JNJ16259685, a highly potent, selective and systemically active mGlu1 receptor antagonist

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.

Neuronal Nitric Oxide Synthase Regulates Cerebellar Parallel Fiber Slow EPSC in Purkinje Neurons by Modulating STIM1-Gated TRPC3-Containing Channels

Responding to burst stimulation of parallel fibers (PFs), cerebellar Purkinje neurons (PNs) generate a convolved synaptic response displaying a fast excitatory postsynaptic current (EPSCFast) followed by a slow EPSC (EPSCSlow). The latter is companied with a rise of intracellular Ca2+ and critical for motor coordination. The genesis of EPSCSlow in PNs results from activation of metabotropic type 1 glutamate receptor (mGluR1), oligomerization of stromal interaction molecule 1 (STIM1) on the membrane of endoplasmic reticulum (ER) and opening of transient receptor potential canonical 3 (TRPC3) channels on the plasma membrane. Neuronal nitric oxide synthase (nNOS) is abundantly expressed in PFs and granule neurons (GNs), catalyzing the production of nitric oxide (NO) hence regulating PF-PN synaptic function. We recently found that nNOS/NO regulates the morphological development of PNs through mGluR1-regulated Ca2+-dependent mechanism. This study investigated the role of nNOS/NO in regulating EPSCSlow. Electrophysiological analyses showed that EPSCSlow in cerebellar slices of nNOS knockout (nNOS-/-) mice was significantly larger than that in wildtype (WT) mice. Activation of mGluR1 in cultured PNs from nNOS-/- mice evoked larger TRPC3-channel mediated currents and intracellular Ca2+ rise than that in PNs from WT mice. In addition, nNOS inhibitor and NO-donor increased and decreased, respectively, the TRPC3-current and Ca2+ rise in PNs. Moreover, the NO-donor effectively decreased TRPC3 currents in HEK293 cells expressing WT STIM1, but not cells expressing a STIM1 with cysteine mutants. These novel findings indicate that nNOS/NO inhibits TRPC3-containig channel mediated cation influx during EPSCSlow, at least in part, by S-nitrosylation of STIM1.

Dataset of eyeblink conditioning in mice treated with the selective mGluR1 antagonist JNJ16259685

Eyeblink conditioning is associated with motor learning, which requires the cerebellum and the brainstem. This article provides behavioral data on whether JNJ16259685, a selective metabotropic glutamate receptor type 1 (mGluR1) antagonist, affects eyeblink conditioning in wild-type mice (C57BL/6 J strain). The dataset contains four types of behavioral outputs pertinent to eyeblink conditioning. We used a t-test and an analysis of variance (ANOVA) to analyze the percentage of conditioned responses (CR%), peak CR latencies, onset CR latencies, and electromyography (EMG) amplitudes. The information obtained in this dataset adds to our knowledge of the molecular mechanisms underlying eyeblink conditioning and can prove beneficial for investigators studying the pharmacological effects of mGluR1 on motor learning. Future research might use this dataset as a basis for conducting experiments with different JNJ16259685 doses, administration methods, and durations than the ones used in the described procedures.

Bidirectional regulation of synaptic SUMOylation by Group 1 metabotropic glutamate receptors

SUMOylation is a post-translational modification essential to cell homeostasis. A tightly controlled equilibrium between SUMOylation and deSUMOylation processes is also critical to the neuronal function including neurotransmitter release and synaptic transmission and plasticity. Disruption of the SUMOylation homeostasis in neurons is associated with several neurological disorders. The balance between the SUMOylation and deSUMOylation of substrate proteins is maintained by a group of deSUMOylation enzymes called SENPs. We previously showed that the activation of type 5 metabotropic glutamate receptors (mGlu5R) first triggers a rapid increase in synaptic SUMOylation and then upon the sustained activation of these receptors, the deSUMOylase activity of SENP1 allows the increased synaptic SUMOylation to get back to basal levels. Here, we combined the use of pharmacological tools with subcellular fractionation and live-cell imaging of individual hippocampal dendritic spines to demonstrate that the synaptic accumulation of the deSUMOylation enzyme SENP1 is bidirectionally controlled by the activation of type 1 mGlu1 and mGlu5 receptors. Indeed, the pharmacological blockade of mGlu1R activation during type 1 mGluR stimulation leads to a faster and greater accumulation of SENP1 at synapses indicating that mGlu1R acts as a brake to the mGlu5R-dependent deSUMOylation process at the post-synapse. Altogether, our findings reveal that type 1 mGluRs work in opposition to dynamically tune the homeostasis of SUMOylation at the mammalian synapse.

Receptor mapping using methoxy phenyl piperazine derivative: Preclinical PET imaging

This study aimed at assessing 2-methoxyphenyl piperazine derivative for its binding specificity and suitability in mapping metabotropic glutamate receptor subtype 1, which is implicated in several neuropsychiatric disorders. N-(2-(4-(2-Methoxyphenyl)piperazin-1-yl)ethyl)-N-methylpyridin-2-amine was synthesised and evaluated for brain imaging subsequent to radiolabelling with [¹¹C] radioisotope via methylation process in 98.9% purity and 52 ± 6% yield (decay corrected). The specific activity was in the range of 72-93 GBq/µmol. The haemolysis of blood was 2-5% for initial 4 hr and remained < 10% after 24 h of incubation indicating low toxicity. In vitro autoradiograms after coincubation with unlabelled ligand confirmed the high uptake of the PET radioligand in the mGluR1 receptor rich regions. The PET as well as biodistribution studies also showed high activity in the brain with a direct correlation between receptor abundance distribution pattern and tracer activity. The biodistribution analyses revealed initial high brain uptake (4.18 ± 0.48). The highest uptake was found in cerebellum (SUV 4.7 ± 0.2), followed by thalamus (SUV 3.5 ± 0.1), and striatum (SUV 3 ± 0.1). In contrast, pons had negligible tracer activity. The high uptake observed in all the regions with known mGluR1 activity indicates suitability of the ligand for mGluR1 imaging.

Subtype-selective mechanisms of negative allosteric modulators binding to group I metabotropic glutamate receptors

Group I metabotropic glutamate receptors (mGlu₁ and mGlu₅) are promising targets for multiple psychiatric and neurodegenerative disorders. Understanding the subtype selectivity of mGlu₁ and mGlu₅ allosteric sites is essential for the rational design of novel modulators with single- or dual-target mechanism of action. In this study, starting from the deposited mGlu₁ and mGlu₅ crystal structures, we utilized computational modeling approaches integrating docking, molecular dynamics simulation, and efficient post-trajectory analysis to reveal the subtype-selective mechanism of mGlu₁ and mGlu₅ to 10 diverse drug scaffolds representing known negative allosteric modulators (NAMs) in the literature. The results of modeling identified six pairs of non-conserved residues and four pairs of conserved ones as critical features to distinguish the selective NAMs binding to the corresponding receptors. In addition, nine pairs of residues are beneficial to the development of novel dual-target NAMs of group I metabotropic glutamate receptors. Furthermore, the binding modes of a reported dual-target NAM (VU0467558) in mGlu₁ and mGlu₅ were predicted to verify the identified residues that play key roles in the receptor selectivity and the dual-target binding. The results of this study can guide rational structure-based design of novel NAMs, and the approach can be generally applicable to characterize the features of selectivity for other G-protein-coupled receptors.

