Expression of the P2Y1Nucleotide Receptor in Chick Muscle: Its Functional Role in the Regulation of Acetylcholinesterase and Acetylcholine Receptor

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Abstract
In vertebrate neuromuscular junctions, ATP is stored at the motor nerve terminals and is co-released with acetylcholine during neural stimulation. Here, we provide several lines of evidence that the synaptic ATP can act as a synapse-organizing factor to induce the expression of acetylcholinesterase (AChE) and acetylcholine receptor (AChR) in muscles, mediated by a metabotropic ATP receptor subtype, the P2Y1 receptor. The activation of the P2Y1receptor by adenine nucleotides stimulated the accumulation of inositol phosphates and intracellular Ca2+ mobilization in cultured chick myotubes. P2Y1 receptor mRNA in chicken muscle is very abundant before hatching and again increases in the adult. The P2Y1 receptor protein is shown to be restricted to the neuromuscular junctions and colocalized with AChRs in adult muscle (chicken, Xenopus, and rat) but not in the chick embryo. In chicks after hatching, this P2Y1 localization develops over ∼3 weeks. Denervation or crush of the motor nerve (in chicken or rat) caused up to 90% decrease in the muscle P2Y1 transcript, which was restored on regeneration, whereas the AChR mRNA greatly increased. Last, mRNAs encoding the AChE catalytic subunit and the AChR α-subunit were induced when the P2Y1 receptors were activated by specific agonists or by overexpression of P2Y1 receptors in cultured myotubes; those agonists likewise induced the activity in the myotubes of promoter–reporter gene constructs for those subunits, actions that were blocked by a P2Y1-specific antagonist. These results provide evidence for a novel function of ATP in regulating the gene expression of those two postsynaptic effectors.