Calcium andcAMP-dependent second messenger systems regulating nicotinic acetylcholine receptor expression.

Abstract

Nicotinic acetylcholine receptors (nAChR) are pentameric integral membrane proteins that function as ligand-gated ion channels. During development of the neuromuscular junction (NMJ), nAChR expression changes dramatically. Prior to muscle innervation these receptors are found throughout the developing myotube; however after innervation they are localized to a small area of the muscle fiber that is contacted by the nerve (the NMJ). The loss of nAChRs from extrajunctional regions of the muscle fiber is a result of nerve-induced muscle activity suppressing nAChR subunit gene expression. Denervation of the adult skeletal muscle reinduces nAChR gene expression in extrajunctional regions of the muscle fiber, resulting in increased sensitivity of the muscle to acetylcholine. Interestingly, these studies have shown that this increased level of gene expression following muscle denervation does not persist beyond a couple of months of denervation, suggesting a mechanism of adaptation. In order to understand how muscle activity regulates nAChR gene expression this research focused on identifying intracellular signaling molecules that may control nAChR gene expression in response to muscle activity. These studies have identified two key signaling molecules. First, calcium release from the sarcoplasmic reticulum (SR) suppresses nAChR subunit RNA levels as a result of decreasing nAChR subunit gene expression. The DNA sequences responsible for mediating the effects of SR calcium on nAChR gene expression map to those that also control expression in response to muscle activity. Second, increasing intracellular cAMP levels prevented depolarization- and calcium-dependent decreases in nAChR RNA levels. However the mechanism by which cAMP establishes this effect is not clear. Preliminary studies suggest cAMP may be affecting both nAChR transcription and RNA stability. These studies provide possible links between muscle activity and nAChR gene expression and suggest these mechanisms play an important role in formation of the NMJ and its modification in the adult.