NEURAL REGULATION OF MUSCLE ACETYLCHOLINE-RECEPTOR EPSILON-SUBUNIT AND ALPHA-SUBUNIT GENE PROMOTERS IN TRANSGENIC MICE

The effects of denervation were investigated in mice with transgenes c ontaining promoter elements from the muscle acetylcholine receptor eps ilon- and alpha-subunit genes. The promoter sequences were coupled to a nuclear localization signal-beta-galactosidase fusion gene (nlacZ) a s a reporter. While many postsynaptic specializations form in the embr yo, expression of the epsilon subunit is induced during the first two postnatal weeks. When muscles were denervated at birth, before the ons et Of epsilon expression, epsilonnlacZ still appeared at the former sy naptic sites on schedule. This result suggests that the nerve leaves a localized ''trace'' in the muscle that can continue to regulate trans cription. An additional finding was that epsilonnlacZ expression was m uch stronger in denervated than in intact muscles. This suggests that the epsilon promoter is similar to the other subunits in containing el ements that are activated on cessation of neural activity. However, ev en after denervation, epsilonnlacZ expression was always confined to t he synaptic region whereas alphanlacZ expression increased in nuclei a long the entire length of the fiber. This suggests that while the epsi lon gene is similar in its activity dependence to other subunit genes, it is unique in that local nerve-derived signals are essential for it s expression. Consequently, inactivity enhances epsilon expression onl y in synaptic nuclei where such signals are present, but enhances expr ession throughout the muscle fiber. Truncations and an internal deleti on of the epsilon promoter indicate that cis-elements essential for th e response to synaptic signals are contained within 280 bp of the tran scription start site. In contrast to these results in young animals, d enervation in older animals leads to an unexpected reduction in nlacZ activity. However, mRNA measurements indicated that transgene expressi on 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.