Shining Light on an mGlu5 Photoswitchable NAM: A Theoretical Perspective

Stepwise activation of a metabotropic glutamate receptor

Metabotropic glutamate receptors belong to a family of G protein-coupled receptors that are obligate dimers and possess a large extracellular ligand-binding domain that is linked via a cysteine-rich domain to their 7-transmembrane domain1. Upon activation, these receptors undergo a large conformational change to transmit the ligand binding signal from the extracellular ligand-binding domain to the G protein-coupling 7-transmembrane domain2. In this manuscript, we propose a model for a sequential, multistep activation mechanism of metabotropic glutamate receptor subtype 5. We present a series of structures in lipid nanodiscs, from inactive to fully active, including agonist-bound intermediate states. Further, using bulk and single-molecule fluorescence imaging, we reveal distinct receptor conformations upon allosteric modulator and G protein binding.

Control of Theta Oscillatory Activity Underlying Fear Expression by mGlu5 Receptors

Metabotropic glutamate 5 receptors (mGlu₅) are thought to play an important role in mediating emotional information processing. In particular, negative allosteric modulators (NAMs) of mGlu₅ have received a lot of attention as potential novel treatments for several neuropsychiatric diseases, including anxiety-related disorders. The aim of this study was to assess the influence of pre- and post-training mGlu₅ inactivation in cued fear conditioned mice on neuronal oscillatory activity during fear retrieval. For this study we used the recently developed mGlu₅ NAM Alloswicth-1 administered systemically. Injection of Alloswicth-1 before, but not after, fear conditioning resulted in a significant decrease in freezing upon fear retrieval. Mice injected with Alloswicth-1 pre-training were also implanted with recording microelectrodes into both the medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC). The recordings revealed a reduction in theta rhythmic activity (4-12 Hz) in both the mPFC and vHPC during fear retrieval. These results indicate that inhibition of mGlu₅ signaling alters local oscillatory activity in principal components of the fear brain network underlying a reduced response to a predicted threat.

Photoswitchable allosteric modulators for metabotropic glutamate receptors

Metabotropic glutamate receptors (mGlu) are a family of class C G protein-coupled receptors (GPCRs) with important biological functions and widespread expression. The mechanisms of mGlu activation and the development of allosteric modulators for these dimeric proteins have attracted singular attention including the use of light regulated ligands. Photopharmacology involves the integration of a photoactive moiety into the ligand structure that following specific illumination undergoes a structural rearrangement and changes its biological activity. The use of light-regulated allosteric ligands offers the opportunity to manipulate mGlu signalling with spatiotemporal precision, unattainable with classical pharmacological approaches. In this review, we will discuss some of the innovations that have been made in the allosteric photopharmacology of mGlu receptors to date. We discuss the prospects of these molecular tools in the control of mGluRs and the new perspectives in understanding mGlu mechanisms, pharmacology and (patho)physiology that can ultimately result in innovative drug discovery concepts.

Mechanistic Insights into Light-Driven Allosteric Control of GPCR Biological Activity

G protein-coupled receptors (GPCR), including the metabotrobic glutamate 5 receptor (mGlu₅), are important therapeutic targets and the development of allosteric ligands for targeting GPCRs has become a desirable approach toward modulating receptor activity. Traditional pharmacological approaches toward modulating GPCR activity are still limited since precise spatiotemporal control of a ligand is lost as soon as it is administered. Photopharmacology proposes the use of photoswitchable ligands to overcome this limitation, since their activity can be reversibly controlled by light with high precision. As this is still a growing field, our understanding of the molecular mechanisms underlying the light-induced changes of different photoswitchable ligand pharmacology is suboptimal. For this reason, we have studied the mechanisms of action of alloswitch-1 and MCS0331; two freely diffusible, mGlu₅ phenylazopyridine photoswitchable negative allosteric modulators. We combined photochemical, cell-based, and in vivo photopharmacological approaches to investigate the effects of trans-cis azobenzene photoisomerization on the functional activity and binding ability of these ligands to the mGlu₅ allosteric pocket. From these results, we conclude that photoisomerization can take place inside and outside the ligand binding pocket, and this leads to a reversible loss in affinity, in part, due to changes in dissociation rates from the receptor. Ligand activity for both photoswitchable ligands deviates from high-affinity mGlu₅ negative allosteric modulation (in the trans configuration) to reduced affinity for the mGlu₅ in their cis configuration. Importantly, this mechanism translates to dynamic and reversible control over pain following local injection and illumination of negative allosteric modulators into a brain region implicated in pain control.