A systems-level study reveals host-targeted repurposable drugs against SARS-CoV-2 infection

Understanding the mechanism of SARS-CoV-2 infection and identifying potential therapeutics are global imperatives. Using a quantitative systems pharmacology approach, we identified a set of repurposable and investigational drugs as potential therapeutics against COVID-19. These were deduced from the gene expression signature of SARS-CoV-2-infected A549 cells screened against Connectivity Map and prioritized by network proximity analysis with respect to disease modules in the viral-host interactome. We also identified immuno-modulating compounds aiming at suppressing hyperinflammatory responses in severe COVID-19 patients, based on the transcriptome of ACE2-overexpressing A549 cells. Experiments with Vero-E6 cells infected by SARS-CoV-2, as well as independent syncytia formation assays for probing ACE2/SARS-CoV-2 spike protein-mediated cell fusion using HEK293T and Calu-3 cells, showed that several predicted compounds had inhibitory activities. Among them, salmeterol, rottlerin, and mTOR inhibitors exhibited antiviral activities in Vero-E6 cells; imipramine, linsitinib, hexylresorcinol, ezetimibe, and brompheniramine impaired viral entry. These novel findings provide new paths for broadening the repertoire of compounds pursued as therapeutics against COVID-19.

Targeting metabotropic glutamate receptors for the treatment of depression and other stress-related disorders

The discovery of robust antidepressant effects of ketamine in refractory patients has led to increasing focus on agents targeting glutamatergic signaling as potential novel antidepressant strategy. Among the agents targeting the glutamatergic system, compounds acting at metabotropic glutamate (mGlu) receptors are among the most promising agents under studies for depressive disorders. Further, the receptor diversity, distinct distribution in the CNS, and ability to modulate the glutamatergic neurotransmission in the brain areas implicated in mood disorders make them an exciting target for stress-related disorders. In preclinical models, antidepressant and anxiolytic effects of mGlu₅ negative allosteric modulators (NAMs) have been reported. Interestingly, mGlu_2/3 receptor antagonists show fast and sustained antidepressant-like effects similar to that of ketamine in rodents. Excitingly, they can also induce antidepressant effects in the animal models of treatment-resistant depression and are devoid of the side-effects associated with ketamine. Unfortunately, clinical trials of both mGlu₅ and mGlu_2/3 receptor NAMs have been inconclusive, and additional trials using other compounds with suitable preclinical and clinical properties are needed. Although group III mGlu receptors have gained less attention, mGlu₇ receptor ligands have been shown to induce antidepressant-like effects in rodents. Collectively, compounds targeting mGlu receptors provide an alternative approach to fill the outstanding clinical need for safer and more efficacious antidepressants. This article is part of the special Issue on "Glutamate Receptors - mGluRs".

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

Metabotropic glutamate (mGlu) receptors respond to glutamate, the major excitatory neurotransmitter in the mammalian brain, mediating a modulatory role that is critical for higher-order brain functions such as learning and memory. Since the first mGlu receptor was cloned in 1992, eight subtypes have been identified along with many isoforms and splice variants. The mGlu receptors are transmembrane-spanning proteins belonging to the class C G protein-coupled receptor family and represent attractive targets for a multitude of central nervous system disorders. Concerted drug discovery efforts over the past three decades have yielded a wealth of pharmacological tools including subtype-selective agents that competitively block or mimic the actions of glutamate or act allosterically via distinct sites to enhance or inhibit receptor activity. Herein, we review the physiologic and pathophysiological roles for individual mGlu receptor subtypes including the pleiotropic nature of intracellular signal transduction arising from each. We provide a comprehensive analysis of the in vitro and in vivo pharmacological properties of prototypical and commercially available orthosteric agonists and antagonists as well as allosteric modulators, including ligands that have entered clinical trials. Finally, we highlight emerging areas of research that hold promise to facilitate rational design of highly selective mGlu receptor-targeting therapeutics in the future. SIGNIFICANCE STATEMENT: The metabotropic glutamate receptors are attractive therapeutic targets for a range of psychiatric and neurological disorders. Over the past three decades, intense discovery efforts have yielded diverse pharmacological tools acting either competitively or allosterically, which have enabled dissection of fundamental biological process modulated by metabotropic glutamate receptors and established proof of concept for many therapeutic indications. We review metabotropic glutamate receptor molecular pharmacology and highlight emerging areas that are offering new avenues to selectively modulate neurotransmission.

Negative Allosteric Modulator of mGluR1 Improves Long-Term Neurologic Deficits after Experimental Subarachnoid Hemorrhage

Aneurysmal subarachnoid hemorrhage (SAH) causes permanent neurological sequelae, but the underlying mechanism needs to be further clarified. Here, we show that inhibition of metabotropic glutamate receptor 1 (mGluR1) with negative allosteric modulator JNJ16259685 improves long-term neurobehavioral outcomes in an endovascular perforation model of SAH. JNJ16259685 improves cerebrovascular dysfunction through attenuation of cerebral blood flow (CBF) reduction, cerebral vasoconstrictio, and microthrombosis formation in a rat SAH model. Moreover, JNJ16259685 reduces experimental SAH-induced long-term neuronal damage through alleviation of neuronal death and degeneration. Mechanically, JNJ16259685 maintains phosphorylation of endothelial NO synthase (eNOS) and vasodilator-stimulated phosphoprotein (VASP) and decreases apoptosis-related factors Bax, active caspase-9, and active caspase-3 following experimental SAH. Altogether, our results suggest JNJ16259685 improves long-term functional impairment through neurovascular protection.

Advances in CNS PET: the state-of-the-art for new imaging targets for pathophysiology and drug development

PURPOSE

A limit on developing new treatments for a number of central nervous system (CNS) disorders has been the inadequate understanding of the in vivo pathophysiology underlying neurological and psychiatric disorders and the lack of in vivo tools to determine brain penetrance, target engagement, and relevant molecular activity of novel drugs. Molecular neuroimaging provides the tools to address this. This article aims to provide a state-of-the-art review of new PET tracers for CNS targets, focusing on developments in the last 5 years for targets recently available for in-human imaging.

METHODS

We provide an overview of the criteria used to evaluate PET tracers. We then used the National Institute of Mental Health Research Priorities list to identify the key CNS targets. We conducted a PubMed search (search period 1st of January 2013 to 31st of December 2018), which yielded 40 new PET tracers across 16 CNS targets which met our selectivity criteria. For each tracer, we summarised the evidence of its properties and potential for use in studies of CNS pathophysiology and drug evaluation, including its target selectivity and affinity, inter and intra-subject variability, and pharmacokinetic parameters. We also consider its potential limitations and missing characterisation data, but not specific applications in drug development. Where multiple tracers were present for a target, we provide a comparison of their properties.

