Metabotropic glutamate receptors as drug targets: what's new?

A "double-edged" role for type-5 metabotropic glutamate receptors in pain disclosed by light-sensitive drugs

Knowing the site of drug action is important to optimize effectiveness and address any side effects. We used light-sensitive drugs to identify the brain region-specific role of mGlu5 metabotropic glutamate receptors in the control of pain. Optical activation of systemic JF-NP-26, a caged, normally inactive, negative allosteric modulator (NAM) of mGlu5 receptors, in cingulate, prelimbic and infralimbic cortices and thalamus inhibited neuropathic pain hypersensitivity. Systemic treatment of alloswitch-1, an intrinsically active mGlu5 receptor NAM, caused analgesia, and the effect was reversed by light-induced drug inactivation in in the prelimbic and infralimbic cortices, and thalamus. This demonstrates that mGlu5 receptor blockade in the medial prefrontal cortex and thalamus is both sufficient and necessary for the analgesic activity of mGlu5 receptor antagonists. Surprisingly, when light was delivered in the basolateral amygdala, local activation of systemic JF-NP-26 reduced pain thresholds, whereas inactivation of alloswitch-1 enhanced analgesia. Electrophysiological analysis showed that alloswitch-1 increased excitatory synaptic responses in prelimbic pyramidal neurons evoked by stimulation of BLA input, and decreased feedforward inhibition of amygdala output neurons by BLA. Both effects were reversed by optical silencing and reinstated by optical reactivation of alloswitch-1. These findings demonstrate for the first time that the action of mGlu5 receptors in the pain neuraxis is not homogenous, and suggest that blockade of mGlu5 receptors in the BLA may limit the overall analgesic activity of mGlu5 receptor antagonists. This could explain the suboptimal effect of mGlu5 NAMs on pain in human studies and validate photopharmacology as an important tool to determine ideal target sites for systemic drugs.

The neuropsychopharmacology of acetyl-L-carnitine (LAC): basic, translational and therapeutic implications

Mitochondrial metabolism can contribute to nuclear histone acetylation among other epigenetic mechanisms. A central aspect of this signaling pathway is acetyl-L-carnitine (LAC), a pivotal mitochondrial metabolite best known for its role in fatty acid oxidation. Work from our and other groups suggested LAC as a novel epigenetic modulator of brain plasticity and a therapeutic target for clinical phenotypes of depression linked to childhood trauma. Aberrant mitochondrial metabolism of LAC has also been implicated in the pathophysiology of Alzheimer's disease. Furthermore, mitochondrial dysfunction is linked to other processes implicated in the pathophysiology of both major depressive disorders and Alzheimer's disease, such as oxidative stress, inflammation, and insulin resistance. In addition to the rapid epigenetic modulation of glutamatergic function, preclinical studies showed that boosting mitochondrial metabolism of LAC protects against oxidative stress, rapidly ameliorates insulin resistance, and reduces neuroinflammation by decreasing proinflammatory pathways such as NFkB in hippocampal and cortical neurons. These basic and translational neuroscience findings point to this mitochondrial signaling pathway as a potential target to identify novel mechanisms of brain plasticity and potential unique targets for therapeutic intervention targeted to specific clinical phenotypes.

Cholesterol in Class C GPCRs: Role, Relevance, and Localization

G-protein coupled receptors (GPCRs), one of the largest superfamilies of cell-surface receptors, are heptahelical integral membrane proteins that play critical roles in virtually every organ system. G-protein-coupled receptors operate in membranes rich in cholesterol, with an imbalance in cholesterol level within the vicinity of GPCR transmembrane domains affecting the structure and/or function of many GPCRs, a phenomenon that has been linked to several diseases. These effects of cholesterol could result in indirect changes by altering the mechanical properties of the lipid environment or direct changes by binding to specific sites on the protein. There are a number of studies and reviews on how cholesterol modulates class A GPCRs; however, this area of study is yet to be explored for class C GPCRs, which are characterized by a large extracellular region and often form constitutive dimers. This review highlights specific sites of interaction, functions, and structural dynamics involved in the cholesterol recognition of the class C GPCRs. We summarize recent data from some typical family members to explain the effects of membrane cholesterol on the structural features and functions of class C GPCRs and speculate on their corresponding therapeutic potential.

Roles of metabotropic glutamate receptor 8 in neuropsychiatric and neurological disorders

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors. Among eight mGlu subtypes (mGlu1-8), mGlu8 has drawn increasing attention. This subtype is localized to the presynaptic active zone of neurotransmitter release and is among the mGlu subtypes with high affinity for glutamate. As a G_i/o-coupled autoreceptor, mGlu8 inhibits glutamate release to maintain homeostasis of glutamatergic transmission. mGlu8 receptors are expressed in limbic brain regions and play a pivotal role in modulating motivation, emotion, cognition, and motor functions. Emerging evidence emphasizes the increasing clinical relevance of abnormal mGlu8 activity. Studies using mGlu8 selective agents and knockout mice have revealed the linkage of mGlu8 receptors to multiple neuropsychiatric and neurological disorders, including anxiety, epilepsy, Parkinson's disease, drug addiction, and chronic pain. Expression and function of mGlu8 receptors in some limbic structures undergo long-lasting adaptive changes in animal models of these disorders, which may contribute to the remodeling of glutamatergic transmission critical for the pathogenesis and symptomatology of brain illnesses. This review summarizes the current understanding of mGlu8 biology and the possible involvement of the receptor in several common psychiatric and neurological disorders.