Exploring the Activation Mechanism of the mGlu5 Transmembrane Domain

As a class C GPCR and regulator of synaptic activity, mGlu5 is an attractive drug target, potentially offering treatment for several neurologic and psychiatric disorders. As little is known about the activation mechanism of mGlu5 at a structural level, potential of mean force calculations linked to molecular dynamics simulations were performed on the mGlu5 transmembrane domain crystal structure to explore various internal mechanisms responsible for its activation. Our results suggest that the hydrophilic interactions between intracellular loop 1 and the intracellular side of TM6 have to be disrupted to reach a theoretically active-like conformation. In addition, interactions between residues that are key for mGlu5 activation (Tyr659^3.44 and Ile751^5.51) and mGlu5 inactivation (Tyr659^3.44 and Ser809^7.39) have been identified. Inasmuch as mGlu5 receptor signaling is poorly understood, potentially showing a complex network of micro-switches and subtle structure-activity relationships, the present study represents a step forward in the understanding of mGlu5 transmembrane domain activation.

GPCR photopharmacology

New technologies for spatial and temporal remote control of G protein-coupled receptors (GPCRs) are necessary to unravel the complexity of GPCR signalling in cells, tissues and living organisms. An effective approach, recently developed, consists on the design of light-operated ligands whereby light-dependent GPCR activity regulation can be achieved. In this context, the use of light provides an advantage as it combines safety, easy delivery, high resolution and it does not interfere with most cellular processes. In this review we summarize the most relevant successful achievements in GPCR photopharmacology. These recent findings constitute a significant advance in research studies on the molecular dynamics of receptor activation and their physiological roles in vivo. Moreover, these molecules hold potential toward clinical uses as light-operated drugs, which can overcome some of the problems of conventional pharmacology.

Synthesis toward Bivalent Ligands for the Dopamine D2 and Metabotropic Glutamate 5 Receptors

In this study, we designed and synthesized heterobivalent ligands targeting heteromers consisting of the metabotropic glutamate 5 receptor (mGluR5) and the dopamine D₂ receptor (D₂R). Bivalent ligand 22a with a linker consisting of 20 atoms showed 4-fold increase in affinity for cells coexpressing D₂R and mGluR5 compared to cells solely expressing D₂R. Likewise, the affinity of 22a for mGluR5 increased 2-fold in the coexpressing cells. Additionally, 22a exhibited a 5-fold higher mGluR5 affinity than its monovalent precursor 21a in cells coexpressing D₂R and mGluR5. These results indicate that 22a is able to bridge binding sites on both receptors constituting the heterodimer. Likewise, cAMP assays revealed that 22a had a 4-fold higher potency in stable D₂R and mGluR5 coexpressing cell lines than 1. Furthermore, molecular modeling reveals that 22a is able to simultaneously bind both receptors by passing between the TM5-TM6 interface and establishing six protein-ligand H-bonds.

Exploring the Binding Mechanism of Metabotropic Glutamate Receptor 5 Negative Allosteric Modulators in Clinical Trials by Molecular Dynamics Simulations

Metabotropic glutamate receptor 5 (mGlu₅) plays a key role in synaptic information storage and memory, which is a well-known target for a variety of psychiatric and neurodegenerative disorders. In recent years, the increasing efforts have been focused on the design of allosteric modulators, and the negative allosteric modulators (NAMs) are the front-runners. Recently, the architecture of the transmembrane (TM) domain of mGlu₅ receptor has been determined by crystallographic experiment. However, it has been not well understood how the pharmacophores of NAMs accommodated into the allosteric binding site. In this study, molecular dynamics (MD) simulations were performed on mGlu₅ receptor bound with NAMs in preclinical or clinical development to shed light on this issue. In order to identify the key residues, the binding free energies as well as per-residue contributions for NAMs binding to mGlu₅ receptor were calculated. Subsequently, the in silico site-directed mutagenesis of the key residues was performed to verify the accuracy of simulation models. As a result, the shared common features of the studied 5 clinically important NAMs (mavoglurant, dipraglurant, basimglurant, STX107, and fenobam) interacting with 11 residues in allosteric site were obtained. This comprehensive study presented a better understanding of mGlu₅ receptor NAMs binding mechanism, which would be further used as a useful framework to assess and discover novel lead scaffolds for NAMs.

