International Union of Basic and Clinical Pharmacology. CXI. Pharmacology, Signaling, and Physiology of Metabotropic Glutamate Receptors

SCA44- and SCAR13-associated GRM1 mutations affect metabotropic glutamate receptor 1 function through distinct mechanisms

BACKGROUND AND PURPOSE

Metabotropic glutamate receptor 1 (mGlu1) is a promising therapeutic target for neurodegenerative CNS disorders including spinocerebellar ataxias (SCAs). Clinical reports have identified naturally-occurring mGlu1 mutations in rare SCA subtypes and linked symptoms to mGlu1 mutations. However, how mutations alter mGlu1 function remains unknown, as does amenability of receptor function to pharmacological rescue. Here, we explored SCA-associated mutation effects on mGlu1 cell surface expression, canonical signal transduction and allosteric ligand pharmacology.

EXPERIMENTAL APPROACH

Orthosteric agonists, positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) were assessed at two functional endpoints (iCa2+ mobilisation and inositol 1-phosphate [IP1] accumulation) in FlpIn Trex HEK293A cell lines expressing five mutant mGlu1 subtypes. Key pharmacological parameters including ligand potency, affinity and cooperativity were derived using operational models of agonism and allostery.

KEY RESULTS

mGlu1 mutants exhibited differential impacts on mGlu1 expression, with a C-terminus truncation significantly reducing surface expression. Mutations differentially influenced orthosteric ligand affinity, efficacy and functional cooperativity between allosteric and orthosteric ligands. Loss-of-function mutations L454F and N885del reduced orthosteric affinity and efficacy, respectively. A gain-of-function Y792C mutant mGlu1 displayed enhanced constitutive activity in IP1 assays, which manifested as reduced orthosteric agonist activity. The mGlu1 PAMs restored glutamate potency in iCa2+ mobilisation for loss-of-function mutations and mGlu1 NAMs displayed enhanced inverse agonist activity at Y792C relative to wild-type mGlu1.

CONCLUSION AND IMPLICATIONS

Collectively, these data highlight distinct mechanisms by which mGlu1 mutations affect receptor function and show allosteric modulators may present a therapeutic strategy to restore aberrant mGlu1 function in rare SCA subtypes.

mGlu4R, mGlu7R, and mGlu8R allosteric modulation for treating acute and chronic neurodegenerative disorders

Neuroprotection, defined as safeguarding neurons from damage and death by inhibiting diverse pathological mechanisms, continues to be a promising approach for managing a range of central nervous system (CNS) disorders, including acute conditions such as ischemic stroke and traumatic brain injury (TBI) and chronic neurodegenerative diseases like Parkinson's disease (PD), Alzheimer's disease (AD), and multiple sclerosis (MS). These pathophysiological conditions involve excessive glutamatergic (Glu) transmission activity, which can lead to excitotoxicity. Inhibiting this excessive Glu transmission has been proposed as a potential therapeutic strategy for treating the CNS disorders mentioned. In particular, ligands of G protein-coupled receptors (GPCRs), including metabotropic glutamatergic receptors (mGluRs), have been recognized as promising options for inhibiting excessive Glu transmission. This review discusses the complex interactions of mGlu receptors with their subtypes, including the formation of homo- and heterodimers, which may vary in function and pharmacology depending on their protomer composition. Understanding these intricate details of mGlu receptor structure and function enhances researchers' ability to develop targeted pharmacological interventions, potentially offering new therapeutic avenues for neurological and psychiatric disorders. This review also summarizes the current knowledge of the neuroprotective potential of ligands targeting group III mGluRs in preclinical cellular (in vitro) and animal (in vivo) models of ischemic stroke, TBI, PD, AD, and MS. In recent years, experiments have shown that compounds, especially those activating mGlu4 or mGlu7 receptors, exhibit protective effects in experimental ischemia models. The discovery of allosteric ligands for specific mGluR subtypes has led to reports suggesting that group III mGluRs may be promising targets for neuroprotective therapy in PD (mGlu4R), TBI (mGlu7R), and MS (mGlu8R).

Delineating the stepwise millisecond allosteric activation mechanism of the class C GPCR dimer mGlu5

Two-thirds of signaling hormones and one-third of approved drugs exert their effects by binding and modulating the G protein-coupled receptors (GPCRs) activation. While the activation mechanism for monomeric GPCRs has been well-established, little is known about GPCRs in dimeric form. Here, by combining transition pathway generation, extensive atomistic simulation-based Markov state models, and experimental signaling assays, we reveal an asymmetric, stepwise millisecond allosteric activation mechanism for the metabotropic glutamate receptor subtype 5 receptor (mGlu5), an obligate dimeric class C GPCR. The dynamic picture is presented that agonist binding induces dimeric ectodomains compaction, amplified by the precise association of the cysteine-rich domains, ultimately loosely bringing the intracellular 7-transmembrane (7TM) domains into proximity and establishing an asymmetric TM6-TM6 interface. The active inter-domain interface enhances their intra-domain flexibility, triggering the activation of micro-switches crucial for downstream signal transduction. Furthermore, we show that the positive allosteric modulator stabilizes both the active inter-domain 7TM interface and an open, extended intra-domain ICL2 conformation. This stabilization leads to the formation of a pseudo-cavity composed of the ICL2, ICL3, TM3, and C-terminus, which facilitates G protein coordination. Our strategy may be generalizable for characterizing millisecond events in other allosteric systems.

