Neurogenic Atrophy‐Induced Nicotinic Acetylcholine Receptor ε to γ Subunit Switching is Impaired in Muscle RING Finger 1 (MuRF1) Null Mice

Abstract

Skeletal muscle atrophy is caused by a multitude of conditions, including denervation, corticosteroids, immobilization and aging. The nicotinic acetylcholine receptors cluster at the neuromuscular junction (NMJ), which receives incoming signals from neurons and relays the stimulus to skeletal muscle cells causing contraction. The acetylcholine receptor (nAChR) is a multimeric transmembrane protein that consists of two α, one β, and one δ subunit and either the γ subunit expressed during embryonic development or the ε subunit expressed in the adult muscle. Microarray analysis of skeletal muscle tissue isolated from wild-type mice confirmed previous observations that the embryonic nAChRγ subunit is up-regulated in response to denervation, while the adult nAChRε subunit is transiently down-regulated. Interestingly, these changes in nAChRγ and nAChRε expression are altered in MuRF1 null mice in response to neurogenic atrophy. In order to further characterize the transcriptional regulation of the nAChRγ and nAChRε subunit genes, fragments of the promoter regions were cloned, fused to a reporter gene, and transfected into C2C12 cells in combination with MyoD1 or myogenin expression plasmids. MyoD1 and myogenin are well characterized myogenic regulatory factors (MRF) that have previously been shown regulate nAChR gene expression. Ectopic expression of MyoD1 or myogenin had no significant effect on the nAChRγ subunit reporter gene. However, overexpression of MyoD1 or myogenin in combination with the nAChRε reporter gene caused a significant increase in ε subunit reporter activity. Furthermore, when MyoD1 or myogenin were overexpressed in combination with MuRF1, the MRF induction of the nAChRε reporter gene was reversed.