Receptor subtype-specific regulation of muscarinic acetylcholine receptor sequestration by dynamin. Distinct sequestration of m2 receptors.

Sustained stimulation of muscarinic acetylcholine receptors (mAChRs) and other G protein-coupled receptors usually leads to a loss of receptor binding sites from the plasma membrane, referred to as receptor sequestration. Receptor sequestration can occur via endocytosis of clathrin-coated vesicles that bud from the plasma membrane into the cell but may also be accomplished by other, as yet ill-defined, mechanisms. Previous work has indicated that the monomeric GTPase dynamin controls the endocytosis of plasma membrane receptors via clathrin-coated vesicles. To investigate whether mAChRs sequester in a receptor subtype-specific manner via dynamin-dependent clathrin-coated vesicles, we tested the effect of overexpressing the dominant-negative dynamin mutant K44A on m1, m2, m3, and m4 mAChR sequestration in HEK-293 cells. The m1, m2, m3, and m4 mAChRs sequestered rapidly in HEK-293 cells following agonist exposure but displayed dissimilar sequestration pathways. Overexpression of dynamin K44A mutant fully blocked m1 and m3 mAChR sequestration, whereas m2 mAChR sequestration was not affected. Also, m4 mAChRs, which like m2 mAChRs preferentially couple to pertussis toxin-sensitive G proteins, sequestered in a completely dynamin-dependent manner. Following agonist removal, sequestered m1 mAChRs fully reappeared on the cell surface, whereas sequestered m2 mAChRs did not. The distinct sequestration of m2 mAChRs was also apparent in COS-7 and Chinese hamster ovary cells. We conclude that the m2 mAChR displays unique subtype-specific sequestration that distinguishes this receptor from the m1, m3, and m4 subtypes. These results are the first to demonstrate that receptor sequestration represents a new type of receptor subtype-specific regulation within the family of mAChRs.

Receptor internalization delays m4 muscarinic acetylcholine receptor resensitization at the plasma membrane.

We analyzed the role of receptor internalization and recycling in muscarinic acetylcholine receptor (mAChR) desensitization and resensitization. Incubation of Chinese hamster ovary cells stably expressing the m4 mAChR with 1 mM carbachol for 1 hr reduced cell surface receptor number by 50-60% with no change in total receptor number. Pretreatment of the cells with 450 mM sucrose, which did not affect the ability of m4 receptors to inhibit forskolin-stimulated cAMP accumulation, completely blocked receptor internalization. On the other hand, the carbachol treatment reduced the ability of m4 receptors to inhibit cAMP accumulation in both sucrose-treated and untreated cells, with a similar onset and to a similar extent. The EC50 value for carbachol was increased approximately 10-fold, and maximal inhibition determined at 100 microM carbachol was reduced approximately 50%. In contrast, thrombin-induced inhibition of cAMP accumulation was not affected. Recycled receptors in cells not treated with sucrose remained refractory to carbachol stimulation for > or = 2 hr after agonist removal, even though cell surface receptor number had recovered completely within 1 hr. In contrast, resensitization of receptor function was very rapid in cells treated with sucrose. Ten minutes on removal of agonist, mAChRs in the plasma membrane of sucrose-treated cells were fully resensitized. Also, an internalization-defective m4 mAChR mutant, T399A, that was found to desensitize similar to the wild-type receptor, resensitized more rapidly than the wild-type receptor. We conclude that desensitization and resensitization of m4 mAChRs in Chinese hamster ovary cells can occur at the plasma membrane and that receptor internalization strongly delays the process of resensitization of desensitized receptors.

Enhanced cAMP accumulation by the human thyrotropin receptor variant with the Pro52Thr substitution in the extracellular domain.

Recently, a naturally occurring variant of the human thyrotropin receptor with a Pro52Thr substitution in the N-terminal extracellular domain of the receptor has been identified. To determine the functional significance of this substitution, cDNAs of wild-type and variant thyrotropin receptors were stably expressed in Chinese hamster ovary cells. The Pro52Thr substitution did not affect synthesis and membrane localization of the receptor, as evidenced by 125I-thyrotropin binding analysis to intact cells. The variant receptor and the wild-type receptor were expressed in equivalent numbers and displayed identical binding affinity for thyrotropin. Strikingly, thyrotropin increased cAMP accumulation to a much greater extent in cells expressing the variant receptor as compared to the wild-type receptor-expressing cells. Basal and cholera toxin-stimulated or forskolin-stimulated cAMP levels were not different. It is concluded that the Pro52Thr substitution in the N-terminal region of the human thyrotropin receptor produces a receptor protein with enhanced coupling to cAMP production. This naturally occurring hyperactive thyrotropin receptor may participate in hyperthyroidism of patients with Graves' disease.