Regulation of muscarinic receptor expression by changes in mRNA stability

Regulation of muscarinic acetylcholine receptor (mAChR) subtype mRNAs was investigated in the human neuroblastoma cell line IMR-32 and in transfected CHO cells. IMR-32 cells express both m1 and m3 subtypes of mAChR. Exposure of IMR-32 cells to the muscarinic agonist, carbamylcholine (CBC) leads to a time dependent down-regulation of mAChRs which was maximal by 9 hours. mAChR activation resulted in a differential regulation of mAChR subtype mRNAs. m1 mAChR mRNA was down-regulated following 12 hours of agonist treatment and was associated with a decreased stability of the receptor transcript. In contrast, the m3 mAChR mRNA was resistant to agonist treatment for up to 24 hours. Using transfected CHO cells, we identified sequence elements within the 3'-untranslated region (3'-UTR) of the m1 mAChR gene which dictate agonist-induced destabilization of the m1 mAChR mRNA. Removal of these sequences abolished the ability of chronic agonist exposure to destabilize m1 mAChR mRNA. These findings suggest that sequence specific differences between m1 and m3 mAChR subtypes, which both preferentially couple to hydrolysis of phosphoinositides, may be responsible for differences in the regulation of mAChR gene expression.