RESULTS AND CONCLUSIONS

Our review shows that multiple new tracers have been developed for proteinopathy targets, particularly tau, as well as the purinoceptor P2X7, phosphodiesterase enzyme PDE10A, and synaptic vesicle glycoprotein 2A (SV2A), amongst others. Some of the most promising of these include 18F-MK-6240 for tau imaging, 11C-UCB-J for imaging SV2A, 11C-CURB and 11C-MK-3168 for characterisation of fatty acid amide hydrolase, 18F-FIMX for metabotropic glutamate receptor 1, and 18F-MNI-444 for imaging adenosine 2A. Our review also identifies recurrent issues within the field. Many of the tracers discussed lack in vivo blocking data, reducing confidence in selectivity. Additionally, late-stage identification of substantial off-target sites for multiple tracers highlights incomplete pre-clinical characterisation prior to translation, as well as human disease state studies carried out without confirmation of test-retest reproducibility.

Selective mGluR1 Negative Allosteric Modulator Reduces Blood-Brain Barrier Permeability and Cerebral Edema After Experimental Subarachnoid Hemorrhage

The blood-brain barrier (BBB) disruption leads to the vasogenic brain edema and contributes to the early brain injury (EBI) after subarachnoid hemorrhage (SAH). However, the mechanisms underlying the BBB damage following SAH are poorly understood. Here we reported that the neurotransmitter glutamate of cerebrospinal fluid (CSF) was dramatically increased in SAH patients with symptoms of cerebral edema. Using the rat SAH model, we found that SAH caused the increase of CSF glutamate level and BBB permeability in EBI, intracerebroventricular injection of exogenous glutamate deteriorated BBB damage and cerebral edema, while intraperitoneally injection of metabotropic glutamate receptor 1(mGluR1) negative allosteric modulator JNJ16259685 significantly attenuated SAH-induced BBB damage and cerebral edema. In an in vitro BBB model, we showed that glutamate increased monolayer permeability of human brain microvascular endothelial cells (HBMEC), whereas JNJ16259685 preserved glutamate-damaged BBB integrity in HBMEC. Mechanically, glutamate downregulated the level and phosphorylation of vasodilator-stimulated phosphoprotein (VASP), decreased the tight junction protein occludin, and increased AQP4 expression at 72 h after SAH. However, JNJ16259685 significantly increased VASP, p-VASP, and occludin, and reduced AQP level at 72 h after SAH. Altogether, our results suggest an important role of glutamate in disruption of BBB function and inhibition of mGluR1 with JNJ16259685 reduced BBB damage and cerebral edema after SAH.

Fragment-Based Approaches for Allosteric Metabotropic Glutamate Receptor (mGluR) Modulators

Metabotropic glutamate receptors (mGluR) are members of the class C G-Protein Coupled Receptors (GPCR-s) and have eight subtypes. These receptors are responsible for a variety of functions in the central and peripheral nervous systems and their modulation has therapeutic utility in neurological and psychiatric disorders. It was previously established that selective orthosteric modulation of these receptors is challenging, and this stimulated the search for allosteric modulators. Fragment-Based Drug Discovery (FBDD) is a viable approach to find ligands binding at allosteric sites owing to their limited size and interactions. However, it was also observed that the structure-activity relationship of allosteric modulators is often sharp and inconsistent. This can be attributed to the characteristics of the allosteric binding site of mGluRs that is a water channel where ligand binding is accompanied with induced fit and interference with the water network, both playing a role in receptor activation. In this review, we summarize fragment-based drug discovery programs on mGluR allosteric modulators and their contribution identifying of new mGluR ligands with better activity and selectivity.

Reproducibility via coordinated standardization: a multi-center study in a Shank2 genetic rat model for Autism Spectrum Disorders

Inconsistent findings between laboratories are hampering scientific progress and are of increasing public concern. Differences in laboratory environment is a known factor contributing to poor reproducibility of findings between research sites, and well-controlled multisite efforts are an important next step to identify the relevant factors needed to reduce variation in study outcome between laboratories. Through harmonization of apparatus, test protocol, and aligned and non-aligned environmental variables, the present study shows that behavioral pharmacological responses in Shank2 knockout (KO) rats, a model of synaptic dysfunction relevant to autism spectrum disorders, were highly replicable across three research centers. All three sites reliably observed a hyperactive and repetitive behavioral phenotype in KO rats compared to their wild-type littermates as well as a dose-dependent phenotype attenuation following acute injections of a selective mGluR1 antagonist. These results show that reproducibility in preclinical studies can be obtained and emphasizes the need for high quality and rigorous methodologies in scientific research. Considering the observed external validity, the present study also suggests mGluR1 as potential target for the treatment of autism spectrum disorders.

Activation of mGluR1 Mediates C1q-Dependent Microglial Phagocytosis of Glutamatergic Synapses in Alzheimer's Rodent Models

Microglia and complements appear to be involved in the synaptic and cognitive deficits in Alzheimer's disease (AD), though the mechanisms remain elusive. In this study, utilizing two types of rodent model of AD, we reported increased complement C1q-mediated microglial phagocytosis of hippocampal glutamatergic synapses, which led to synaptic and cognitive deficits. We also found increased activity of the metabotropic glutamate receptor 1 (mGluR1) in hippocampal CA1 in the modeled rodents. Artificial activation of mGluR1 signaling promoted dephosphorylation of fragile X mental retardation protein (FMRP) and facilitated the local translation machinery of synaptic C1q mRNA, thus mimicking the C1q-mediated microglial phagocytosis of hippocampal glutamatergic synapses and synaptic and cognitive deficiency in the modeled rodents. However, suppression of mGluR1 signaling inhibited the dephosphorylation of FMRP and repressed the local translation of synaptic C1q mRNA, which consequently alleviated microglial phagocytosis of synapses and restored the synaptic and cognitive function in the rodent models. These findings illustrate a novel molecular mechanism underlying C1q-mediated microglial phagocytosis of hippocampal glutamatergic synapses in AD.

Hippocampal mGluR1-dependent long-term potentiation requires NAADP-mediated acidic store Ca2+ signaling

Acidic organelles, such as endosomes and lysosomes, store Ca²⁺ that is released in response to intracellular increases in the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP). In neurons, NAADP and Ca²⁺ signaling contribute to synaptic plasticity, a process of activity-dependent long-term potentiation (LTP) [or, alternatively, long-term depression (LTD)] of synaptic strength and neuronal transmission that is critical for neuronal function and memory formation. We explored the function of and mechanisms regulating acidic Ca²⁺ store signaling in murine hippocampal neurons. We found that metabotropic glutamate receptor 1 (mGluR1) was coupled to NAADP signaling that elicited Ca²⁺ release from acidic stores. In turn, this released Ca²⁺-mediated mGluR1-dependent LTP by transiently inhibiting SK-type K⁺ channels, possibly through the activation of protein phosphatase 2A. Genetically removing two-pore channels (TPCs), which are endolysosomal-specific ion channels, switched the polarity of plasticity from LTP to LTD, indicating the importance of specific receptor store coupling and providing mechanistic insight into how mGluR1 can produce both synaptic potentiation and synaptic depression.