Evolutionary and functional analysis of metabotropic glutamate receptors in lampreys

The metabotropic glutamate receptor (mGluR, GRM) family is involved in multiple signaling pathways and regulates neurotransmitter release. However, the evolutionary history, distribution, and function of the mGluRs family in lampreys have not been determined. Therefore, we identified the mGluRs gene family in the genome of Lethenteron reissneri, which has been conserved throughout vertebrate evolution. We confirmed that Lr-GRM3, Lr-GRM5, and Lr-GRM7 encode three types of mGluRs in lamprey. Additionally, we investigated the distribution of Lr-GRM3 within this species by qPCR and Western blotting. Furthermore, we conducted RNA sequencing to investigate the molecular function of Lr-GRM3 in lamprey. Our gene expression profile revealed that, similar to that in jawed vertebrates, Lr-GRM3 participates in multiple signal transduction pathways and influences synaptic excitability in lampreys. Moreover, it also affects intestinal motility and the inflammatory response in lampreys. This study not only enhances the understanding of mGluRs' gene evolution but also highlights the conservation of GRM3's role in signal transduction while expanding our knowledge of its functions specifically within lampreys. In summary, our experimental findings provide valuable insights for studying both the evolution and functionality of the mGluRs family.

Structural basis of positive allosteric modulation of metabotropic glutamate receptor activation and internalization

The metabotropic glutamate receptors (mGluRs) are neuromodulatory family C G protein coupled receptors which assemble as dimers and allosterically couple extracellular ligand binding domains (LBDs) to transmembrane domains (TMDs) to drive intracellular signaling. Pharmacologically, mGluRs can be targeted at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric compounds as therapeutics, an understanding of the functional and structural basis of their effects is limited. Here we use multiple approaches to dissect the functional and structural effects of orthosteric versus allosteric ligands. We find, using electrophysiological and live cell imaging assays, that both agonists and positive allosteric modulators (PAMs) can drive activation and internalization of group II and III mGluRs. The effects of PAMs are pleiotropic, boosting the maximal response to orthosteric agonists and serving independently as internalization-biased agonists across mGluR subtypes. Motivated by this and intersubunit FRET analyses, we determine cryo-electron microscopy structures of mGluR3 in the presence of either an agonist or antagonist alone or in combination with a PAM. These structures reveal PAM-driven re-shaping of intra- and inter-subunit conformations and provide evidence for a rolling TMD dimer interface activation pathway that controls G protein and beta-arrestin coupling.

Photoswitchable positive allosteric modulators of metabotropic glutamate receptor 4 to improve selectivity

Metabotropic glutamate receptors (mGlu) regulate multiple functions in the nervous systems and are involved in several neurological disorders. However, selectively targeting individual mGlu subtypes with spatiotemporal precision is still an unmet need. Photopharmacology can address this concern through the utilization of photoswitchable compounds such as optogluram, which is a positive allosteric modulator (PAM) of mGlu4 that enables the precise control of physiological responses using light but does not have an optimal selectivity profile. Optogluram analogs were developed to obtain photoswitchable PAMs of mGlu4 receptor with an improved selectivity. Among them, optogluram-2 emerged as a photoswitchable ligand for mGlu4 receptor with activity as both PAM and allosteric agonists. It presents a higher selectivity and offers improved photoswitching of mGlu4 activity. These improved properties make optogluram-2 an excellent candidate to study the role of mGlu4 with a high spatiotemporal precision in systems where mGlu4 can be co-expressed with other mGlu receptors.

The influence of sleep disorders on perioperative neurocognitive disorders among the elderly: A narrative review

This review comprehensively assesses the epidemiology, interaction, and impact on patient outcomes of perioperative sleep disorders (SD) and perioperative neurocognitive disorders (PND) in the elderly. The incidence of SD and PND during the perioperative period in older adults is alarmingly high, with SD significantly contributing to the occurrence of postoperative delirium. However, the clinical evidence linking SD to PND remains insufficient, despite substantial preclinical data. Therefore, this study focuses on the underlying mechanisms between SD and PND, underscoring that potential mechanisms driving SD-induced PND include uncontrolled central nervous inflammation, blood-brain barrier disruption, circadian rhythm disturbances, glial cell dysfunction, neuronal and synaptic abnormalities, impaired central metabolic waste clearance, gut microbiome dysbiosis, hippocampal oxidative stress, and altered brain network connectivity. Additionally, the review also evaluates the effectiveness of various sleep interventions, both pharmacological and nonpharmacological, in mitigating PND. Strategies such as earplugs, eye masks, restoring circadian rhythms, physical exercise, noninvasive brain stimulation, dexmedetomidine, and melatonin receptor agonists have shown efficacy in reducing PND incidence. The impact of other sleep-improvement drugs (e.g., orexin receptor antagonists) and methods (e.g., cognitive-behavioral therapy for insomnia) on PND is still unclear. However, certain drugs used for treating SD (e.g., antidepressants and first-generation antihistamines) may potentially aggravate PND. By providing valuable insights and references, this review aimed to enhance the understanding and management of PND in older adults based on SD.

