Vibrational spectroscopy analysis of ligand efficacy in human M2 muscarinic acetylcholine receptor (M2R)

Discovering Key Activation Hotspots in the M2 Muscarinic Receptor

The M2 muscarinic receptor (M2R) is a prototypical G protein-coupled receptor (GPCR) that serves as a model system for understanding ligand recognition and GPCR activation. Here, using vibrational spectroscopy, we identify the mechanisms governing M2R activation by its native agonist, acetylcholine. Combined with mutagenesis, computational chemistry, and organic synthetic chemistry, our analyses found that the precise distance between acetylcholine and Asn404, one of the amino acids constituting the ligand-binding site, is important for M2R activation and that the N404Q mutant undergoes partial active state-like conformational changes. We discovered that a water molecule bridging acetylcholine and Asn404 forms a precise and flexible hydrogen bond network, triggering the outward movement of transmembrane helix 6 in M2R. Consistent with this observation, disruptions in this hydrogen bond network via chemical modification at the α- or β-position of acetylcholine failed to activate M2R. Collectively, our findings pinpoint Asn404 as a critical residue that both senses acetylcholine binding and induces M2R activation.

Ligand bias at the muscarinic acetylcholine receptor family: Opportunities and challenges

Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors (GPCRs) that are activated by the endogenous neurotransmitter, acetylcholine (ACh). Disruption of mAChR signalling has been associated with a variety of neurological disorders and non-neurological diseases. Consequently, the development of agonists and antagonists of the mAChRs has been a major avenue in drug discovery. Unfortunately, mAChR ligands are often associated with on-target side effects for two reasons. The first reason is due to the high sequence conservation at the orthosteric ACh binding site among all five receptor subtypes (M1-M5), making on-target subtype selectivity a major challenge. The second reason is due to on-target side effects of mAChR drugs that are associated with the pleiotropic nature of mAChR signalling at the level of a single mAChR subtype. Indeed, there is growing evidence that within the myriad of signalling events produced by mAChR ligands, some will have therapeutic benefits, whilst others may promote cholinergic side effects. This paradigm of drug action, known as ligand bias or biased agonism, is an attractive feature for next-generation mAChR drugs, as it holds the promise of developing drugs devoid of on-target adverse effects. Although relatively simple to detect and even quantify in vitro, ligand bias, as observed in recombinant systems, does not always translate to in vivo systems, which remains a major hurdle in GPCR drug discovery, including the mAChR family. Here we report recent studies that have attempted to detect and quantify ligand bias at the mAChR family, and briefly discuss the challenges associated with biased agonist drug development. This article is part of the Special Issue on "Ligand Bias".

Extracellular polymeric substances sustain photoreduction of Cr(VI) by Shewanella oneidensis-CdS biohybrid system

Photosensitized biohybrid system (PBS) enables bacteria to exploit light energy harvested by semiconductors for rapid pollutants transformation, possessing a promising future for water reclamation. Maintaining a biocompatible environment under photocatalytic conditions is the key to developing PBS-based treatment technologies. Natural microbial cells are surrounded by extracellular polymeric substances (EPS) that either be tightly bound to the cell wall (i.e., tightly bound EPS, tbEPS) or loosely associated with cell surface (i.e., loosely bound EPS, lbEPS), which provide protection from unfavorable environment. We hypothesized that providing EPS fractions can enhance bacterial viability under adverse environment created by photocatalytic reactions. We constructed a model PBS consisting of Shewanella oneidensis and CdS using Cr(VI) as the target pollutant. Results showed complete removal of 25 mg/L Cr(VI) within 90 min without an electron donor, which may mainly rely on the synergistic effect of CdS and bacteria on photoelectron transfer. Long-term cycling experiment of pristine PBS and PBS with extra EPS fractions (including lbEPS and tbEPS) for Cr(VI) treatment showed that PBS with extra lbEPS achieved efficient Cr(VI) removal within five consecutive batch treatment cycles, compared to the three cycles both in pristine PBS and PBS with tbEPS. After addition of lbEPS, the accumulation of reactive oxygen species (ROS) was greatly reduced via the EPS-capping effect and quenching effect, and the toxic metal internalization potential was lowered by complexation with Cd and Cr, resulting in enhanced bacterial viability during photocatalysis. This facile and efficient cytoprotective method helps the rational design of PBS for environmental remediation.

