Toward an understanding of the structural basis of allostery in muscarinic acetylcholine receptors

Abstract

Recent breakthroughs and developments in structural biology have led to a spate of crystal structures for G protein-coupled receptors (GPCRs). This is the case for the muscarinic acetylcholine receptors (mAChRs) where inactive-state structures for four of the five subtypes and two active-state structures for one subtype are available. These mAChR crystal structures have provided new insights into receptor mechanisms, dynamics, and allosteric modulation. This is highly relevant to the mAChRs given that these receptors are an exemplar model system for the study of GPCR allostery. Allosteric mechanisms of the mAChRs are predominantly consistent with a two-state model, albeit with some notable recent exceptions. Herein, we discuss the mechanisms for positive and negative allosteric modulation at the mAChRs and compare and contrast these to evidence offered by pharmacological, biochemical, and computational approaches. This analysis provides insight into the fundamental pharmacological properties exhibited by GPCR allosteric modulators, such as enhanced subtype selectivity, probe dependence, and biased modulation while highlighting the current challenges that remain. Though complex, enhanced molecular understanding of allosteric mechanisms will have considerable influence on our understanding of GPCR activation and signaling and development of therapeutic interventions.

Crystal structure of the M5 muscarinic acetylcholine receptor

“…Taken together, these findings indicate that the differential targeting of ECL residues may be a path forward for creating selective mAChR ligands. This is well supported by the fact that many mAChR-selective allosteric modulators interact with the ECL regions (27, 35), and suggest that designing orthosteric ligands linked to allosteric pharmacophores, known as bitopic ligands, is a potential strategy for future structure-based drug design.…”

“…This has been extensively explored for the mAChR family where a palette of both positive and negative allosteric modulators has been identified (33, 34). Structural and mutagenesis studies have established that many of these ligands bind to a “common” allosteric site that is located above the orthosteric site and within an ECV (Figure 3, Supplementary Figure 4) (35). In fact, the M 5 mAChR has often served as model system for early research into understanding the binding mode and mechanism of selectivity for prototypical modulators, such as the bis-ammonium alkane ligands (Figure 1b), that have higher sensitivity for modulating the M 2 mAChR and lower sensitivity for the M 5 mAChR (26, 36–39).…”

Current Advances in Allosteric Modulation of Muscarinic Receptors

“…In contrast, tyrosine in the o2 loop (Y179 in M 1 , Y177 in M 2 ) and tryptophan in TM7 (W 7.35 , Ballesteros-Weinstein numbering [45]) have been identified as key residues, defining the magnitude of binding cooperativity of M 1 -selective PAM of acetylcholine (BQCA) as well as M 2 -selective PAM of iperoxo (LY2119620, Figure 4) [4,29,34]. The common feature found for all PAMs is shrinkage of the vestibule to the orthosteric binding site that is accompanied by the closure of the binding pocket [46]. Divergent mechanisms underlying how shrinkage of the vestibule is achieved probably represent the molecular basis of PAM receptor subtype selectivity.…”

Allosteric modulation of β-cell M3muscarinic acetylcholine receptors greatly improves glucose homeostasis in lean and obese mice

“…During the past 10 to 15 y, novel drugs have been developed that can modulate the function of the M3R and other muscarinic ACh receptor (mAChR) subtypes by binding to a receptor site distinct from the conventional (orthosteric) ACh-binding site (12)(13)(14)(15). This so-called "allosteric site" is located on the extracellular receptor surface and shows significant sequence variation among the 5 mAChR subtypes (16,17).…”

“…Over the past few years, considerable progress has been made in developing subtype-selective mAChR modulators (12)(13)(14)(15). Interestingly, these efforts have led to the identification of VU0119498, a positive allosteric modulator (PAM) that facilitates ACh signaling through M3Rs expressed by cultured CHO cells (18,19).…”