Effect of Novel Allosteric Modulators of Metabotropic Glutamate Receptors on Drug Self-administration and Relapse: A Review of Preclinical Studies and Their Clinical Implications

Results from preclinical rodent studies during the last 20 years implicated glutamate neurotransmission in different brain regions in drug self-administration and rodent models of relapse. These results, along with evidence for drug-induced neuroadaptations in glutamatergic neurons and receptors, suggested that addiction might be treatable by medications that inhibit glutamatergic responses to drugs of abuse, drug-associated cues, and stressors. This idea is supported by findings in rodent and primate models that drug self-administration and relapse are reduced by systemic injections of antagonists of ionotropic glutamate receptors or metabotropic glutamate receptors (mGluRs) or orthosteric agonists of mGluR2/3. However, these compounds have not advanced to clinical use because of potential side effects and other factors. This state of affairs has led to the development of positive allosteric modulators (PAMs) and negative allosteric modulators (NAMs) of mGluRs. PAMs and NAMs of mGluRs, either of which can inhibit evoked glutamate release, may be suitable for testing in humans. We reviewed results from recent studies of systemically injected PAMs and NAMs of mGluRs in rodents and monkeys, focusing on whether they reduce drug self-administration, reinstatement of drug seeking, and incubation of drug craving. We also review results from rat studies in which PAMs or NAMs of mGluRs were injected intracranially to reduce drug self-administration and reinstatement. We conclude that PAMs and NAMs of mGluRs should be considered for clinical trials.

The Role of Metabotropic Glutamate Receptor 1 Dependent Signaling in Glioma Viability

Glioma refers to malignant central nervous system tumors that have histologic characteristics in common with glial cells. The most prevalent type, glioblastoma multiforme, is associated with a poor prognosis and few treatment options. On the basis of reports of aberrant expression of mGluR1 mRNA in glioma, evidence that melanoma growth is directly influenced by glutamate metabotropic receptor 1 (mGluR1), and characterization of β-arrestin-dependent prosurvival signaling by this receptor, this study investigated the hypothesis that glioma cell lines aberrantly express mGluR1 and depend on mGluR1-mediated signaling to maintain viability and proliferation. Three glioma cell lines (Hs683, A172, and U87) were tested to confirm mGluR1 mRNA expression and the dependence of glioma cell viability on glutamate. Pharmacologic and genetic evidence is presented that suggests mGluR1 signaling specifically supports glioma proliferation and viability. For example, selective noncompetitive antagonists of mGluR1, CPCCOEt and JNJ16259685, decreased the viability of these cells in a dose-dependent manner, and glutamate metabotropic receptor 1 gene silencing significantly reduced glioma cell proliferation. Also, results of an anchorage-independent growth assay suggested that noncompetitive antagonism of mGluR1 may decrease the tumorigenic potential of Hs683 glioma cells. Finally, data are provided that support the hypothesis that a β-arrestin-dependent signaling cascade may be involved in glutamate-stimulated viability in glioma cells and that ligand bias may exist at mGluR1 expressed in these cells. Taken together, the results strongly suggest that mGluR1 may act as a proto-oncogene in glioma and be a viable drug target in glioma treatment.

Hyperactivity and Hypermotivation Associated With Increased Striatal mGluR1 Signaling in a Shank2 Rat Model of Autism

Mutations in the SHANK family of genes have been consistently identified in genetic and genomic screens of autism spectrum disorder (ASD). The functional overlap of SHANK with several other ASD-associated genes suggests synaptic dysfunction as a convergent mechanism of pathophysiology in ASD. Although many ASD-related mutations result in alterations to synaptic function, the nature of those dysfunctions and the consequential behavioral manifestations are highly variable when expressed in genetic mouse models. To investigate the phylogenetic conservation of phenotypes resultant of Shank2 loss-of-function in a translationally relevant animal model, we generated and characterized a novel transgenic rat with a targeted mutation of the Shank2 gene, enabling an evaluation of gene-associated phenotypes, the elucidation of complex behavioral phenotypes, and the characterization of potential translational biomarkers. The Shank2 loss-of-function mutation resulted in a notable phenotype of hyperactivity encompassing hypermotivation, increased locomotion, and repetitive behaviors. Mutant rats also expressed deficits in social behavior throughout development and in the acquisition of operant tasks. The hyperactive phenotype was associated with an upregulation of mGluR1 expression, increased dendritic branching, and enhanced long-term depression (LTD) in the striatum but opposing morphological and cellular alterations in the hippocampus (HP). Administration of the mGluR1 antagonist JNJ16259685 selectively normalized the expression of striatally mediated repetitive behaviors and physiology but had no effect on social deficits. Finally, Shank2 mutant animals also exhibited alterations in electroencephalography (EEG) spectral power and event-related potentials, which may serve as translatable EEG biomarkers of synaptopathic alterations. Our results show a novel hypermotivation phenotype that is unique to the rat model of Shank2 dysfunction, in addition to the traditional hyperactive and repetitive behaviors observed in mouse models. The hypermotivated and hyperactive phenotype is associated with striatal dysfunction, which should be explored further as a targetable mechanism for impairment in ASD.

Ionotropic and metabotropic glutamate receptors regulate protein translation in co-cultured nucleus accumbens and prefrontal cortex neurons

The regulation of protein translation by glutamate receptors and its role in plasticity have been extensively studied in the hippocampus. In contrast, very little is known about glutamatergic regulation of translation in nucleus accumbens (NAc) medium spiny neurons (MSN), despite their critical role in addiction-related plasticity and recent evidence that protein translation contributes to this plasticity. We used a co-culture system, containing NAc MSNs and prefrontal cortex (PFC) neurons, and fluorescent non-canonical amino acid tagging (FUNCAT) to visualize newly synthesized proteins in neuronal processes of NAc MSNs and PFC pyramidal neurons. First, we verified that the FUNCAT signal reflects new protein translation. Next, we examined the regulation of translation by group I metabotropic glutamate receptors (mGluRs) and ionotropic glutamate receptors by incubating co-cultures with agonists or antagonists during the 2-h period of non-canonical amino acid labeling. In NAc MSNs, basal translation was modestly reduced by blocking Ca²⁺-permeable AMPARs whereas blocking all AMPARs or suppressing constitutive mGluR5 signaling enhanced translation. Activating group I mGluRs with dihydroxyphenylglycine increased translation in an mGluR1-dependent manner in NAc MSNs and PFC pyramidal neurons. Disinhibiting excitatory transmission with bicuculline also increased translation. In MSNs, this was reversed by antagonists of mGluR1, mGluR5, AMPARs or NMDARs. In PFC neurons, AMPAR or NMDAR antagonists blocked bicuculline-stimulated translation. Our study, the first to examine glutamatergic regulation of translation in MSNs, demonstrates regulatory mechanisms specific to MSNs that depend on the level of neuronal activation. This sets the stage for understanding how translation may be altered in addiction.