Analysis of positive and negative allosteric modulation in metabotropic glutamate receptors 4 and 5 with a dual ligand

As class C GPCRs and regulators of synaptic activity, human metabotropic glutamate receptors (mGluRs) 4 and 5 are prime targets for allosteric modulation, with mGlu5 inhibition or mGlu4 stimulation potentially treating conditions like chronic pain and Parkinson’s disease. As an allosteric modulator that can bind both receptors, 2-Methyl-6-(phenylethynyl)pyridine (MPEP) is able to negatively modulate mGlu5 or positively modulate mGlu4. At a structural level, how it elicits these responses and how mGluRs undergo activation is unclear. Here, we employ homology modelling and 30 µs of atomistic molecular dynamics (MD) simulations to probe allosteric conformational change in mGlu4 and mGlu5, with and without docked MPEP. Our results identify several structural differences between mGlu4 and mGlu5, as well as key differences responsible for MPEP-mediated positive and negative allosteric modulation, respectively. A novel mechanism of mGlu4 activation is revealed, which may apply to all mGluRs in general. This involves conformational changes in TM3, TM4 and TM5, separation of intracellular loop 2 (ICL2) from ICL1/ICL3, and destabilization of the ionic-lock. On the other hand, mGlu5 experiences little disturbance when MPEP binds, maintaining its inactive state with reduced conformational fluctuation. In addition, when MPEP is absent, a lipid molecule can enter the mGlu5 allosteric pocket.

Synapses in the spotlight with synthetic optogenetics

Membrane receptors and ion channels respond to various stimuli and relay that information across the plasma membrane by triggering specific and timed processes. These include activation of second messengers, allowing ion permeation, and changing cellular excitability, to name a few. Gaining control over equivalent processes is essential to understand neuronal physiology and pathophysiology. Recently, new optical techniques have emerged proffering new remote means to control various functions of defined neuronal populations by light, dubbed optogenetics. Still, optogenetic tools do not typically address the activity of receptors and channels native to neurons (or of neuronal origin), nor gain access to their signaling mechanisms. A related method-synthetic optogenetics-bridges this gap by endowing light sensitivity to endogenous neuronal receptors and channels by the appending of synthetic, light-receptive molecules, or photoswitches. This provides the means to photoregulate neuronal receptors and channels and tap into their native signaling mechanisms in select regions of the neurons, such as the synapse. This review discusses the development of synthetic optogenetics as a means to study neuronal receptors and channels remotely, in their natural environment, with unprecedented spatial and temporal precision, and provides an overview of tool design, mode of action, potential clinical applications and insights and achievements gained.

Illuminating Phenylazopyridines To Photoswitch Metabotropic Glutamate Receptors: From the Flask to the Animals

Phenylazopyridines are photoisomerizable compounds with high potential to control biological functions with light. We have obtained a series of phenylazopyridines with light dependent activity as negative allosteric modulators (NAM) of metabotropic glutamate receptor subtype 5 (mGlu₅). Here we describe the factors needed to achieve an operational molecular photoisomerization and its effective translation into in vitro and in vivo receptor photoswitching, which includes zebrafish larva motility and the regulation of the antinociceptive effects in mice. The combination of light and some specific phenylazopyridine ligands displays atypical pharmacological profiles, including light-dependent receptor overactivation, which can be observed both in vitro and in vivo. Remarkably, the localized administration of light and a photoswitchable compound in the peripheral tissues of rodents or in the brain amygdalae results in an illumination-dependent analgesic effect. The results reveal a robust translation of the phenylazopyridine photoisomerization to a precise photoregulation of biological activity.