Postweaning social isolation and autism-like phenotype: a biochemical and behavioral comparative analysis

Adolescence is a critical period for brain development. In most mammalian species, disturbances experienced during adolescence constitute a risk factor for several neuropsychiatric disorders. In this study, we compared the biochemical and behavioral profile induced by postweaning social isolation (PWSI) in inbred C57BL/6N mice with that of BTBR mice, a rodent model of autism spectrum disorders. Male C57BL/6N mice were either housed in groups of four or isolated from weaning (postnatal day 21) for four weeks before experimental analyses. After weaning, male BTBR mice were housed four per cage and analyzed at 48 days of age. PWSI reduced hippocampal levels of type 2 metabotropic glutamate (mGlu2) receptors, and glucocorticoid and mineralocorticoid receptors. A similar reduction was seen in group-housed BTBR mice. Plasma corticosterone levels in basal conditions were not influenced by PWSI, but were increased in BTBR mice. Social investigation (total and head sniffing) and the number of ultrasonic vocalizations were reduced in both PWSI mice and age-matched group-housed BTBR mice, indicating a lower social responsiveness in both groups of mice. These results suggest that absence of social stimuli during adolescence induces an endophenotype with social deficit features, which mimics the phenotype of a mouse model of autism spectrum disorders.

Multidimensional predictors of antidepressant responses: Integrating mitochondrial, genetic, metabolic and environmental factors with clinical outcomes

Major depressive disorder (MDD) is a primary psychiatric illness worldwide; there is a dearth of new mechanistic models for the development of better therapeutic strategies. Although we continue to discover individual biological factors, a major challenge is the identification of integrated, multidimensional traits underlying the complex heterogeneity of depression and treatment outcomes. Here, we set out to ascertain the emergence of the novel mitochondrial mediator of epigenetic function acetyl-L-carnitine (LAC) in relation to previously described individual predictors of antidepressant responses to the insulin-sensitizing agent pioglitazone. Herein, we report that i) subjects with MDD and shorter leukocyte telomere length (LTL) show decreased levels of LAC, increased BMI, and a history of specific types of childhood trauma; and that ii) these multidimensional factors spanning mitochondrial metabolism, cellular aging, metabolic function, and childhood trauma provide more detailed signatures to predict longitudinal changes in depression severity in response to pioglitazone than individual factors. The findings of multidimensional signatures involved in the pathophysiology of depression and their role in predicting treatment outcomes provide a starting point for the development of a mechanistic framework linking biological networks and environmental factors to clinical outcomes in pursuit of personalized medicine strategies to effectively treat MDD.

The Effects of N-Acetylcysteine on the Rat Mesocorticolimbic Pathway: Role of mGluR5 Receptors and Interaction with Ethanol

N-acetylcysteine (NAC) is a prodrug that is marketed as a mucolytic agent and used for the treatment of acetaminophen overdose. Over the last few decades, evidence has been gathered that suggests the potential use of NAC as a new pharmacotherapy for alcohol use disorder (AUD), although its mechanism of action is already being debated. In this paper, we set out to assess both the potential involvement of the glutamate metabotropic receptors (mGluR) in the possible dual effect of NAC administered at two different doses and NAC's effect on ethanol-induced activation. To this aim, 30 or 120 mg/kg of NAC was intraperitoneally administered to rats with the presence or absence of the negative allosteric modulator of mGluR5 (MTEP 0.1 mg/kg). Thereafter, the cFOS IR-cell expression was analyzed. Secondly, we explored the effect of 120 mg/kg of NAC on the neurochemical and behavioral activation induced by intra-VTA ethanol administration (150 nmol). Our results showed that the high NAC dose stimulated cFOS expression in the NAcc, and that this effect was suppressed in the presence of MTEP, thus suggesting the implication of mGluR5. Additionally, high doses could attenuate the ethanol-induced increase in cFOS-expression in the NAcc, probably due to a phenomenon based on the long-term depression of the MSNs. Additional experiments are required to corroborate our hypothesis.

Presynaptic Release-regulating Metabotropic Glutamate Receptors: An Update

Metabotropic glutamate (mGlu) receptors represent the largest family of glutamate receptors in mammals and act as fine tuners of the chemical transmission in central nervous system (CNS). In the last decade, results concerning the expression and the subcellular localization of mGlu receptors further clarified their role in physio-pathological conditions. Concomitantly, their pharmacological characterization largely improved thanks to the identification of new compounds (chemical ligands and antibodies recognizing epitopic sequences of the receptor proteins) that allowed to decipher the protein compositions of the naive receptors. mGlu receptors are expressed at the presynaptic site of chemical synapses. Here, they modulate intraterminal enzymatic pathways controlling the migration and the fusion of vesicles to synaptic membranes as well as the phosphorylation of colocalized receptors. Both the control of transmitter exocytosis and the phosphorylation of colocalized receptors elicited by mGlu receptors are relevant events that dictate the plasticity of nerve terminals, and account for the main role of presynaptic mGlu receptors as modulators of neuronal signalling. The role of the presynaptic mGlu receptors in the CNS has been the matter of several studies and this review aims at briefly summarizing the recent observations obtained with isolated nerve endings (we refer to as synaptosomes). We focus on the pharmacological characterization of these receptors and on their receptor-receptor interaction / oligo-dimerization in nerve endings that could be relevant to the development of new therapeutic approaches for the cure of central pathologies.