Physiological activation of mGlu5 receptors supports the ion channel function of NMDA receptors in hippocampal LTD induction in vivo

Synaptic long-term depression (LTD) is believed to underlie critical mnemonic processes in the adult hippocampus. The roles of the metabotropic and ionotropic actions of glutamate in the induction of synaptic LTD by electrical low-frequency stimulation (LFS) in the living adult animal is poorly understood. Here we examined the requirement for metabotropic glutamate (mGlu) and NMDA receptors in LTD induction in anaesthetized adult rats. LTD induction was primarily dependent on NMDA receptors and required the involvement of both the ion channel function and GluN2B subunit of the receptor. Endogenous mGlu5 receptor activation necessitated the local application of relatively high doses of either competitive or non-competitive NMDA receptor antagonists to block LTD induction. Moreover, boosting endogenous glutamate activation of mGlu5 receptors with a positive allosteric modulator lowered the threshold for NMDA receptor-dependent LTD induction by weak LFS. The present data provide support in the living animal that NMDA receptor-dependent LTD is boosted by endogenously released glutamate activation of mGlu5 receptors. Given the predominant perisynaptic location of mGlu5 receptors, the present findings emphasize the need to further evaluate the contribution and mechanisms of these receptors in NMDA receptor-dependent synaptic plasticity in the adult hippocampus in vivo.

Identification of Allosteric Modulators of Metabotropic Glutamate 7 Receptor Using Proteochemometric Modeling

Proteochemometric modeling (PCM) is a computational approach that can be considered an extension of quantitative structure-activity relationship (QSAR) modeling, where a single model incorporates information for a family of targets and all the associated ligands instead of modeling activity versus one target. This is especially useful for situations where bioactivity data exists for similar proteins but is scarce for the protein of interest. Here we demonstrate the application of PCM to identify allosteric modulators of metabotropic glutamate (mGlu) receptors. Given our long-running interest in modulating mGlu receptor function we compiled a matrix of compound-target bioactivity data. Some members of the mGlu family are well explored both internally and in the public domain, while there are much fewer examples of ligands for other targets such as the mGlu7 receptor. Using a PCM approach mGlu7 receptor hits were found. In comparison to conventional single target modeling the identified hits were more diverse, had a better confirmation rate, and provide starting points for further exploration. We conclude that the robust structure-activity relationship from well explored target family members translated to better quality hits for PCM compared to virtual screening (VS) based on a single target.

Prolonged Type 1 Metabotropic Glutamate Receptor Dependent Synaptic Signaling Contributes to Spino-Cerebellar Ataxia Type 1

Type 1 metabotropic glutamate receptor (mGluR1)-dependent signaling at parallel fiber to Purkinje neuron synapses is critical for cerebellar function. In a mouse model of human spino-cerebellar ataxia type 1 (early SCA1, 12 weeks) we find prolonged parallel fiber mGluR1-dependent synaptic currents and calcium signaling. Acute treatment with a low dose of the potent and specific activity-dependent mGluR1-negative allosteric modulator JNJ16259685 shortened the prolonged mGluR1 currents and rescued the moderate ataxia. Our results provide exciting new momentum for developing mGluR1-based pharmacology to treat ataxia.

SIGNIFICANCE STATEMENT

Ataxia is a progressive and devastating degenerative movement disorder commonly associated with loss of cerebellar function and with no known cure. In the early stages of a mouse model of human spinocerebellar ataxia type 1, SCA1, where mice exhibit only moderate motor impairment, we detect excess "gain of function" of metabotropic glutamate receptor signaling at an important cerebellar synapse. Because careful control of this type of signaling is critical for cerebellar function in mice and humans, we sought to remove the excess signaling with a powerful, readily available pharmacological modulator. Remarkably, this pharmacological treatment acutely restored normal motor function in the ataxic mice. Our results pave the way for exploring a new avenue for early treatment of human ataxias.

Industrial medicinal chemistry insights: neuroscience hit generation at Janssen

The role of medicinal chemistry has changed over the past 10 years. Chemistry had become one step in a process; funneling the output of high-throughput screening (HTS) on to the next stage. The goal to identify the ideal clinical compound remains, but the means to achieve this have changed. Modern medicinal chemistry is responsible for integrating innovation throughout early drug discovery, including new screening paradigms, computational approaches, novel synthetic chemistry, gene-family screening, investigating routes of delivery, and so on. In this Foundation Review, we show how a successful medicinal chemistry team has a broad impact and requires multidisciplinary expertise in these areas.

Preclinical anaylses of [18F]cEFQ as a PET tracer for imaging metabotropic glutamate receptor type 1 (mGluR1)

Metabotropic glutamate receptor type 1 (mGluR1) is related with various neurological and psychiatric diseases, such as anxiety, depression, epilepsy, Parkinson's disease, and neuropathic pain. Hence, mGluR1 is an important target for drug development and imaging. We synthesized [¹⁸F]cEFQ (3-ethyl-2-[¹⁸F]fluoroquinolin-6-yl cis-(4-methoxycyclohexyl)methanone) as a PET tracer for selective mGluR1 imaging and evaluated its properties in rodents. A chloroquinoline precursor was labeled by a nucleophilic substitution reaction, and the resulting [¹⁸F]cEFQ was obtained with high radiochemical purity (>99%) and specific activity (63-246 GBq/µmol). The log D value was 3.24, and the initial brain uptake at 10 min was over 4% of injected dose per gram in BALB/c mice. According to PET/CT and autoradiography in SD rats, [¹⁸F]cEFQ showed wide distribution in the whole brain and the highest uptake in the cerebellum. Pre-treatment with unlabeled cEFQ or the mGluR1-specific antagonist JNJ16259685 blocked the uptake of [¹⁸F]cEFQ. However, the uptake was not blocked by pre-treatment with the mGluR5-specific antagonist ABP688. The trans isomer [¹⁸F]tEFQ did not show high uptake in the mGluR1-rich region. [¹⁸F]cEFQ was straightforwardly prepared using a chloro-derivative precursor. Its feasibility as a specific and selective PET agent for imaging mGluR1 was proved by in vitro and in vivo experiments using rodents.

mGlu1 receptor as a drug target for treatment of substance use disorders: time to gather stones together?

Modulation of the mGlu1 receptor was repeatedly shown to inhibit various phenomena associated with exposure to abused drugs. Efficacy in preclinical models was observed with both positive and negative allosteric modulators (PAMs and NAMs, respectively) using essentially non-overlapping sets of experimental methods. Taken together, these data indicate that the mGlu1 receptor certainly plays a significant role in the plasticity triggered by the exposure to abused drugs and is involved in the maintenance of drug-seeking and drug-taking behaviors. Understanding whether modulation of the mGlu1 receptor activity can also affect drug-seeking and drug-taking in humans could have a significant impact on the future development of medications in this field. We argue that the mGlu1 receptor NAMs have a significant value as potential tools for human experimental pharmacology that could help to validate methods used in preclinical research. Compared with the PAMs, the mGlu1 receptor NAMs appear to be better candidates for this role due to the following: (1) a number of highly potent, selective, and chemically diverse mGlu1 receptor NAMs to choose from; (2) availability of high-quality PET ligands to monitor target exposure; and (3) a rich pharmacological profile with a number of effects that can complement anti-addictive action (e.g., anxiolytic/antidepressant) and may also serve as additional pharmacodynamic readouts during the preclinical-to-clinical translation. We believe that the mGlu1 receptor NAMs have a significant value as potential tools for human experimental pharmacology that could help to validate methods used in preclinical research.

