Structure–Function Studies of Muscarinic Acetylcholine Receptors

Abstract

There has been great interest in the structure-function relationships of the muscarinic acetylcholine receptors (mAChRs) because these prototypical Family A/class 1 G protein-coupled receptors (GPCRs) are attractive therapeutic targets for both peripheral and central nervous system disorders. A multitude of drugs that act at the mAChRs have been identified over the years, but many of these show minimal selectivity for any one of the five mAChR subtypes over the others, which has hampered their development into therapeutics due to adverse side effects. The lack of drug specificity is primarily due to high sequence similarity in this family of receptor, especially in the orthosteric binding pocket. Thus, there remains an ongoing need for a molecular understanding of how mAChRs bind their ligands, and how selectivity in binding and activation can be achieved. Unfortunately, there remains a paucity of solved high-resolution structures of GPCRs, including the mAChRs, and thus most of our knowledge of structure-function mechanisms related to this receptor family to date has been obtained indirectly through approaches such as mutagenesis. Nonetheless, such studies have revealed a wealth of information that has led to novel insights and may be used to guide future rational drug design campaigns.

A constitutively active G protein-coupled acetylcholine receptor regulates motility of larval Schistosoma mansoni

“…However the analysis also revealed potentially important amino acid substitutions that could affect binding affinity and/or signaling activity. For example, the signature TM6 asparagine of mammalian muscarinic receptors (Asn 6.52 ) [50, 51], which is directly involved in hydrogen bonding within the ligand binding pocket [42] is replaced with a histidine in SmGAR (His870 6.52) . Other important substitutions were detected at the TM3/il2 interface and the cytoplasmic end of TM6 (Fig.…”

“…Other important substitutions were detected at the TM3/il2 interface and the cytoplasmic end of TM6 (Fig. 4B), two regions known to play a key role in the conformational activation of GPCRs and subsequent G protein-coupling [reviewed in 50]. Of note are substitutions involving residues of the so-called “ionic lock” between TM3 and TM6, which stabilizes the inactive conformation of some GPCRs.…”

“…Of note are substitutions involving residues of the so-called “ionic lock” between TM3 and TM6, which stabilizes the inactive conformation of some GPCRs. The lock is formed in part by a salt bridge between an invariant arginine of TM3 (Arg 3.50 ) and an acidic residue at the cytosolic interface of TM6 (Glu 6.30 ) [50]. The TM3 arginine is conserved in SmGAR (Arg248 3.50) but there is a non-conservative substitution near the interacting site (Phe →Cys250 3.52 ) and the acidic residue of TM6 is replaced with an alanine (Glu →Ala848 6.30 ) (Fig.…”

Advances in G Protein-Coupled Receptor Allostery: From Function to Structure

“…One can use this to address questions around how much (and what type of) cooperativity, bias, and allosteric agonism are required to achieve in vivo efficacy and/ or avoid on-target side effects. Similar analytical methods can be applied to mutational analysis of receptor allostery and bias to enrich structure-function analyses, in addition to SAR (Gregory et al, 2010;Nawaratne et al, 2010;Tschammer et al, 2011a;Koole et al, 2012;Leach et al, 2012Leach et al, , 2013Valant et al, 2012b;Abdul-Ridha et al, 2014).…”

What Can Crystal Structures of Aminergic Receptors Tell Us about Designing Subtype-Selective Ligands?

“…Another strategy to improve subtypeselectivity is to extend the orthosteric or primary pharmacophore (PP) of a ligand to take advantage of the divergence in the SBPs. In particular, a novel strategy is to link orthosteric and allosteric pharmacophores to create so-called "bitopic" ligands , which has been successfully applied to improve ligand affinity and selectivity in muscarinic acetylcholine receptors (Leach et al, 2012). Moreover, a SBP that has not yet been successfully targeted by an allosteric ligand can still be targeted by an extended moiety from the PP to gain selectivity, such as in the case of selective compounds for individual D2-like receptors described below.…”