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.

Group II Metabotropic Glutamate Receptors Modulate Sound Evoked and Spontaneous Activity in the Mouse Inferior Colliculus

Little is known about the functions of Group II metabotropic glutamate receptors (mGluRs2/3) in the inferior colliculus (IC), a midbrain structure that is a major integration region of the central auditory system. We investigated how these receptors modulate sound-evoked and spontaneous firing in the mouse IC in vivo. We first performed immunostaining and tested hearing thresholds to validate vesicular GABA transporter (VGAT)-ChR2 transgenic mice on a mixed CBA/CaJ x C57BL/6J genetic background. Transgenic animals allowed for optogenetic cell-type identification. Extracellular single neuron recordings were obtained before and after pharmacological mGluR2/3 activation. We observed increased sound-evoked firing, as assessed by the rate-level functions (RLFs), in a subset of both GABAergic and non-GABAergic IC neurons following mGluR2/3 pharmacological activation. These neurons also displayed elevated spontaneous excitability and were distributed throughout the IC area tested, suggesting a widespread mGluR2/3 distribution in the mouse IC.

Cholesterol Localization around the Metabotropic Glutamate Receptor 2

The metabotropic glutamate receptor (mGluR) 2 plays a key role in the central nervous system. mGluR2 has been shown to be regulated by its surrounding lipid environment, especially by cholesterol, by an unknown mechanism. Here, using a combination of biochemical approaches, photo-cross-linking experiments, and molecular dynamics simulations we show the interaction of cholesterol with at least two, but potentially five more, preferential sites on the mGluR2 transmembrane domain. Our simulations demonstrate that surface matching, rather than electrostatic interactions with specific amino acids, is the main factor defining cholesterol localization. Moreover, the cholesterol localization observed here is similar to the sterol-binding pattern previously described in silico for other members of the mGluR family. Biochemical assays suggest little influence of cholesterol on trafficking or dimerization of mGluR2. Nevertheless, simulations revealed a significant reduction of residue-residue contacts together with an alteration in the internal mechanical stress at the cytoplasmic side of the helical bundle when cholesterol was present in the membrane. These alterations may be related to destabilization of the basal state of mGluR2. Due to the high sequence conservation of the transmembrane domains of mGluRs, the molecular interaction of cholesterol and mGluR2 described here is also likely to be relevant for other members of the mGLuR family.

N-Acetylaspartylglutamate (NAAG) Pretreatment Reduces Hypoxic-Ischemic Brain Damage and Oxidative Stress in Neonatal Rats

N-acetylaspartylglutamate (NAAG), the most abundant peptide transmitter in the mammalian nervous system, activates mGluR3 at presynaptic sites, inhibiting the release of glutamate, and acts on mGluR3 on astrocytes, stimulating the release of neuroprotective growth factors (TGF-β). NAAG can also affect N-methyl-d-aspartate (NMDA) receptors in both synaptic and extrasynaptic regions. NAAG reduces neurodegeneration in a neonatal rat model of hypoxia-ischemia (HI), although the exact mechanism is not fully recognized. In the present study, the effect of NAAG application 24 or 1 h before experimental birth asphyxia on oxidative stress markers and the potential mechanisms of neuroprotection on 7-day old rats was investigated. The intraperitoneal application of NAAG at either time point before HI significantly reduced the weight deficit of the ischemic brain hemisphere, radical oxygen species (ROS) content and activity of antioxidant enzymes, and increased the concentration of reduced glutathione (GSH). No additional increase in the TGF-β concentration was observed after NAAG application. The fast metabolism of NAAG and the decrease in TGF-β concentration that resulted from NAAG pretreatment, performed up to 24 h before HI, excluded the involvement mGluR3 in neuroprotection. The observed effect may be explained by the activation of NMDA receptors induced by NAAG pretreatment 24 h before HI. Inhibition of the NAAG effect by memantine supports this conclusion. NAAG preconditioning 1 h before HI results in a mixture of mGluR3 and NMDA receptor activation. Preconditioning with NAAG induces the antioxidative defense system triggered by mild excitotoxicity in neurons. Moreover, this response to NAAG pretreatment is consistent with the commonly accepted mechanism of preconditioning. However, this theory requires further investigation.

Small-Molecule Ligands that Bind the RET Receptor Activate Neuroprotective Signals Independent of but Modulated by Coreceptor GFRα1

Glial cell line-derived neurotrophic factor (GDNF) binds the GFRα1 receptor, and the GDNF-GFRα1 complex binds to and activates the transmembrane RET tyrosine kinase to signal through intracellular Akt/Erk pathways. To dissect the GDNF-GFRα1-RET signaling complex, agents that bind and activate RET directly and independently of GFRα1 expression are valuable tools. In a focused naphthalenesulfonic acid library from the National Cancer Institute database, we identified small molecules that are genuine ligands binding to the RET extracellular domain. These ligands activate RET tyrosine kinase and afford trophic signals irrespective of GFRα1 coexpression. However, RET activation by these ligands is constrained by GFRα1, likely via an allosteric mechanism that can be overcome by increasing RET ligand concentration. In a mouse model of retinitis pigmentosa, monotherapy with a small-molecule RET agonist activates survival signals and reduces neuronal death significantly better than GDNF, suggesting therapeutic potential. SIGNIFICANCE STATEMENT: A genuine ligand of RET receptor ectodomain was identified, which acts as an agonist. Binding and agonism are independent of a coreceptor glial cell line-derived neurotrophic factor family receptor α, which is required by the natural growth factor glial cell line-derived neurotrophic factor, and are selective for cells expressing RET. The lead agent protects neurons from death in vivo. This work validates RET receptor as a druggable therapeutic target and provides for potential leads to evaluate in neurodegenerative states. We also report problems that arise when screening chemical libraries.

