased in nuclei along the entire length of the fiber. This suggests that while the ε gene is similar in its activity dependence to other subunit genes, it is unique in that local nerve-derived signals are essential for its expression. Consequently, inactivity enhances ε expression only in synaptic nuclei where such signals are present, but enhances expression throughout the muscle fiber. Truncations and an internal deletion of the ε promoter indicate that cis-elements essential for the response to synaptic signals are contained within 280 bp of the transcription start site. In contrast to these results in young animals, denervation in older animals leads to an unexpected reduction in nlacZ activity. However, mRNA measurements indicated that transgene expression was increased in these animals. This discordance between nlacZ mRNA and enzyme activity, demonstrates a previously unknown limitation of nlacZ as a reporter gene in transgenic animals. © The Rockefeller University Press.