Intracellular trafficking of the muscarinic acetylcholine receptor: importance of subtype and cell type.

Agonist-induced decrease of surface muscarinic receptor number occurs in a number of cell lines. Recent work has suggested that some muscarinic receptor subtypes undergo internalization, whereas others do not. We investigated the agonist-induced trafficking of various muscarinic receptor subtypes transfected into CHO cells and compared it with the trafficking of receptors expressed natively in neuronal cells, fibroblasts, or epithelial cells. SH-SY5Y neuroblastoma cells, which express predominantly the m3 receptor subtype, show qualitatively similar changes in surface receptor number in response to agonist stimulation to those occurring in NG108-15 cells, which express predominantly the m4 subtype. The rate constants for internalization, however, were considerably different, indicating that receptors in SH-SY5Y cells show a much faster turnover than those in NG108-15 cells. In the transfected cells, the muscarinic receptor subtypes m1 and m3, which are coupled to second messenger systems via Gq/11, showed little agonist-induced loss of surface receptors. In contrast, the muscarinic receptor subtypes m2 and m4, which are coupled via Gi or G(o), showed a substantial loss of surface receptors after treatment with agonist. An interesting implication of this result is that agonist-induced receptor trafficking can still occur efficiently, even at very high receptor densities. Significant agonist-induced internalization also occurs in a fibroblast line (HeLa) and an epithelial cell line (HT29), both of which express predominantly m3 receptors. Our results suggest that the extent and rate of the loss of receptors from the cell surface in response to agonist stimulation are governed by both the receptor subtype and the cell type in which it is expressed.