Detailed In Vitro Pharmacological Characterization of Clinically Tested Negative Allosteric Modulators of the Metabotropic Glutamate Receptor 5

Negative allosteric modulation of the metabotropic glutamate 5 (mGlu₅) receptor has emerged as a potential strategy for the treatment of neurologic disorders. Despite the success in preclinical studies, many mGlu₅ negative allosteric modulators (NAMs) that have reached clinical trials failed due to lack of efficacy. In this study, we provide a detailed in vitro pharmacological characterization of nine clinically and preclinically tested NAMs. We evaluated inhibition of l-glutamate-induced signaling with Ca²⁺ mobilization, inositol monophosphate (IP₁) accumulation, extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation, and real-time receptor internalization assays on rat mGlu₅ expressed in HEK293A cells. Moreover, we determined association rates (k_on) and dissociation rates (k_off), as well as NAM affinities with [³H]methoxy-PEPy binding experiments. k_on and k_off values varied greatly between the nine NAMs (34- and 139-fold, respectively) resulting in long receptor residence times (>400 min) for basimglurant and mavoglurant, medium residence times (10-30 min) for AZD2066, remeglurant, and (RS)-remeglurant, and low residence times (<10 mins) for dipraglurant, F169521, F1699611, and STX107. We found that all NAMs inhibited l-glutamate-induced mGlu₅ receptor internalization, generally with a similar potency to IP₁ accumulation and ERK1/2 phosphorylation, whereas Ca²⁺ mobilization was less potently inhibited. Operational model of allosterism analyses revealed that dipraglurant and (RS)-remeglurant were biased toward (affinity) receptor internalization and away (cooperativity) from the ERK1/2 phosphorylation pathway, respectively. Our study is the first to measure mGlu₅ NAM binding kinetics and negative allosteric modulation of mGlu₅ receptor internalization and adds significant new knowledge about the molecular pharmacology of a diverse range of clinically relevant NAMs. SIGNIFICANCE STATEMENT: The metabotropic glutamate 5 (mGlu₅) receptor is important in many brain functions and implicated in several neurological pathologies. Negative allosteric modulators (NAMs) have shown promising results in preclinical models but have so far failed in human clinical trials. Here we provide the most comprehensive and comparative molecular pharmacological study to date of nine preclinically/clinically tested NAMs at the mGlu₅ receptor, which is also the first study to measure ligand binding kinetics and negative allosteric modulation of mGlu₅ receptor internalization.

Exploring the Activation Mechanism of the mGlu5 Transmembrane Domain

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

mGlu5 in GABAergic neurons modulates spontaneous and psychostimulant-induced locomotor activity

RATIONALE

A role of group I metabotropic glutamate receptor 5 (mGlu5) in regulating spontaneous locomotion and psychostimulant-induced hyperactivity has been proposed.

OBJECTIVES

This study aims to determine if mGlu5 in GABAergic neurons regulates spontaneous or psychostimulant-induced locomotion.

METHODS

We generated mice specifically lacking mGlu5 in forebrain GABAergic neuron by crossing DLX-Cre mice with mGlu5^flox/flox mice to generate DLX-mGlu5 KO mice. The locomotion of adult mice was examined in the open-field assay (OFA) and home cage setting. The effects of the mGlu5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP), cocaine, and methylphenidate on acute motor behaviors in DLX-mGlu5 KO and littermate control mice were assessed in OFA. Striatal synaptic plasticity of these mice was examined with field potential electrophysiological recordings.

RESULTS

Deleting mGlu5 from forebrain GABAergic neurons results in failure to induce long-term depression (LTD) in the dorsal striatum and absence of habituated locomotion in both novel and familiar settings. In a familiar environment (home cage), DLX-mGlu5 KO mice were hyperactive. In the OFA, DLX-mGlu5 KO mice exhibited initial hypo-activity, and then gradually increased their locomotion with time, resulting in no habituation response. DLX-mGlu5 KO mice exhibited almost no locomotor response to MPEP (40 mg/kg), while the same dose elicited hyperlocomotion in control mice. The DLX-mGlu5 KO mice also showed reduced hyperactivity response to cocaine, while they retained normal hyperactivity response to methylphenidate, albeit with delayed onset.

CONCLUSION

mGlu5 in forebrain GABAergic neurons is critical to trigger habituation upon the initiation of locomotion as well as to mediate MPEP-induced hyperlocomotion and modulate psychostimulant-induced hyperactivity.

