Putative RNA editing of a nicotinic receptor increases acetylcholine sensitivity

Regulation of the agonist sensitivity of neurotransmitter receptors is critical for proper functioning of neuronal circuits and is, therefore, conserved across evolutionary time. Mutations that alter agonist sensitivity are often pathological in humans. A brain-expressing nicotinic acetylcholine receptor (nAChR) from the frog Xenopus tropicalis shows ∼20× greater sensitivity to ACh as orthologs from human, chickens, and other frogs prompt us to examine the molecular basis for this extreme sensitivity. We identified a single amino acid substitution in the third transmembrane domain (M3) of the X. tropicalis α4 nAChR subunit, F294 (S in other vertebrate orthologs), that confers the high sensitivity. Surprisingly, we noted variation at this site in sequences deposited in NCBI, suggesting either allelic variation or RNA editing. By sequencing genomic DNA and mRNA (cDNA) from the same individuals from two different colonies of X. tropicalis, we determined that a possible source of this variation is RNA editing. The unedited receptor from X. tropicalis (S294) has a similar ACh sensitivity as those from other vertebrates. Further work must be done to examine possible adaptations of edited receptors and if the frog's brain compensates for an increase in sensitivity since increases in agonist sensitivity lead to pathology in humans.