Pharmacological hallmarks of allostery at the M4 muscarinic receptor elucidated through structure and dynamics

Allosteric modulation of G protein-coupled receptors (GPCRs) is a major paradigm in drug discovery. Despite decades of research, a molecular-level understanding of the general principles that govern the myriad pharmacological effects exerted by GPCR allosteric modulators remains limited. The M₄ muscarinic acetylcholine receptor (M₄ mAChR) is a validated and clinically relevant allosteric drug target for several major psychiatric and cognitive disorders. In this study, we rigorously quantified the affinity, efficacy, and magnitude of modulation of two different positive allosteric modulators, LY2033298 (LY298) and VU0467154 (VU154), combined with the endogenous agonist acetylcholine (ACh) or the high-affinity agonist iperoxo (Ipx), at the human M₄ mAChR. By determining the cryo-electron microscopy structures of the M₄ mAChR, bound to a cognate G_i1 protein and in complex with ACh, Ipx, LY298-Ipx, and VU154-Ipx, and applying molecular dynamics simulations, we determine key molecular mechanisms underlying allosteric pharmacology. In addition to delineating the contribution of spatially distinct binding sites on observed pharmacology, our findings also revealed a vital role for orthosteric and allosteric ligand-receptor-transducer complex stability, mediated by conformational dynamics between these sites, in the ultimate determination of affinity, efficacy, cooperativity, probe dependence, and species variability. There results provide a holistic framework for further GPCR mechanistic studies and can aid in the discovery and design of future allosteric drugs.

Specific zinc binding to heliorhodopsin

Heliorhodopsins (HeRs), a recently discovered family of rhodopsins, have an inverted membrane topology compared to animal and microbial rhodopsins. The slow photocycle of HeRs suggests a light-sensor function, although the actual function remains unknown. Although HeRs exhibit no specific binding of monovalent cations or anions, recent ATR-FTIR spectroscopy studies have demonstrated the binding of Zn²⁺ to HeR from Thermoplasmatales archaeon (TaHeR) and 48C12. Even though ion-specific FTIR spectra were observed for many divalent cations, only helical structural perturbations were observed for Zn²⁺-binding, suggesting a possible modification of the HeR function by Zn²⁺. The present study shows that Zn²⁺-binding lowers the thermal stability of TaHeR, and slows back proton transfer to the retinal Schiff base (M decay) during its photocycle. Zn²⁺-binding was similarly observed for a TaHeR opsin that lacks the retinal chromophore. We then studied the Zn²⁺-binding site by means of the ATR-FTIR spectroscopy of site-directed mutants. Among five and four mutants of His and Asp/Glu, respectively, only E150Q exhibited a completely different spectral feature of the α-helix (amide-I) in ATR-FTIR spectroscopy, suggesting that E150 is responsible for Zn²⁺-binding. Molecular dynamics (MD) simulations built a coordination structure of Zn²⁺-bound TaHeR, where E150 and protein bound water molecules participate in direct coordination. It was concluded that the specific binding site of Zn²⁺ is located at the cytoplasmic side of TaHeR, and that Zn²⁺-binding affects the structure and structural dynamics, possibly modifying the unknown function of TaHeR.

Vibrational spectroscopy analysis of ligand efficacy in human M2 muscarinic acetylcholine receptor (M2R)

The intrinsic efficacy of ligand binding to G protein-coupled receptors (GPCRs) reflects the ability of the ligand to differentially activate its receptor to cause a physiological effect. Here we use attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy to examine the ligand-dependent conformational changes in the human M2 muscarinic acetylcholine receptor (M2R). We show that different ligands affect conformational alteration appearing at the C=O stretch of amide-I band in M2R. Notably, ATR-FTIR signals strongly correlated with G-protein activation levels in cells. Together, we propose that amide-I band serves as an infrared probe to distinguish the ligand efficacy in M2R and paves the path to rationally design ligands with varied efficacy towards the target GPCR.