Dopaminergic control of ADAMTS2 expression through cAMP/CREB and ERK: molecular effects of antipsychotics

A better understanding of the molecular mechanisms that participate in the development and clinical manifestations of schizophrenia can lead to improve our ability to diagnose and treat this disease. Previous data strongly associated the levels of deregulated ADAMTS2 expression in peripheral blood mononuclear cells (PBMCs) from patients at first episode of psychosis (up) as well as in clinical responders to treatment with antipsychotic drugs (down). In this current work, we performed an independent validation of such data and studied the mechanisms implicated in the control of ADAMTS2 gene expression. Using a new cohort of drug-naïve schizophrenia patients with clinical follow-up, we confirmed that the expression of ADAMTS2 was highly upregulated in PBMCs at the onset (drug-naïve patients) and downregulated, in clinical responders, after treatment with antipsychotics. Mechanistically, ADAMTS2 expression was activated by dopaminergic signalling (D1-class receptors) and downstream by cAMP/CREB and mitogen-activated protein kinase (MAPK)/ERK signalling. Incubation with antipsychotic drugs and selective PKA and MEK inhibitors abrogated D1-mediated activation of ADAMTS2 in neuronal-like cells. Thus, D1 receptors signalling towards CREB activation might participate in the onset and clinical responses to therapy in schizophrenia patients, by controlling ADAMTS2 expression and activity. The unbiased investigation of molecular mechanisms triggered by antipsychotic drugs may provide a new landscape of novel targets potentially associated with clinical efficacy.

Group I metabotropic glutamate receptors in the primate motor thalamus: subsynaptic association with cortical and sub-cortical glutamatergic afferents

Preclinical evidence indicates that mGluR5 is a potential therapeutic target for Parkinson's disease and L-DOPA-induced dyskinesia. However, the mechanisms through which these therapeutic benefits are mediated remain poorly understood. Although the regulatory role of mGluR5 on glutamatergic transmission has been examined in various basal ganglia nuclei, very little is known about the localization and function of mGluR5 in the ventral motor and intralaminar thalamic nuclei, the main targets of basal ganglia output in mammals. Thus, we used immuno-electron microscopy to map the cellular and subcellular localization of group I mGluRs (mGluR1a and mGluR5) in the ventral motor and caudal intralaminar thalamic nuclei in rhesus monkeys. Furthermore, using double immuno-electron microscopy, we examined the subsynaptic localization of mGluR5 in relation to cortical and sub-cortical glutamatergic afferents. Four major conclusions can be drawn from these data. First, mGluR1a and mGluR5 are expressed postsynaptically on the plasma membrane of dendrites of projection neurons and GABAergic interneurons in the basal ganglia- and cerebellar-receiving regions of the ventral motor thalamus and in CM. Second, the plasma membrane-bound mGluR5 immunoreactivity is preferentially expressed perisynaptically at the edges of cortical and sub-cortical glutamatergic afferents. Third, the mGluR5 immunoreactivity is more strongly expressed in the lateral than the medial tiers of CM, suggesting a preferential association with thalamocortical over thalamostriatal neurons in the primate CM. Overall, mGluR5 is located to subserve powerful modulatory role of cortical and subcortical glutamatergic transmission in the primate ventral motor thalamus and CM.

The developmental expression of metabotropic glutamate receptor 4 in prenatal human frontal lobe and neurogenesis regions

BACKGROUNDS

Metabotropic glutamate receptors, besides ionotropic receptors, mediate the complicated effect of glutamate on neurogenesis. Previous studies showed that metabotropic glutamate receptor 4 (mGluR4) regulated the proliferation and differentiation of neural stem/progenitor cells in vitro. However, little is known about the expression pattern of mGluR4 on prenatal central nervous system in vivo, especially the human being.

METHODS

The normal brain tissues of human fetus were collected and divided into 4 groups according to the gestational age: 9-11 W, 14-16 W, 22-24 W and 32-36 W. Then the expression of mGluR4 was evaluated at mRNA and protein levels by means of PCR or immunohistochemistry method, respectively. The type of cell expressing mGluR4 was further investigated using double-labeling immunofluorescence.

RESULTS

RT-PCR showed that the mRNA of mGluR4 could be detected in frontal lobe from 9 W to 32 W and real-time PCR quantificationally demonstrated the mRNA increased with development. Similarly, immnoreactivity was found in all layers of frontal lobe, VZ/SVZ. The intensity scores analysis showed that the staining became stronger and the range extended gradually with development. The double-labeling immunofluorescence showed that mGluR4 was present in neural stem/progenitor cells (nestin-positive cells after 9 W), young neurons (DCX-positive cells after 9 W), mature neurons (NeuN-positive cells in cortex after 32 W), as well as typical astrocytes (GFAP-positive cells in medulla after 32 W).

CONCLUSION

These results supply an important evidence that mGluR4 is expressed in prenatal human cerebrum, and main kinds of cells related to neurogenesis are involved in its expression.