Molecular Basis for Modulation of Metabotropic Glutamate Receptors and Their Drug Actions by Extracellular Ca2

Metabotropic glutamate receptors (mGluRs) associated with the slow phase of the glutamatergic signaling pathway in neurons of the central nervous system have gained importance as drug targets for chronic neurodegenerative diseases. While extracellular Ca²⁺ was reported to exhibit direct activation and modulation via an allosteric site, the identification of those binding sites was challenged by weak binding. Herein, we review the discovery of extracellular Ca²⁺ in regulation of mGluRs, summarize the recent developments in probing Ca²⁺ binding and its co-regulation of the receptor based on structural and biochemical analysis, and discuss the molecular basis for Ca²⁺ to regulate various classes of drug action as well as its importance as an allosteric modulator in mGluRs.

Practical Strategies and Concepts in GPCR Allosteric Modulator Discovery: Recent Advances with Metabotropic Glutamate Receptors

Allosteric modulation of GPCRs has initiated a new era of basic and translational discovery, filled with therapeutic promise yet fraught with caveats. Allosteric ligands stabilize unique conformations of the GPCR that afford fundamentally new receptors, capable of novel pharmacology, unprecedented subtype selectivity, and unique signal bias. This review provides a comprehensive overview of the basics of GPCR allosteric pharmacology, medicinal chemistry, drug metabolism, and validated approaches to address each of the major challenges and caveats. Then, the review narrows focus to highlight recent advances in the discovery of allosteric ligands for metabotropic glutamate receptor subtypes 1-5 and 7 (mGlu1-5,7) highlighting key concepts ("molecular switches", signal bias, heterodimers) and practical solutions to enable the development of tool compounds and clinical candidates. The review closes with a section on late-breaking new advances with allosteric ligands for other GPCRs and emerging data for endogenous allosteric modulators.

Are Type 1 metabotropic glutamate receptors a viable therapeutic target for the treatment of cerebellar ataxia?

The cerebellum is a key brain structure for accurate coordination of sensory and motor function. Compared with other brain regions, the cerebellum expresses a particularly high level of Type 1 metabotropic glutamate receptors (mGluR1). In this review we aim to explore the significance of these receptors for cerebellar synapse function and their potential for treating cerebellar ataxia, a poorly treated degenerative motor disorder that is often hereditary. We find a significant and historical literature showing pivotal mechanisms linking mGluR1 activity with healthy cerebellar synaptic function and motor coordination. This is best illustrated by the impaired motor behaviour in mGluR1 knockout mice that bears strong resemblance to human ataxias. More recent literature also indicates that an imbalance of mGluR1 signalling is as critical as its removal. Too much, as well as too little, mGluR1 activity contributes to ataxia in several clinically relevant mouse models, and perhaps also in humans. Given the availability and ongoing refinement of selective pharmacological tools to either reduce (negative allosteric modulation) or boost (positive allosteric modulation) mGluR1 activity, our findings suggest that pharmacological manipulation of these receptors should be explored as an exciting new approach for the treatment of a variety of human cerebellar ataxias.

[carbonyl-11C]4-Fluoro-N-methyl-N-(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide ([11C]FIMX) is an effective radioligand for PET imaging of metabotropic glutamate receptor 1 (mGluR1) in monkey brain

INTRODUCTION

Metabotropic glutamate subtype receptor 1 (mGluR1) is implicated in several neuropsychiatric disorders and is a target for drug development. [(18)F]FIMX ([(18)F]4-fluoro--N-methyl-N--(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide) is an effective radioligand for imaging brain mGluR1 with PET. A similarly effective radioligand with a shorter half-life would usefully allow PET studies of mGluR1 at baseline and after pharmacological or other challenge on the same day. Here we describe the preparation of [(11)C]FIMX for evaluation in monkey with PET.

METHODS

[(11)C]FIMX was prepared via Pd-promoted carbonylation of 1-fluoro-4-iodobenzene with [(11)C]carbon monoxide, aminolysis of the [(11)C]acyl-palladium complex with the requisite Boc-protected amine, and deprotection with HCl in THF. PET scans of [(11)C]FIMX injected into a monkey were performed at baseline and after preblock of mGluR1 with measurement of the arterial input function.

RESULTS

The radiosynthesis required 42 min and gave [(11)C]FIMX in about 5% overall decay-corrected radiochemical yield and with a specific activity of about 100 GBq/μmol. PET in rhesus monkey at baseline showed that radioactivity peaked high in receptor-rich cerebellum and much lower in receptor-poor occipital cortex. Radioactivity in cerebellum declined to 32% of peak at 85 min. VT at baseline appeared stable in all brain regions after 60 min. Under mGluR1 pre-blocked condition, radioactivity uptake in all regions declined more rapidly to a low level. Receptor pre-block reduced VT from 13.0 to 1.5 in cerebellum and from 2.9 to 1.4 in occipital cortex.

CONCLUSION

[(11)C]FIMX is an effective radioligand for imaging mGluR1 in monkey with PET.

Atypical signaling of metabotropic glutamate receptor 1 in human melanoma cells

The metabotropic glutamate 1 (mGlu1) receptor has emerged as a novel target for the treatment of metastatic melanoma and various other cancers. Our laboratory has demonstrated that a selective, non-competitive mGlu1 receptor antagonist slows human melanoma growth in vitro and in vivo. In this study, we sought to determine if the activation of a canonical G protein-dependent signal transduction cascade, which is often used as an output of mGlu1 receptor activity in neuronal cells, correlated with mGlu1 receptor-mediated melanoma cell viability. Glutamate, the endogenous ligand of mGlu1 receptors, significantly increased melanoma cell viability, but did not stimulate phosphoinositide (PI) hydrolysis in several human melanoma cell lines. In contrast, melanoma cell viability was not increased by quisqualate, a highly potent mGlu1 receptor agonist, or DHPG, a selective group I mGlu receptor agonist. Similarly to glutamate, quisqualate also failed to stimulate PI hydrolysis in mGlu1 receptor-expressing melanoma cells. These results suggest that the canonical G protein-dependent signal transduction cascade is not coupled to mGlu1 receptors in all human melanoma cells. On the other hand, dynamin inhibition selectively decreased viability of mGlu1 receptor-expressing melanoma cells, suggesting that a mechanism requiring internalization may control melanoma cell viability. Taken together, these data demonstrate that the approaches commonly used to study mGlu1 receptor function and signaling in other systems may be inappropriate for studying mGlu1 receptor-mediated melanoma cell viability.

