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Helix 8 of the M1 muscarinic acetylcholine receptor: scanning mutagenesis delineates a G protein recognition site.

ABSTRACT: We have used alanine-scanning mutagenesis followed by functional expression and molecular modeling to analyze the roles of the 14 residues, Asn422 to Cys435, C-terminal to transmembrane (TM) helix 7 of the M(1) muscarinic acetylcholine receptor. The results suggest that they form an eighth (H8) helix, associated with the cytoplasmic surface of the cell membrane in the active state of the receptor. We suggest that the amide side chain of Asn422 may act as a cap to the C terminus of TM7, stabilizing its junction with H8, whereas the side chain of Phe429 may restrict the relative movements of H8 and the C terminus of TM7 in the inactive ground state of the receptor. We have identified four residues, Phe425, Arg426, Thr428, and Leu432, which are important for G protein binding and signaling. These may form a docking site for the C-terminal helix of the G protein ? subunit, and collaborate with G protein recognition residues elsewhere in the cytoplasmic domain of the receptor to form a coherent surface for G protein binding in the activated state of the receptor.

SUBMITTER: Kaye RG

PROVIDER: S-EPMC3063726 | biostudies-literature | 2011 Apr

REPOSITORIES: biostudies-literature

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Helix 8 of the M1 muscarinic acetylcholine receptor: scanning mutagenesis delineates a G protein recognition site.

Kaye Robert G RG Saldanha José W JW Lu Zhi-Liang ZL Hulme Edward C EC

Molecular pharmacology 20110119 4

We have used alanine-scanning mutagenesis followed by functional expression and molecular modeling to analyze the roles of the 14 residues, Asn422 to Cys435, C-terminal to transmembrane (TM) helix 7 of the M(1) muscarinic acetylcholine receptor. The results suggest that they form an eighth (H8) helix, associated with the cytoplasmic surface of the cell membrane in the active state of the receptor. We suggest that the amide side chain of Asn422 may act as a cap to the C terminus of TM7, stabilizi ...[more]

PMID: 21247934

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Project description:Aggregation of amyloid beta and loss of cholinergic innervation in the brain are predominant components of Alzheimer's disease pathology and likely underlie cognitive impairment. Acetylcholinesterase inhibitors are one of the few treatment options for Alzheimer's disease, where levels of available acetylcholine are enhanced to counteract the cholinergic loss. However, these inhibitors show limited clinical efficacy. One potential explanation for this is a concomitant dysregulation of cholinergic receptors themselves as a consequence of the amyloid beta pathology. We tested this hypothesis by examining levels of M1 muscarinic acetylcholine receptors in the temporal cortex from seven Alzheimer's disease and seven non-disease age-matched control brain tissue samples (control: 85 ± 2.63 years old, moderate Alzheimer's disease: 84 ± 2.32 years old, P-value = 0.721; eight female and six male patients). The samples were categorized into two groups: 'control' (Consortium to Establish a Registry for Alzheimer's Disease diagnosis of 'No Alzheimer's disease', and Braak staging pathology of I-II) and 'moderate Alzheimer's disease' (Consortium to Establish a Registry for Alzheimer's Disease diagnosis of 'possible/probable Alzheimer's disease', and Braak staging pathology of IV). We find that in comparison to age-matched controls, there is a loss of M1 muscarinic acetylcholine receptors in moderate Alzheimer's disease tissue (control: 2.17 ± 0.27 arbitrary units, n = 7, Mod-AD: 0.83 ± 0.16 arbitrary units, n = 7, two-tailed t-test, t = 4.248, P = 0.00113). Using a functional rat cortical brain slice model, we find that postsynaptic muscarinic acetylcholine receptor function is dysregulated by aberrant amyloid beta-mediated activation of metabotropic glutamate receptor 5. Crucially, blocking metabotropic glutamate receptor 5 restores muscarinic acetylcholine receptor function and object recognition memory in 5XFAD transgenic mice. This indicates that the amyloid beta-mediated activation of metabotropic glutamate receptor 5 negatively regulates muscarinic acetylcholine receptor and illustrates the importance of muscarinic acetylcholine receptors as a potential disease-modifying target in the moderate pathological stages of Alzheimer's disease.

Project description:Background and objectiveMuscarinic acetylcholine receptors (mAChRs) are 7-transmembrane, G protein-coupled receptors that regulate a variety of physiological processes and represent potentially important targets for therapeutic intervention. mAChRs can be stimulated by full and partial orthosteric and allosteric agonists, however the relative abilities of such ligands to induce conformational changes in the receptor remain unclear. To gain further insight into the actions of mAChR agonists, we have developed a fluorescently tagged M(1) mAChR that reports ligand-induced conformational changes in real-time by changes in Förster resonance energy transfer (FRET).MethodsVariants of CFP and YFP were inserted into the third intracellular loop and at the end of the C-terminus of the mouse M(1) mAChR, respectively. The optimized FRET receptor construct (M(1)-cam5) was expressed stably in HEK293 cells.ResultsThe variant CFP/YFP-receptor chimera expressed predominantly at the plasma membrane of HEK293 cells and displayed ligand-binding affinities comparable with those of the wild-type receptor. It also retained an ability to interact with G?(q/11) proteins and to stimulate phosphoinositide turnover, ERK1/2 phosphorylation and undergo agonist-dependent internalization. Addition of the full agonist methacholine caused a reversible decrease in M(1) FRET (F(EYFP)/F(ECFP)) that was prevented by atropine pre-addition and showed concentration-dependent amplitude and kinetics. Partial orthosteric agonists, arecoline and pilocarpine, as well as allosteric agonists, AC-42 and 77-LH-28-1, also caused atropine-sensitive decreases in the FRET signal, which were smaller in amplitude and significantly slower in onset compared to those evoked by methacholine.ConclusionThe M(1) FRET-based receptor chimera reports that allosteric and orthosteric agonists induce similar conformational changes in the third intracellular loop and/or C-terminus, and should prove to be a valuable molecular reagent for pharmacological and structural investigations of M(1) mAChR activation.

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Helix 8 of the M1 muscarinic acetylcholine receptor: scanning mutagenesis delineates a G protein recognition site.

Publications

Helix 8 of the M1 muscarinic acetylcholine receptor: scanning mutagenesis delineates a G protein recognition site.

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