Allosteric modulation and G-protein selectivity of the Ca2+-sensing receptor

The calcium-sensing receptor (CaSR) is a family C G-protein-coupled receptor1 (GPCR) that has a central role in regulating systemic calcium homeostasis2,3. Here we use cryo-electron microscopy and functional assays to investigate the activation of human CaSR embedded in lipid nanodiscs and its coupling to functional Gi versus Gq proteins in the presence and absence of the calcimimetic drug cinacalcet. High-resolution structures show that both Gi and Gq drive additional conformational changes in the activated CaSR dimer to stabilize a more extensive asymmetric interface of the seven-transmembrane domain (7TM) that involves key protein-lipid interactions. Selective Gi and Gq coupling by the receptor is achieved through substantial rearrangements of intracellular loop 2 and the C terminus, which contribute differentially towards the binding of the two G-protein subtypes, resulting in distinct CaSR-G-protein interfaces. The structures also reveal that natural polyamines target multiple sites on CaSR to enhance receptor activation by zipping negatively charged regions between two protomers. Furthermore, we find that the amino acid L-tryptophan, a well-known ligand of CaSR extracellular domains, occupies the 7TM bundle of the G-protein-coupled protomer at the same location as cinacalcet and other allosteric modulators. Together, these results provide a framework for G-protein activation and selectivity by CaSR, as well as its allosteric modulation by endogenous and exogenous ligands.

Metabotropic glutamate receptors (mGluRs) in epileptogenesis: an update on abnormal mGluRs signaling and its therapeutic implications

Epilepsy is a neurological disorder characterized by high morbidity, high recurrence, and drug resistance. Enhanced signaling through the excitatory neurotransmitter glutamate is intricately associated with epilepsy. Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors activated by glutamate and are key regulators of neuronal and synaptic plasticity. Dysregulated mGluR signaling has been associated with various neurological disorders, and numerous studies have shown a close relationship between mGluRs expression/activity and the development of epilepsy. In this review, we first introduce the three groups of mGluRs and their associated signaling pathways. Then, we detail how these receptors influence epilepsy by describing the signaling cascades triggered by their activation and their neuroprotective or detrimental roles in epileptogenesis. In addition, strategies for pharmacological manipulation of these receptors during the treatment of epilepsy in experimental studies is also summarized. We hope that this review will provide a foundation for future studies on the development of mGluR-targeted antiepileptic drugs.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

Allosteric crosstalk in modular proteins: Function fine-tuning and drug design

Modular proteins are regulatory proteins that carry out more than one function. These proteins upregulate or downregulate a biochemical cascade to establish homeostasis in cells. To switch the function or alter the efficiency (based on cellular needs), these proteins require different facilitators that bind to a site different from the catalytic (active/orthosteric) site, aka 'allosteric site', and fine-tune their function. These facilitators (or effectors) are allosteric modulators. In this Review, we have discussed the allostery, characterized them based on their mechanisms, and discussed how allostery plays an important role in the activity modulation and function fine-tuning of proteins. Recently there is an emergence in the discovery of allosteric drugs. We have also emphasized the role, significance, and future of allostery in therapeutic applications.

Neonatal Maternal Separation Induces Sexual Dimorphism in Brain Development: The Influence on Amino Acid Levels and Cognitive Disorders

Repeated maternal separation (MS) is a useful experimental model in rodents for studying the long-term influence of early-life stress on brain neurophysiology. In our work, we assessed the effect of repeated MS (postnatal day (PND)1-21, 180 min/day) on the postnatal development of rat brain regions involved in memory using proton magnetic resonance spectroscopy (1HMRS) for tissue volume and the level of amino acids such as glutamate, aspartate, glutamine, glycine and gamma-aminobutyric acid (GABA) in the hippocampus. We assessed whether these effects are sex dependent. We also use novel object recognition (NOR) task to examine the effect of MS on memory and the effect of ethanol on it. Finally, we attempted to ameliorate postnatal stress-induced memory deficits by using VU-29, a positive allosteric modulator (PAM) of the metabotropic glutamate type 5 (mGlu5) receptor. In males, we noted deficits in the levels of glutamate, glycine and glutamine and increases in GABA in the hippocampus. In addition, the values of perirhinal cortex, prefrontal cortex and insular cortex and CA3 were decreased in these animals. MS females, in contrast, demonstrated significant increase in glutamate levels and decrease in GABA levels in the hippocampus. Here, the CA1 values alone were increased. VU-29 administration ameliorated these cognitive deficits. Thus, MS stress disturbs amino acids levels mainly in the hippocampus of adult male rats, and enhancement of glutamate neurotransmission reversed recognition memory deficits in these animals.