Shedding Light on Metabotropic Glutamate Receptors

In this issue of Cell Chemical Biology, the elegant manuscript by Rovira et al. (2016) describes a negative allosteric modulator (NAM) of mGlu4 metabotropic glutamate receptors with in vivo activity. This compound is rapidly and reversibly inactivated by light and represents a powerful pharmacological tool for the study of mGlu4 receptors in their native environment.

mGlu5 Positive Allosteric Modulators Facilitate Long-Term Potentiation via Disinhibition Mediated by mGlu5-Endocannabinoid Signaling

Metabotropic glutamate (mGlu) receptor type 5 (mGlu₅) positive allosteric modulators (PAMs) enhance hippocampal long-term potentiation (LTP) and have cognition-enhancing effects in animal models. These effects were initially thought to be mediated by potentiation of mGlu₅ modulation of N-methyl-d-aspartate receptor (NMDAR) currents. However, a biased mGlu₅ PAM that potentiates Gα_q-dependent mGlu₅ signaling, but not mGlu₅ modulation of NMDAR currents, retains cognition-enhancing effects in animal models, suggesting that potentiation of NMDAR currents is not required for these in vivo effects of mGlu₅ PAMs. However, it is not clear whether the potentiation of NMDAR currents is critical for the ability of mGlu₅ PAMs to enhance hippocampal LTP. We now report the characterization of effects of two structurally distinct mGlu₅ PAMs, VU-29 and VU0092273, on NMDAR currents and hippocampal LTP. As with other mGlu₅ PAMs that do not display observable bias for potentiation of NMDAR currents, VU0092273 enhanced both mGlu₅ modulation of NMDAR currents and induction of LTP at the hippocampal Schaffer collateral (SC)-CA1 synapse. In contrast, VU-29 did not potentiate mGlu₅ modulation of NMDAR currents but induced robust potentiation of hippocampal LTP. Interestingly, both VU-29 and VU0092273 suppressed evoked inhibitory postsynaptic currents (eIPSCs) in CA1 pyramidal cells, and this effect was blocked by the cannabinoid receptor type 1 (CB1) antagonist AM251. Furthermore, AM251 blocked the ability of both mGlu₅ PAMs to enhance LTP. Finally, both PAMs failed to enhance LTP in mice with the restricted genetic deletion of mGlu₅ in CA1 pyramidal cells. Taken together with previous findings, these results suggest that enhancement of LTP by mGlu₅ PAMs does not depend on mGlu₅ modulation of NMDAR currents but is mediated by a previously established mechanism in which mGlu₅ in CA1 pyramidal cells induces endocannabinoid release and CB1-dependent disinhibition.

Assessment of mGluR5 KO mice under conditions of low stress using a rodent touchscreen apparatus reveals impaired behavioural flexibility driven by perseverative responses

Genetic and pharmacological manipulations targeting metabotropic glutamate receptor 5 (mGluR5) affect performance in behavioural paradigms that depend on cognitive flexibility. Many of these studies involved exposing mice to highly stressful conditions including electric foot shocks or water immersion and forced swimming. Because mGluR5 is also implicated in resilience and stress responses, however, apparent impairments in inhibitory learning may have been an artifact of manipulation-induced changes in affective state. To address this, we present here a characterization of cognitive flexibility in mGluR5 knockout (KO) mice conducted with a rodent touchscreen cognitive assessment apparatus in which the animals experience significantly less stress.

Our results indicate a significant reversal learning impairment relative to wild-type (WT) controls in the two-choice Visual Discrimination-Reversal (VDR) paradigm. Upon further analysis, we found that this deficit is primarily driven by a prolonged period of perseveration in the early phase of reversal. We also observed a similar perseveration phenotype in the KO mice in the Extinction (EXT) paradigm. In addition, mGluR5 KO mice show higher breakpoints in the touchscreen Progressive Ratio (PR) and altered decision making in the Effort-related Choice (ERC) tasks. Interestingly, this impairment in PR is an additional manifestation of an increased propensity to perseverate on the emission of relatively simplistic behavioural outputs.

Together, these findings suggest that under conditions of low stress, mGluR5 KO mice exhibit a pronounced perseverative phenotype that blunts cognitive flexibility.

Targeting metabotropic glutamate receptors in the treatment of epilepsy: rationale and current status

INTRODUCTION

Several drugs targeting the GABAergic system are used in the treatment of epilepsy, but only one drug targeting glutamate receptors is on the market. This is surprising because an imbalance between excitatory and inhibitory neurotransmission lies at the core of the pathophysiology of epilepsy. One possible explanation is that drug development has been directed towards the synthesis of molecules that inhibit the activity of ionotropic glutamate receptors. These receptors mediate fast excitatory synaptic transmission in the central nervous system (CNS) and their blockade may cause severe adverse effects such as sedation, cognitive impairment, and psychotomimetic effects. Metabotropic glutamate (mGlu) receptors are more promising drug targets because these receptors modulate synaptic transmission rather than mediate it. Areas covered: We review the current evidence that links mGlu receptor subtypes to the pathophysiology and experimental treatment of convulsive and absence seizures. Expert opinion: While mGlu₅ receptor negative allosteric modulators have the potential to be protective against convulsive seizures and hyperactivity-induced neurodegeneration, drugs that enhance mGlu₅ and mGlu₇ receptor function may have beneficial effects in the treatment of absence epilepsy. Evidence related to the other mGlu receptor subtypes is more fragmentary; further investigations are required for an improved understanding of their role in the generation and propagation of seizures.

Computational Drug Design Applied to the Study of Metabotropic Glutamate Receptors

Metabotropic glutamate (mGlu) receptors are a family of eight GPCRs that are attractive drug discovery targets to modulate glutamate action and response. Here we review the application of computational methods to the study of this family of receptors. X-ray structures of the extracellular and 7-transmembrane domains have played an important role to enable structure-based modeling approaches, whilst we also discuss the successful application of ligand-based methods. We summarize the literature and highlight the areas where modeling and experiment have delivered important understanding for mGlu receptor drug discovery. Finally, we offer suggestions of future areas of opportunity for computational work.

