Transcriptional Activation of Neuromuscular Junction Genes is Altered in Muscle RING Finger 1 (MuRF1) Null Mice

Abstract

Skeletal muscle atrophy occurs as a result of numerous conditions, including disruption of the neuromuscular junction (NMJ), which is the integration point between muscle cells and the neurons that regulate their activity. MuRF1 is an E3 ubiquitin ligase that is activated under must atrophy conditions and is believed to target substrate proteins for degradation during muscle wasting. However, data presented here suggests that MuRF1 may also regulate the transcriptional activity of a subset of genes during neurogenic atrophy, including MuSK and the nicotinic acetylcholine receptor (nAChR) subunit genes. The MuSK and nAChR proteins are essential for proper NMJ function and are rapidly and robustly activated in the muscle of wild-type mice following neuron damage. Interestingly, MuSK and the nAChR genes show blunted activation in MuRF1-null mice in response to denervation. To further investigate a potential transcriptional regulatory role for MuRF1, the promoter regions of MuSK and the nAChR subunit genes were cloned and fused to a reporter gene. These reporter constructs were then transfected into C2C12 mouse muscle cells alone or in combination with expression plasmids for MuRF1 and/or the myogenic regulatory factors (MRF) MyoD1 or myogenin. The results from reporter assays support previous findings demonstrating that the MRFs significantly up-regulate MuSK and nAChR subunit gene expression. Furthermore, when MuRF1 is co-transfected with MyoD1 or myogenin, the MRF-mediated MuSK and nAChR subunit reporter gene induction is significantly repressed. While the mechanism by which MuRF1 represses MRF-mediated gene activation remains unknown, the data presented here provides evidence that MuRF1 may modulate the transcriptional activity of skeletal muscle genes.