Metabotropic glutamate receptor 1 acts as a dependence receptor creating a requirement for glutamate to sustain the viability and growth of human melanomas

Metabotropic glutamate 1 (mGlu) receptor has been proposed as a target for the treatment of metastatic melanoma. Studies have demonstrated that inhibiting the release of glutamate (the natural ligand of mGlu1 receptors), results in a decrease of melanoma tumor growth in mGlu1 receptor-expressing melanomas. Here we demonstrate that mGlu1 receptors, which have been previously characterized as oncogenes, also behave like dependence receptors by creating a dependence on glutamate for sustained cell viability. In the mGlu1 receptor-expressing melanoma cell lines SK-MEL-2 (SK2) and SK-MEL-5 (SK5), we show that glutamate is both necessary and sufficient to maintain cell viability, regardless of underlying genetic mutations. Addition of glutamate increased DNA synthesis, whereas removal of glutamate not only suppressed DNA synthesis but also promoted cell death in SK2 and SK5 melanoma cells. Using genetic and pharmacological inhibitors, we established that this effect of glutamate is mediated by the activation of mGlu1 receptors. The stimulatory potential of mGlu1 receptors was further confirmed in vivo in a melanoma cell xenograft model. In this model, subcutaneous injection of SK5 cells with short hairpin RNA-targeted downregulation of mGlu1 receptors resulted in a decrease in the rate of tumor growth relative to control. We also demonstrate for the first time that a selective mGlu1 receptor antagonist JNJ16259685 ((3,4-Dihydro-2H-pyrano[2,3-b]quinolin-7-yl)-(cis-4-methoxycyclohexyl)-methanone) slows SK2 and SK5 melanoma tumor growth in vivo. Taken together, these data suggest that pharmacological inhibition of mGlu1 receptors may be a novel approach for the treatment of metastatic melanoma.

Molecular Insights into Metabotropic Glutamate Receptor Allosteric Modulation

The metabotropic glutamate (mGlu) receptors are a group of eight family C G protein-coupled receptors that are expressed throughout the central nervous system (CNS) and periphery. Within the CNS the different subtypes are found in neurons, both pre- and/or postsynaptically, where they mediate modulatory roles and in glial cells. The mGlu receptor family provides attractive targets for numerous psychiatric and neurologic disorders, with the majority of discovery programs focused on targeting allosteric sites, with allosteric ligands now available for all mGlu receptor subtypes. However, the development of allosteric ligands remains challenging. Biased modulation, probe dependence, and molecular switches all contribute to the complex molecular pharmacology exhibited by mGlu receptor allosteric ligands. In recent years we have made significant progress in our understanding of this molecular complexity coupled with an increased understanding of the structural basis of mGlu allosteric modulation.

Pharmacological characterization of mGlu1 receptors in cerebellar granule cells reveals biased agonism

The majority of existing research on the function of metabotropic glutamate (mGlu) receptor 1 focuses on G protein-mediated outcomes. However, similar to other G protein-coupled receptors (GPCR), it is becoming apparent that mGlu1 receptor signaling is multi-dimensional and does not always involve G protein activation. Previously, in transfected CHO cells, we showed that mGlu1 receptors activate a G protein-independent, β-arrestin-dependent signal transduction mechanism and that some mGlu1 receptor ligands were incapable of stimulating this response. Here we set out to investigate the physiological relevance of these findings in a native system using primary cultures of cerebellar granule cells. We tested the ability of a panel of compounds to stimulate two mGlu1 receptor-mediated outcomes: (1) protection from decreased cell viability after withdrawal of trophic support and (2) G protein-mediated phosphoinositide (PI) hydrolysis. We report that the commonly used mGlu1 receptor ligands quisqualate, DHPG, and ACPD are completely biased towards PI hydrolysis and do not induce mGlu1 receptor-stimulated neuroprotection. On the other hand, endogenous compounds including glutamate, aspartate, cysteic acid, cysteine sulfinic acid, and homocysteic acid stimulate both responses. These results show that some commonly used mGlu1 receptor ligands are biased agonists, stimulating only a fraction of mGlu1 receptor-mediated responses in neurons. This emphasizes the importance of utilizing multiple agonists and assays when studying GPCR function.

Epsilon toxin from Clostridium perfringens acts on oligodendrocytes without forming pores, and causes demyelination

Epsilon toxin (ET) is produced by Clostridium perfringens types B and D and causes severe neurological disorders in animals. ET has been observed binding to white matter, suggesting that it may target oligodendrocytes. In primary cultures containing oligodendrocytes and astrocytes, we found that ET (10(-9) M and 10(-7) M) binds to oligodendrocytes, but not to astrocytes. ET induces an increase in extracellular glutamate, and produces oscillations of intracellular Ca(2+) concentration in oligodendrocytes. These effects occurred without any change in the transmembrane resistance of oligodendrocytes, underlining that ET acts through a pore-independent mechanism. Pharmacological investigations revealed that the Ca(2+) oscillations are caused by the ET-induced rise in extracellular glutamate concentration. Indeed, the blockade of metabotropic glutamate receptors type 1 (mGluR1) prevented ET-induced Ca(2+) signals. Activation of the N-methyl-D-aspartate receptor (NMDA-R) is also involved, but to a lesser extent. Oligodendrocytes are responsible for myelinating neuronal axons. Using organotypic cultures of cerebellar slices, we found that ET induced the demyelination of Purkinje cell axons within 24 h. As this effect was suppressed by antagonizing mGluR1 and NMDA-R, demyelination is therefore caused by the initial ET-induced rise in extracellular glutamate concentration. This study reveals the novel possibility that ET can act on oligodendrocytes, thereby causing demyelination. Moreover, it suggests that for certain cell types such as oligodendrocytes, ET can act without forming pores, namely through the activation of an undefined receptor-mediated pathway.

Preclinical evaluation and test-retest studies of [(18)F]PSS232, a novel radioligand for targeting metabotropic glutamate receptor 5 (mGlu5)

PURPOSE

A novel, (18)F-labelled metabotropic glutamate receptor subtype 5 (mGlu5) derivative of [(11)C]ABP688 ([(11)C]1), [(18)F]PSS232 ([(18)F] ]5), was evaluated in vitro and in vivo for its potential as a PET agent and was used in test-retest reliability studies

METHODS

The radiosynthesis of [(18)F]5 was accomplished via a one-step reaction using a mesylate precursor. In vitro stability was determined in PBS and plasma, and with liver microsomal enzymes. Metabolite studies were performed using rat brain extracts, blood and urine. In vitro autoradiography was performed on horizontal slices of rat brain using 1 and 8, antagonists for mGlu5 and mGlu1, respectively. Small-animal PET, biodistribution, and test-retest studies were performed in Wistar rats. In vivo, dose-dependent displacement studies were performed using 6 and blocking studies with 7.

