Neuromuscular synapses form as a result of inductive interactions between motor neurons and muscle fibers. Following contact with the growth cone of a developing motor neuron, developing muscle fibers undergo a complex differentiation program in the synaptic region, and signals from the muscle in turn regulate the differentiation of the presynaptic terminals. Two different transcriptional signaling pathways lead to the localization of acetylcholine receptors (AChRs) at synaptic sites. The signal for one pathway acts focally to stimulate expression of certain genes, including AChR genes in myofiber nuclei near the synaptic site. The second signaling pathway is activated by propagated electrical activity, and this signaling pathway regulates gene expression in all nuclei of the myofiber. The signals that regulate synapse-specific transcription remain elusive. Experiments described here are designed to determine whether muscle-derived Neuregulin-1 (NRG-1) or motor neuron-derived NRG-2 is required for synapse-specific transcription. An Ets-binding site in the AChR d subunit gene is critical for synapse-specific and NRG-1-induced gene expression; this Ets-site binds GABP, a heterodimer of GABPa, an Ets protein, and GABPb. Experiments described here are designed to determine whether GABPa is required for synapse-specific and NRG-induced transcription and how NRG increases the transcriptional activity of GABP. The pathway that couples changes in the pattern of electrical activity to changes in gene expression is not known. Myogenin is a good candidate for a transcriptional mediator of electrical activity-dependent transcription, as myogenin can promote expression of genes that are regulated by electrical activity and response elements for electrical activity-dependent transcription have been mapped to E-boxes in AChR genes. Electrical activity can increase the activity of PKC, and PKC can phosphorylate a threonine residue (T87) in myogenin, thereby inhibiting DNA-binding. Experiments described here are designed to determine whether myogenin is phosphorylated at T87 in electrically active muscle and whether T87 phosphorylation is necessary in vivo to inactivate myogenin and AChR expression. These experiments should provide a better understanding of the signals and mechanisms that regulate gene expression in skeletal muscle and may provide insight into the mechanisms of synaptic- and electrical activity-dependent signaling in the central nervous system.
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