Electron Acceptive Mass Tag for Mass Spectrometric Imaging-Guided Synergistic Targeting to Mice Brain Glutamate Receptors

Dysfunctional glutamate receptors (GluRs) have been implicated in neurological disorders and injuries. Hetero-tetrameric assemblies of different GluR subunits or splicing variants have distinct spatiotemporal expression patterns and pharmacological properties. Mass spectrometric imaging of GluRs-targeted small molecules is important for determining the regional preferences of these compounds. We report herein the development of a mass tag covalently bonded with glutamate or N-methyl-d-aspartate that functions as both an electron acceptor to generate mass spectrometric signals on irradiated (Bi₂O₃)_0.07(CoO)_0.03(ZnO)_0.9 nanoparticles with the third harmonic (355 nm) of Nd³⁺:YAG laser and as the core component to target bilobed clamshell-like structures of GluRs. In this approach, different molecules produce the same tag ion. It provides a new avenue for quantitative assessment of spatial densities of different compounds, which cannot be achieved with well-established stable isotope labeling technique due to different ionization efficiency of different compounds. Various coexisting endogenous molecules are also simultaneously detected for investigation of overall physiological changes induced by these compounds. Because semiconductors do not generate background peaks, this method eliminates interferences from organic matrix materials that are used in regular MALDI (matrix assisted laser desorption ionization). The localized ionization provides high spatial resolution that can be down to sub-micrometers.

Glutamatergic Deficits in Schizophrenia - Biomarkers and Pharmacological Interventions within the Ketamine Model

Background:

The observation that N-methyl-D-aspartate glutamate receptor (NMDAR) antagonists such as ketamine transiently induce schizophrenia-like positive, negative and cognitive symptoms has led to a paradigm shift from dopaminergic to glutamatergic dysfunction in pharmacological models of schizophrenia. NMDAR hypofunction can explain many schizophrenia symptoms directly due to excitatory-to-inhibitory (E/I) imbalance, but also dopaminergic dysfunction itself. However, so far no new drug targeting the NMDAR has been successfully approved. In the search for possible biomarkers it is interesting that ketamine-induced psychopathological changes in healthy participants were accompanied by altered electro-(EEG), magnetoencephalographic (MEG) and functional magnetic resonance imaging (fMRI) signals.

Methods:

We systematically searched PubMed/Medline and Web of Knowledge databases (January 2006 to July 2017) to identify EEG/MEG and fMRI studies of the ketamine model of schizophrenia with human subjects. The search strategy identified 209 citations of which 46 articles met specified eligibility criteria.

Results:

In EEG/MEG studies, ketamine induced changes of event-related potentials, such as the P300 potential and the mismatch negativity, similar to alterations observed in schizophrenia patients. In fMRI studies, alterations of activation were observed in different brain regions, most prominently within the anterior cingulate cortex and limbic structures as well as task-relevant brain regions. These alterations were accompanied by changes in functional connectivity, indicating a balance shift of the underlying brain networks. Pharmacological treatments did alter ketamine-induced changes in EEG/MEG and fMRI studies to different extents.

Conclusion:

This review highlights the potential applicability of the ketamine model for schizophrenia drug development by offering the possibility to assess the effect of pharmacological agents on schizophrenia-like symptoms and to find relevant neurophysiological and neuroimaging biomarkers.

Group II Metabotropic Glutamate Receptors: Role in Pain Mechanisms and Pain Modulation

Glutamate is the main excitatory neurotransmitter in the nervous system and plays a critical role in nociceptive processing and pain modulation. G-protein coupled metabotropic glutamate receptors (mGluRs) are widely expressed in the central and peripheral nervous system, and they mediate neuronal excitability and synaptic transmission. Eight different mGluR subtypes have been identified so far, and are classified into Groups I-III. Group II mGluR2 and mGluR3 couple negatively to adenylyl cyclase through Gi/Go proteins, are mainly expressed presynaptically, and typically inhibit the release of neurotransmitters, including glutamate and GABA. Group II mGluRs have consistently been linked to pain modulation; they are expressed in peripheral, spinal and supraspinal elements of pain-related neural processing. Pharmacological studies have shown anti-nociceptive/analgesic effects of group II mGluR agonists in preclinical models of acute and chronic pain, although much less is known about mechanisms and sites of action for mGluR2 and mGluR3 compared to other mGluRs. The availability of orthosteric and new selective allosteric modulators acting on mGluR2 and mGluR3 has provided valuable tools for elucidating (subtype) specific contributions of these receptors to the pathophysiological mechanisms of pain and other disorders and their potential as therapeutic targets. This review focuses on the important role of group II mGluRs in the neurobiology of pain mechanisms and behavioral modulation, and discusses evidence for their therapeutic potential in pain.