β-Arrestins: Structure, Function, Physiology, and Pharmacological Perspectives

The two β-arrestins, β-arrestin-1 and -2 (systematic names: arrestin-2 and -3, respectively), are multifunctional intracellular proteins that regulate the activity of a very large number of cellular signaling pathways and physiologic functions. The two proteins were discovered for their ability to disrupt signaling via G protein-coupled receptors (GPCRs) via binding to the activated receptors. However, it is now well recognized that both β-arrestins can also act as direct modulators of numerous cellular processes via either GPCR-dependent or -independent mechanisms. Recent structural, biophysical, and biochemical studies have provided novel insights into how β-arrestins bind to activated GPCRs and downstream effector proteins. Studies with β-arrestin mutant mice have identified numerous physiologic and pathophysiological processes regulated by β-arrestin-1 and/or -2. Following a short summary of recent structural studies, this review primarily focuses on β-arrestin-regulated physiologic functions, with particular focus on the central nervous system and the roles of β-arrestins in carcinogenesis and key metabolic processes including the maintenance of glucose and energy homeostasis. This review also highlights potential therapeutic implications of these studies and discusses strategies that could prove useful for targeting specific β-arrestin-regulated signaling pathways for therapeutic purposes. SIGNIFICANCE STATEMENT: The two β-arrestins, structurally closely related intracellular proteins that are evolutionarily highly conserved, have emerged as multifunctional proteins able to regulate a vast array of cellular and physiological functions. The outcome of studies with β-arrestin mutant mice and cultured cells, complemented by novel insights into β-arrestin structure and function, should pave the way for the development of novel classes of therapeutically useful drugs capable of regulating specific β-arrestin functions.

Fundamental neurochemistry review: Glutamatergic dysfunction as a central mechanism underlying flavivirus-induced neurological damage

The Flaviviridae family comprises positive-sense single-strand RNA viruses mainly transmitted by arthropods. Many of these pathogens are especially deleterious to the nervous system, and a myriad of neurological symptoms have been associated with infections by Zika virus (ZIKV), West Nile virus (WNV), and Japanese encephalitis virus (JEV) in humans. Studies suggest that viral replication in neural cells and the massive release of pro-inflammatory mediators lead to morphological alterations of synaptic spine structure and changes in the balance of excitatory/inhibitory neurotransmitters and receptors. Glutamate is the predominant excitatory neurotransmitter in the brain, and studies propose that either enhanced release or impaired uptake of this amino acid contributes to brain damage in several conditions. Here, we review existing evidence suggesting that glutamatergic dysfunction-induced by flaviviruses is a central mechanism for neurological damage and clinical outcomes of infection. We also discuss current data suggesting that pharmacological approaches that counteract glutamatergic dysfunction show benefits in animal models of such viral diseases.

Distribution of [11C]-JNJ-42491293 in the marmoset brain: a positron emission tomography study

JNJ-42491293 is a metabotropic glutamate 2 (mGlu2) positive allosteric modulator (PAM) that was radiolabelled with [11C]- to serve as a positron emission tomography (PET) ligand. Indeed, in vitro, the molecule displays high selectivity at mGlu2 receptors. However, PET experiments performed in rats, macaques and humans, have suggested that [11C]-JNJ-42491293 could interact with an unidentified, non-mGlu2 receptor binding site. The brain distribution of [11C]-JNJ-42491293 has not been determined in the brain of the common marmoset, a small non-human primate increasingly used in neuroscience research. Here, we investigated the distribution of [11C]-JNJ-42491293 in the marmoset brain. Three marmosets underwent brain magnetic resonance imaging (MRI) and 90-min dynamic PET scans with [11C]-JNJ-42491293 in combination with vehicle or the mGlu2 PAM AZD8529 (0.1, 1 and 10 mg/kg). In the scans in which [11C]-JNJ-42491293 was co-administered with vehicle, the brain areas with the highest standardised uptake values (SUVs) were the midbrain, cerebellum and thalamus, while the lowest SUVs were found in the pons. The addition of AZD8529 (0.1, 1 and 10 mg/kg) to [11C]-JNJ-42491293 did not modify the SUVs obtained with [11C]-JNJ-42491293 alone, and ex vivo blocking autoradiography with PAM AZD8529 (10, 100, 300 µM) on marmoset brain sections showed increased signals in the blocking conditions compared to vehicle, suggesting that no competition occurred between the 2 ligands. The results we obtained here do not suggest that [11C]-JNJ-42491293 interacts selectively, or even at all, with mGlu2 receptors in the marmoset, in agreement with findings previously reported in macaque and human.

Heterodimerization of Chemoreceptors TAS1R3 and mGlu2 in Human Blood Leukocytes

The expression of canonical chemosensory receptors of the tongue, such as the heteromeric sweet taste (TAS1R2/TAS1R3) and umami taste (TAS1R1/TAS1R3) receptors, has been demonstrated in many extra-oral cells and tissues. Gene expression studies have revealed transcripts for all TAS1 and metabotropic glutamate (mGlu) receptors in different types of immune cells, where they are involved, for example, in the chemotaxis of human neutrophils and the protection of T cells from activation-induced cell death. Like other class-C G protein-coupling receptors (GPCRs), TAS1Rs and mGlu receptors form heteromers within their families. Since mGlu receptors and TAS1R1/TAS1R3 share the same ligand, monosodium glutamate (MSG), we hypothesized their hitherto unknown heteromerization across receptor families in leukocytes. Here we show, by means of immunocytochemistry and co-IP/Western analysis, that across class-C GPCR families, mGlu2 and TAS1R3 co-localize and heterodimerize in blood leukocytes. Expressing the recombinant receptors in HEK-293 cells, we validated their heterodimerization by bioluminescence resonance energy transfer. We demonstrate MSG-induced, mGlu2/TAS1R3 heteromer-dependent gain-of-function and pertussis toxin-sensitive signaling in luminescence assays. Notably, we show that mGlu2/TAS1R3 is necessary and sufficient for MSG-induced facilitation of N-formyl-methionyl-leucyl-phenylalanine-stimulated IL-8 secretion in neutrophils, using receptor-specific antagonists. In summary, our results demonstrate mGlu2/TAS1R3 heterodimerization in leukocytes, suggesting cellular function-tailored chemoreceptor combinations to modulate cellular immune responses.