RESULTS

[(18)F]5 was obtained in decay-corrected maximal radiochemical yield of 37 % with a specific activity of 80 - 400 GBq/μmol. Treatment with rat and human microsomal enzymes in vitro for 60 min resulted in 20 % and 4 % of hydrophilic radiometabolites, respectively. No hydrophilic decomposition products or radiometabolites were found in PBS or plasma. In vitro autoradiography on rat brain slices showed a heterogeneous distribution consistent with the known distribution of mGlu5 with high binding to hippocampal and cortical regions, and negligible radioactivity in the cerebellum. Similar distribution of radioactivity was found in PET images. Under displacement conditions with 6, reduced [(18)F]5 binding was found in all brain regions except the cerebellum. 7 reduced binding in the striatum by 84 % on average. Test-retest studies were reproducible with a variability ranging from 6.8 % to 8.2 %. An extended single-dose toxicity study in Wistar rats showed no compound-related adverse effects.

CONCLUSION

The new mGlu5 radiotracer, [(18)F]5, showed specific and selective in vitro and in vivo properties and is a promising radioligand for PET imaging of mGlu5 in humans.

A novel class of succinimide-derived negative allosteric modulators of metabotropic glutamate receptor subtype 1 provides insight into a disconnect in activity between the rat and human receptors

Recent progress in the discovery of mGlu₁ allosteric modulators has suggested the modulation of mGlu₁ could offer possible treatment for a number of central nervous system disorders; however, the available chemotypes are inadequate to fully investigate the therapeutic potential of mGlu₁ modulation. To address this issue, we used a fluorescence-based high-throughput screening assay to screen an allosteric modulator-biased library of compounds to generate structurally diverse mGlu₁ negative allosteric modulator hits for chemical optimization. Herein, we describe the discovery and characterization of a novel mGlu₁ chemotype. This series of succinimide negative allosteric modulators, exemplified by VU0410425, exhibited potent inhibitory activity at rat mGlu₁ but was, surprisingly, inactive at human mGlu₁. VU0410425 and a set of chemically diverse mGlu₁ negative allosteric modulators previously reported in the literature were utilized to examine this species disconnect between rat and human mGlu₁ activity. Mutation of the key transmembrane domain residue 757 and functional screening of VU0410425 and the literature compounds suggests that amino acid 757 plays a role in the activity of these compounds, but the contribution of the residue is scaffold specific, ranging from critical to minor. The operational model of allosterism was used to estimate the binding affinities of each compound to compare to functional data. This novel series of mGlu₁ negative allosteric modulators provides valuable insight into the pharmacology underlying the disconnect between rat and human mGlu₁ activity, an issue that must be understood to progress the therapeutic potential of allosteric modulators of mGlu₁.

Estradiol mediates dendritic spine plasticity in the nucleus accumbens core through activation of mGluR5

Accumulating evidence from human and rodent studies suggests that females are more sensitive to the motivating and rewarding properties of drugs of abuse. Numerous reports implicate estradiol in enhancing drug-related responses in females, yet the neurobiological mechanisms underlying this effect of estradiol are unknown. Because dendritic spine plasticity in the nucleus accumbens (NAc) is linked to the addictive effects of drugs, we examined the influence of estradiol on dendritic spines in this region. Previously our laboratory demonstrated that in female medium spiny neurons, estradiol activates metabotropic glutamate receptor subtype five (mGluR5), a G protein-coupled receptor already implicated in the etiology of drug addiction. Thus, we sought to determine whether mGluR5 is a part of the mechanism by which estradiol affects dendritic spine density in the NAc. To test this hypothesis, ovariectomized female rats were treated with the mGluR5 antagonist, MPEP, or vehicle prior to estradiol (or oil) treatment and 24 h later dendritic spine density was evaluated by DiI labeling and confocal microscopy. We found that estradiol decreased dendritic spine density in the NAc core and that pretreatment with MPEP blocked this effect. In contrast, MPEP had no effect on dendritic spine density in the NAc shell or CA1 region of the hippocampus, two regions in which estradiol increased the density of dendritic spines. As dendritic spine plasticity in the NAc core has behavioral consequences for drug addiction, these data provide a clue as to how estradiol acts in females to enhance behavioral responses to drugs of abuse.

Progress toward advanced understanding of metabotropic glutamate receptors: structure, signaling and therapeutic indications

The metabotropic glutamate (mGlu) receptors are a group of Class C seven-transmembrane spanning/G protein-coupled receptors (7TMRs/GPCRs). These receptors are activated by glutamate, one of the standard amino acids and the major excitatory neurotransmitter. By activating G protein-dependent and non-G protein-dependent signaling pathways, mGlus modulate glutamatergic transmission both in the periphery and throughout the central nervous system. Since the discovery of the first mGlu receptor, and especially during the last decade, a great deal of progress has been made in understanding the signaling, structure, pharmacological manipulation and therapeutic indications of the 8 mGlu members.

Molecular recognition of metabotropic glutamate receptor type 1 (mGluR1): synergistic understanding with free energy perturbation and linear response modeling

Metabotropic glutamate receptors (mGluRs) constitute an important family of the G-protein coupled receptors. Due to their widespread distribution in the central nervous system (CNS), these receptors are attractive candidates for understanding the molecular basis of various cognitive processes as well as for designing inhibitors for relevant psychiatric and neurological disorders. Despite many studies on drugs targeting the mGluR receptors to date, the molecular level details on the ligand binding dynamics still remain unclear. In this study, we performed in silico experiments for mGluR1 with 29 different ligands including known synthetic agonists and antagonists as well as natural amino acids. The ligand-receptor binding affinities were estimated by the use of atomistic simulations combined with the mathematically rigorous, Free Energy Perturbation (FEP) method, which successfully recognized the native agonist l-glutamate among the highly favorable binders, and also accurately distinguished antagonists from agonists. Comparative contact analysis also revealed the binding mode differences between natural and non-natural amino acid-based ligands. Several factors potentially affecting the ligand binding affinity and specificity were identified including net charges, dipole moments, and the presence of aromatic rings. On the basis of these findings, linear response models (LRMs) were built for different sets of ligands that showed high correlations (R(2) > 0.95) to the corresponding FEP binding affinities. These results identify some key factors that determine ligand-mGluR1 binding and could be used for future inhibitor designs and support a role for in silico modeling for understanding receptor ligand interactions.

Structure of a class C GPCR metabotropic glutamate receptor 1 bound to an allosteric modulator

The excitatory neurotransmitter glutamate induces modulatory actions via the metabotropic glutamate receptors (mGlus), which are class C G protein-coupled receptors (GPCRs). We determined the structure of the human mGlu1 receptor seven-transmembrane (7TM) domain bound to a negative allosteric modulator, FITM, at a resolution of 2.8 angstroms. The modulator binding site partially overlaps with the orthosteric binding sites of class A GPCRs but is more restricted than most other GPCRs. We observed a parallel 7TM dimer mediated by cholesterols, which suggests that signaling initiated by glutamate's interaction with the extracellular domain might be mediated via 7TM interactions within the full-length receptor dimer. A combination of crystallography, structure-activity relationships, mutagenesis, and full-length dimer modeling provides insights about the allosteric modulation and activation mechanism of class C GPCRs.

The Concise Guide to PHARMACOLOGY 2013/14: G protein-coupled receptors

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. G protein-coupled receptors are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.