Activity-Linked Regulation of Acetylcholinesterase mRNA Levels Involves Distinct Molecular Mechanisms in Developing Versus Adult Skeletal Muscles

Abstract

Despite the wealth of information available on the plasticity of acetylcholinesterase (AChE) molecular forms subjected to altered levels of neuromuscular activation, our knowledge of the cellular and molecular mechanisms involved in the localization and activity-linked regulation of AChE in skeletal muscle is still rudimentary. Levels of AChE mRNA for instance, are known to be highly sensitive to nerve-evoked electrical activity (1,2) but the underlying molecular events have yet to be clearly determined. In the present study, we therefore examined the contribution of transcriptional and post-transcriptional regulatory mechanisms in the control of AChE mRNA levels in denervated neo-natal and adult rat skeletal muscles. Our results show that in comparison to the sharp increase (20-fold) in the levels of transcripts encoding the α-subunit of the acetylcholine receptor (AChR), denervation of adult muscle induced a rapid (within 2 days) and large (8-fold) decrease in the abundance of AChE mRNAs. Northern blot analysis also revealed that the two predominant species of AChE T transcripts expressed in adult muscle are reduced to a similar extent by muscle denervation. Furthermore, nuclear run-on assays showed that the transcriptional activity of the AChE gene and, unexpectedly, of the AChR α-subunit gene remained essentially unchanged following denervation indicating that post-transcriptional events are responsible for the observed modifications in AChE and AChR α-subunit transcript levels. In separate experiments, we examined whether denervation resulted in significant changes in the half-life of these transcripts by injecting animals with actinomycin D. Quantitative analysis revealed that Egr-1 transcripts, used as a positive control in these experiments, was turning over rapidly (~ 2.5 hr) in skeletal muscle.