Structural basis of allosteric modulation of metabotropic glutamate receptor activation and desensitization

The metabotropic glutamate receptors (mGluRs) are neuromodulatory family C G protein coupled receptors which assemble as dimers and allosterically couple extracellular ligand binding domains (LBDs) to transmembrane domains (TMDs) to drive intracellular signaling. Pharmacologically, mGluRs can be targeted either at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric TMD-targeting compounds as therapeutics, an understanding of the functional and structural basis of their effects on mGluRs is limited. Here we use a battery of approaches to dissect the distinct functional and structural effects of orthosteric versus allosteric ligands. We find using electrophysiological and live cell imaging assays that both agonists and positive allosteric modulators (PAMs) can drive activation and desensitization of mGluRs. The effects of PAMs are pleiotropic, including both the ability to boost the maximal response to orthosteric agonists and to serve independently as desensitization-biased agonists across mGluR subtypes. Conformational sensors reveal PAM-driven inter-subunit re-arrangements at both the LBD and TMD. Motivated by this, we determine cryo-electron microscopy structures of mGluR3 in the presence of either an agonist or antagonist alone or in combination with a PAM. These structures reveal PAM-driven re-shaping of intra- and inter-subunit conformations and provide evidence for a rolling TMD dimer interface activation pathway that controls G protein and beta-arrestin coupling.

Highlights

-Agonists and PAMs drive mGluR activation, desensitization, and endocytosis-PAMs are desensitization-biased and synergistic with agonists-Four combinatorial ligand conditions reveal an ensemble of full-length mGluR structures with novel interfaces-Activation and desensitization involve rolling TMD interfaces which are re-shaped by PAM.

Asymmetric activation of dimeric GABAB and metabotropic glutamate receptors

G protein-coupled receptors (GPCRs) represent the largest family of membrane proteins and are important drug targets. GPCRs are allosteric machines that transduce an extracellular signal to the cell by activating heterotrimeric G proteins. Herein, we summarize the recent advancements in the molecular activation mechanism of the γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors, the most important class C GPCRs that modulate synaptic transmission in the brain. Both are mandatory dimers, this quaternary structure being needed for their function The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of mGlu heterodimers, where the eight mGlu subunits can form specific and functional heterodimers. Finally, the development of allosteric modulators has revealed new possibilities for regulating the function of these receptors by targeting the transmembrane dimer interface. This family of receptors never ceases to astonish and serve as models to better understand the diversity and asymmetric functioning of GPCRs.NEW & NOTEWORTHY γ-aminobutyric acid type B (GABAB) and metabotropic glutamate (mGlu) receptors form constitutive dimers, which are required for their function. They serve as models to better understand the diversity and activation of G protein-coupled receptors (GPCRs). The structures of these receptors in different conformations and in complexes with G proteins have revealed their asymmetric activation. This asymmetry is further highlighted by the recent discovery of specific and functional mGlu heterodimers. Allosteric modulators can be developed to target the transmembrane interface and modulate the asymmetry.

Photopharmacological manipulation of amygdala metabotropic glutamate receptor mGlu4 alleviates neuropathic pain

Neuropathic pain is a common health problem resulting in exacerbated response to noxious and non noxious stimuli, as well as impaired emotional and cognitive responses. Unfortunately, neuropathic pain is also one of the most difficult pain syndromes to manage, highlighting the importance of better understanding the brain regions and neuromodulatory mechanisms involved in its regulation. Among the many interconnected brain areas which process pain, the amygdala is known to play an important role in the integration of sensory and emotional pain signals. Here we questioned the ability of a recently identified neuromodulatory mechanism associated with the metabotropic glutamate receptors mGlu₄ in the amygdala to modulate neuropathic pain. In a murine model of peripheral mononeuropathy, we demonstrate that pharmacological activation of amygdala mGlu₄ efficiently alleviates sensory and depressive-like symptoms in both male and female mice. Moreover, we reveal a differential modulation of these symptoms. Activating mGlu₄ in the contralateral amygdala relative to the side of the mononeuropathy, is necessary and sufficient to relieve both sensory and depressive-like symptoms, while ipsilateral activation solely reduces depressive-like symptoms. Furthermore, using photopharmacology, a recent strategy allowing precise photocontrol of endogenous proteins, we further demonstrate the dynamic alleviation of neuropathic pain through light-dependent facilitation of mGlu₄ by a photoswitchable positive allosteric modulator. Finally, coupling photopharmacology and analgesic conditioned place preference, we show a significant pain-reducing effect of mGlu4 activation. Taken together, these data highlight the analgesic potential of enhancing amygdala mGlu₄ activity to counteract neuropathy reinforcing its therapeutic interest for the treatment of pathological pain.

Structure-Based Discovery of Negative Allosteric Modulators of the Metabotropic Glutamate Receptor 5

Recently determined structures of class C G protein-coupled receptors (GPCRs) revealed the location of allosteric binding sites and opened new opportunities for the discovery of novel modulators. In this work, molecular docking screens for allosteric modulators targeting the metabotropic glutamate receptor 5 (mGlu₅) were performed. The mGlu₅ receptor is activated by the main excitatory neurotransmitter of the nervous central system, L-glutamate, and mGlu₅ receptor activity can be allosterically modulated by negative or positive allosteric modulators. The mGlu₅ receptor is a promising target for the treatment of psychiatric and neurodegenerative diseases, and several allosteric modulators of this GPCR have been evaluated in clinical trials. Chemical libraries containing fragment- (1.6 million molecules) and lead-like (4.6 million molecules) compounds were docked to an allosteric binding site of mGlu₅ identified in X-ray crystal structures. Among the top-ranked compounds, 59 fragments and 59 lead-like compounds were selected for experimental evaluation. Of these, four fragment- and seven lead-like compounds were confirmed to bind to the allosteric site with affinities ranging from 0.43 to 8.6 μM, corresponding to a hit rate of 9%. The four compounds with the highest affinities were demonstrated to be negative allosteric modulators of mGlu₅ signaling in functional assays. The results demonstrate that virtual screens of fragment- and lead-like chemical libraries have complementary advantages and illustrate how access to high-resolution structures of GPCRs in complex with allosteric modulators can accelerate lead discovery.

Global trends in research of glutamate in epilepsy during past two decades: A bibliometric analysis

Epilepsy affects more than 70 million people in the world. It is characterized by recurrent spontaneous seizures, and it is related to many neurological, cognitive, and psychosocial consequences. Glutamate neurotransmitter dysfunction has essential functions in the pathophysiology of epilepsy. In this work, bibliometric analysis was conducted to explore the trends, frontiers, and hotspots of the global scientific output of glutamate in epilepsy research in the past 20 years. The Science Citation Index Expanded of the Web of Science Core Collection (WoSCC) was searched to obtain information on publications and records published between 2002 and 2021. VOSviewer and CiteSpace were used to conduct bibliometric and visual analyses on the overall distribution of annual output, major countries, active institutions, journals, authors, commonly cited literature, and keywords. The impact and quality of the papers were assessed using the global citation score (GCS). Four thousand eight hundred ninety-one publications were retrieved in total. During the past two decades, the number of publications (Np) associated with glutamate in epilepsy has risen yearly. The United States has published the most papers; its H-index and number of citations are also the highest. The League of European Research Universities (LERU) was the most productive institution. In 2016, the total score of the paper written by Zhang Y was 854, ranking first. The keywords that appear most frequently are “epilepsy,” “glutamate,” “temporal lobe epilepsy (TLE),” “hippocampus,” and “seizures.” This study showed that although the publications related to epileptic glutamate fluctuated slightly, the Np increased overall. The United States is a great creator and influential country in this field. The first three authors are Eid, T., Aronica, E., and Smolders, I. “spectrum,” “animal model,” “inflammation,” “mutation,” “dysfunction,” and “prefrontal cortex” are increasing research hotspots. By recognizing the most critical indicators (researchers, countries, research institutes, and journals of glutamate release in epilepsy research), the research hotspot of glutamate in epilepsy could help countries, scholars, and policymakers in this field enhance their understanding of the role of glutamate in epilepsy and make decisions.

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.

Evaluation of the effects of the mGlu2/3 antagonist LY341495 on dyskinesia and psychosis-like behaviours in the MPTP-lesioned marmoset

BACKGROUND

We have previously demonstrated that the metabotropic glutamate 2 and 3 (mGlu_2/3) antagonist LY341495 reverses the anti-dyskinetic and anti-psychotic benefits conferred by mGlu₂ activation and serotonin 2A (5-HT_2A) antagonism. Here, we hypothesised that a higher dose of LY341495, associated with a higher antagonistic effect at mGlu₃ receptors, would result in a reduction of the reversal of mGlu₂ activation and 5-HT_2A blockade on dyskinesia, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset.

METHODS

After induction of parkinsonism with MPTP, marmosets entered 3 streams of experiments, in which the following treatments were administered, in combination with l-3,4-dihydroxyphenylalanine (L-DOPA), after which dyskinesia, psychosis-like behaviours (PLBs) and parkinsonism were rated: 1. vehicle/vehicle, LY354740 (mGlu_2/3 orthosteric agonist)/vehicle, LY354740/LY341495 1 mg/kg and LY354740/LY341495 3 mg/kg; 2. vehicle/vehicle, LY487379 (mGlu₂ positive allosteric modulator)/vehicle, LY487379/LY341495 1 mg/kg and LY487379/LY341495 3 mg/kg; 3. vehicle/vehicle, EMD-281,014 (5-HT_2A antagonist)/vehicle, EMD-281,014/LY341495 1 mg/kg and EMD-281,014/LY341495 3 mg/kg.

RESULTS

Each of LY354740, LY487379 and EMD-281,014 reduced the severity of L-DOPA-induced dyskinesia, by 55%, 39% and 40%, respectively (all p < 0.001), as well as the severity of PLBs, by 48%, 36% and 41%, respectively (all p < 0.001). Adding LY341495 1 and 3 mg/kg to each of LY354740, LY487379 and EMD-281,014 resulted in a dose-dependent reversal of their anti-dyskinetic and anti-psychotic actions. No effect on the anti-parkinsonian action of L-DOPA was noted with any treatment combination.

CONCLUSION

These results suggest that an antagonistic effect at mGlu₃ receptors may not be sufficient to overcome the deleterious effect of mGlu₂ blockade on dyskinesia in PD. It remains to be seen whether similar effects would have been obtained with a selective mGlu₃ antagonist.

[Recent advances in the structural biology of the class C G protein-coupled receptors: The metabotropic Glutamate receptor 5]

Class C GPCRs, that include metabotropic glutamate receptors (mGlu), taste receptors, GABA_B receptor and Calcium-sensing receptor, are unusual in terms of their molecular architecture and allosteric regulation. They all form obligatory dimers, dimerization being fundamental for their function. More specifically, the mGlu are activated by the main excitatory neurotransmitter, L-glutamate. mGlu activation by glutamate binding in the venus flytrap domain (VFT) triggers conformational changes that are transmitted, through the Cystein-Rich Domain (CRD), to the conserved fold of 7 transmembrane helices (7TM), that couples to intracellular G protein. mGlu activity can also be allosterically modulated by positive (PAM) or negative (NAM) allosteric modulators binding to the 7TM. Recent progress in cryo-electron microscopy (cryoEM) has allowed unprecedented advances in deciphering the structural and molecular basis of their activation mechanism. The agonist induces a large movement between the subunits, bringing the 7TMs together and stabilizing a 7TM conformation structurally similar to the inactive state. The diversity of inactive conformations for the class C was unexpected but allows PAM stabilising a 7TM active conformation independent of the conformational changes induced by agonists, representing an alternative mode of mGlu activation. Here we present and discuss recent structural characterisation of mGlu receptors, highlighting findings that make the class C of GPCR unique. Understanding the structural basis of mGlu dimer signaling represents a landmark achievement and paves the way for structural investigation of GPCR dimer signaling in general. Structural information will open new avenues for structure-based drug design.

Group I Metabotropic Glutamate Receptors and Interacting Partners: An Update

Group I metabotropic glutamate (mGlu) receptors (mGlu1/5 subtypes) are G protein-coupled receptors and are broadly expressed in the mammalian brain. These receptors play key roles in the modulation of normal glutamatergic transmission and synaptic plasticity, and abnormal mGlu1/5 signaling is linked to the pathogenesis and symptomatology of various mental and neurological disorders. Group I mGlu receptors are noticeably regulated via a mechanism involving dynamic protein-protein interactions. Several synaptic protein kinases were recently found to directly bind to the intracellular domains of mGlu1/5 receptors and phosphorylate the receptors at distinct amino acid residues. A variety of scaffolding and adaptor proteins also interact with mGlu1/5. Constitutive or activity-dependent interactions between mGlu1/5 and their interacting partners modulate trafficking, anchoring, and expression of the receptors. The mGlu1/5-associated proteins also finetune the efficacy of mGlu1/5 postreceptor signaling and mGlu1/5-mediated synaptic plasticity. This review analyzes the data from recent studies and provides an update on the biochemical and physiological properties of a set of proteins or molecules that interact with and thus regulate mGlu1/5 receptors.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Allosteric modulators enhance agonist efficacy by increasing the residence time of a GPCR in the active state

Much hope in drug development comes from the discovery of positive allosteric modulators (PAM) that display target subtype selectivity and act by increasing agonist potency and efficacy. How such compounds can allosterically influence agonist action remains unclear. Metabotropic glutamate receptors (mGlu) are G protein-coupled receptors that represent promising targets for brain diseases, and for which PAMs acting in the transmembrane domain have been developed. Here, we explore the effect of a PAM on the structural dynamics of mGlu2 in optimized detergent micelles using single molecule FRET at submillisecond timescales. We show that glutamate only partially stabilizes the extracellular domains in the active state. Full activation is only observed in the presence of a PAM or the G_i protein. Our results provide important insights on the role of allosteric modulators in mGlu activation, by stabilizing the active state of a receptor that is otherwise rapidly oscillating between active and inactive states.

Here, the authors use smFRET to assess the structural dynamics of metabotropic glutamate receptor mGlu2 and show that a positive allosteric modulator or the Gi protein stabilize mGlu2 in the glutamate-induced active state, leading to the full activation of the receptor.

Agonists and allosteric modulators promote signaling from different metabotropic glutamate receptor 5 conformations

Metabotropic glutamate receptors (mGluRs) are dimeric G-protein-coupled receptors activated by the main excitatory neurotransmitter, L-glutamate. mGluR activation by agonists binding in the venus flytrap domain is regulated by positive (PAM) or negative (NAM) allosteric modulators binding to the 7-transmembrane domain (7TM). We report the cryo-electron microscopy structures of fully inactive and intermediate-active conformations of mGlu₅ receptor bound to an antagonist and a NAM or an agonist and a PAM, respectively, as well as the crystal structure of the 7TM bound to a photoswitchable NAM. The agonist induces a large movement between the subunits, bringing the 7TMs together and stabilizing a 7TM conformation structurally similar to the inactive state. Using functional approaches, we demonstrate that the PAM stabilizes a 7TM active conformation independent of the conformational changes induced by agonists, representing an alternative mode of mGlu activation. These findings provide a structural basis for different mGluR activation modes.

•

Cryo-EM analysis of thermostabilized mGlu₅ receptor bound to inhibitors or activators

•

X-ray structure of trans-Alloswitch-1 bound to thermostable mGlu₅ 7TMs

•

Photopharmacology provides insight into allosteric regulation of mGlu₅ 7TMs

•

Multiple conformations of mGlu₅ receptor activate G protein

5-Hydroxytryptamine, Glutamate, and ATP: Much More Than Neurotransmitters

5-hydroxytryptamine (5-HT) is derived from the essential amino acid L-tryptophan. Although the compound has been studied extensively for its neuronal handling and synaptic actions, serotonin 5-HT receptors can be found extra-synaptically and not only in neurons but in many types of mammalian cells, inside and outside the central nervous system (CNS). In sharp contrast, glutamate (Glu) and ATP are better known as metabolism-related molecules, but they also are neurotransmitters, and their receptors are expressed on almost any type of cell inside and outside the nervous system. Whereas 5-hydroxytryptamine and Glu are key regulators of the immune system, ATP actions are more general. 5-hydroxytryptamine, ATP and Glu act through both G protein-coupled receptors (GPCRs), and ionotropic receptors, i.e., ligand gated ion channels. These are the three examples of neurotransmitters whose actions as holistic regulatory molecules are briefly put into perspective here.

Hippocampal glucocorticoid target genes associated with enhancement of memory consolidation

Glucocorticoids enhance memory consolidation of emotionally arousing events via largely unknown molecular mechanisms. This glucocorticoid effect on the consolidation process also requires central noradrenergic neurotransmission. The intracellular pathways of these two stress mediators converge on two transcription factors: the glucocorticoid receptor (GR) and phosphorylated cAMP response element‐binding protein (pCREB). We therefore investigated, in male rats, whether glucocorticoid effects on memory are associated with genomic interactions between the GR and pCREB in the hippocampus. In a two‐by‐two design, object exploration training or no training was combined with post‐training administration of a memory‐enhancing dose of corticosterone or vehicle. Genomic effects were studied by chromatin immunoprecipitation followed by sequencing (ChIP‐seq) of GR and pCREB 45 min after training and transcriptome analysis after 3 hr. Corticosterone administration induced differential GR DNA‐binding and regulation of target genes within the hippocampus, largely independent of training. Training alone did not result in long‐term memory nor did it affect GR or pCREB DNA‐binding and gene expression. No strong evidence was found for an interaction between GR and pCREB. Combination of the GR DNA‐binding and transcriptome data identified a set of novel, likely direct, GR target genes that are candidate mediators of corticosterone effects on memory consolidation. Cell‐specific expression of the identified target genes using single‐cell expression data suggests that the effects of corticosterone reflect in part non‐neuronal cells. Together, our data identified new GR targets associated with memory consolidation that reflect effects in both neuronal and non‐neuronal cells.

Positive Allosteric Modulators of Metabotropic Glutamate Receptor 5 as Tool Compounds to Study Signaling Bias

Positive allosteric modulation of metabotropic glutamate subtype 5 (mGlu₅) receptor has emerged as a potential new therapeutic strategy for the treatment of schizophrenia and cognitive impairments. However, positive allosteric modulator (PAM) agonist activity has been associated with adverse side effects, and neurotoxicity has also been observed for pure PAMs. The structural and pharmacological basis of therapeutic versus adverse mGlu₅ PAM in vivo effects remains unknown. Thus, gaining insights into the signaling fingerprints, as well as the binding kinetics of structurally diverse mGlu₅ PAMs, may help in the rational design of compounds with desired properties. We assessed the binding and signaling profiles of N-methyl-5-(phenylethynyl)pyrimidin-2-amine (MPPA), 3-cyano-N-(2,5-diphenylpyrazol-3-yl)benzamide (CDPPB), and 1-[4-(4-chloro-2-fluoro-phenyl)piperazin-1-yl]-2-(4-pyridylmethoxy)ethenone [compound 2c, a close analog of 1-(4-(2-chloro-4-fluorophenyl)piperazin-1-yl)-2-(pyridin-4-ylmethoxy)ethanone] in human embryonic kidney 293A cells stably expressing mGlu₅ using Ca²⁺ mobilization, inositol monophosphate (IP₁) accumulation, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, and receptor internalization assays. Of the three allosteric ligands, only CDPPB had intrinsic agonist efficacy, and it also had the longest receptor residence time and highest affinity. MPPA was a biased PAM, showing higher positive cooperativity with orthosteric agonists in ERK1/2 phosphorylation and Ca²⁺ mobilization over IP₁ accumulation and receptor internalization. In primary cortical neurons, all three PAMs showed stronger positive cooperativity with (S)-3,5-dihydroxyphenylglycine (DHPG) in Ca²⁺ mobilization over IP₁ accumulation. Our characterization of three structurally diverse mGlu₅ PAMs provides further molecular pharmacological insights and presents the first assessment of PAM-mediated mGlu₅ internalization. SIGNIFICANCE STATEMENT: Enhancing metabotropic glutamate receptor subtype 5 (mGlu₅) activity is a promising strategy to treat cognitive and positive symptoms in schizophrenia. It is increasingly evident that positive allosteric modulators (PAMs) of mGlu₅ are not all equal in preclinical models; there remains a need to better understand the molecular pharmacological properties of mGlu₅ PAMs. This study reports detailed characterization of the binding and functional pharmacological properties of mGlu₅ PAMs and is the first study of the effects of mGlu₅ PAMs on receptor internalization.