New evidences for a role of mGluR7 in astrocyte survival: Possible implications for neuroprotection

A specific activation of metabotropic glutamate receptor 7 (mGluR7) has been shown to be neuroprotective in various models of neuronal cell damage, however, its role in glia cell survival has not been studied, yet. Thus, we performed comparative experiments estimating protective effects of the mGluR7 allosteric agonist AMN082 in glia, neuronal and neuronal-glia cell cultures against various harmful stimuli. First, the transcript levels of mGluR7 and other subtypes of group II and III mGluRs in cortical neuronal, neuronal-glia and glia cell cultures have been measured by qPCR method. Next, we demonstrated that AMN082 with similar efficiency attenuated the glia cell damage evoked by staurosporine (St) and doxorubicin (Dox). The AMN082-mediated glioprotection was mGluR7-dependent and associated with decreased DNA fragmentation without involvement of caspase-3 inhibition. Moreover, the inhibitors of PI3K/Akt and MAPK/ERK1/2 pathways blocked the protective effect of AMN082. In neuronal and neuronal-glia cell cultures in the model of glutamate (Glu)- but not St-evoked cell damage, we showed a significant glia contribution to mGluR7-mediated neuroprotection. Finally, by using glia and neuronal cells derived from mGluR7+/+ and mGluR7-/- mice we demonstrated a higher cell-damaging effect of St and Dox in mGluR7-deficient glia but not in neurons (cerebellar granule cells). Our present data showed for the first time a glioprotective potential of AMN082 underlain by mechanisms involving the activation of PI3K/Akt and MAPK/ERK1/2 pathways and pro-survival role of mGluR7 in glia cells. These findings together with the confirmed neuroprotective properties of AMN082 justify further research on mGluR7-targeted therapies for various CNS disorders.

Chloride ions stabilize the glutamate-induced active state of the metabotropic glutamate receptor 3

Due to the essential roles of glutamate, detection and response to a large range of extracellular concentrations of this excitatory amino acid are necessary for the fine-tuning of brain functions. Metabotropic glutamate receptors (mGluRs) are implicated in shaping the activity of many synapses in the central nervous system. Among the eight mGluR subtypes, there is increasing interest in studying the mGlu₃ receptor which has recently been linked to various diseases, including psychiatric disorders. This receptor displays striking functional properties, with a high and, often, full basal activity, making its study elusive in heterologous systems. Here, we demonstrate that Cl^- ions exert strong positive allosteric modulation of glutamate on the mGlu₃ receptor. We have also identified the molecular and structural determinants lying behind this allostery: a unique interactive "chloride-lock" network. Indeed, Cl^- ions dramatically stabilize the glutamate-induced active state of the extracellular domain of the mGlu₃ receptor. Thus, the mGlu₃ receptors' large basal activity does not correspond to a constitutive activity in absence of agonist. Instead, it results mostly from a Cl^-mediated amplified response to low ambient glutamate concentrations, such as those measured in cell media. This strong interaction between glutamate and Cl^- ions allows the mGlu₃ receptor to sense and efficiently react to sub-micromolar concentrations of glutamate, making it the most sensitive member of mGluR family.

In Vivo Non-radioactive Assessment of mGlu5 Receptor-Activated Polyphosphoinositide Hydrolysis in Response to Systemic Administration of a Positive Allosteric Modulator

mGlu5 receptor-mediated polyphosphoinositide (PI) hydrolysis is classically measured by determining the amount of radioactivity incorporated in inositolmonophosphate (InsP) after labeling of membrane phospholipids with radioactive inositol. Although this method is historically linked to the study of mGlu receptors, it is inappropriate for the assessment of mGlu5 receptor signaling in vivo. Using a new ELISA kit we showed that systemic treatment with the selective positive allosteric modulator (PAM) of mGlu5 receptors VU0360172 enhanced InsP formation in different brain regions of CD1 or C57Black mice. The action of VU0360172 was sensitive to the mGlu5 receptor, negative allosteric modulator (NAM), MTEP, and was abolished in mice lacking mGlu5 receptors. In addition, we could demonstrate that endogenous activation of mGlu5 receptors largely accounted for the basal PI hydrolysis particularly in the prefrontal cortex. This method offers opportunity for investigation of mGlu5 receptor signaling in physiology and pathology, and could be used for the functional screening of mGlu5 receptor PAMs in living animals.

The activation of group II metabotropic glutamate receptors protects neonatal rat brains from oxidative stress injury after hypoxia-ischemia

Birth asphyxia resulting in brain hypoxia-ischemia (H-I) can cause neonatal death or lead to persistent brain damage. Recent investigations have shown that group II metabotropic glutamate receptor (mGluR2/3) activation can provide neuroprotection against H-I but the mechanism of this effect is not clear. The aim of this study was to investigate whether mGluR2/3 agonists applied a short time after H-I reduce brain damage in an experimental model of birth asphyxia, and whether a decrease in oxidative stress plays a role in neuroprotection. Neonatal H-I in 7-day-old rats was used as an experimental model of birth asphyxia. Rats were injected intra peritoneally with mGluR2 (LY 379268) or mGluR3 (NAAG) agonists 1 h or 6 h after H-I (5 mg/kg). The weight deficit of the ischemic brain hemisphere, radical oxygen species (ROS) content levels, antioxidant enzymes activity and the concentrations of reduced glutathione (GSH) were measured. Both agonists reduced weight loss in the ischemic hemisphere and mitigated neuronal degeneration in the CA1 hippocampal region and cerebral cortex. Both agonists reduced the elevated levels of ROS in the ipsilateral hemisphere observed after H-I and prevented an increase in antioxidant enzymes activity in the injured hemisphere restoring them to control levels. A decrease in GSH level was also restored after agonists application. The results show that the activation of mGluR2 and mGluR3 a short time after H-I triggers neuroprotective mechanisms that act through the inhibition of oxidative stress and ROS production. The prevention of ROS production by the inhibition of glutamate release and decrease in its extracellular concentration is likely the main mechanism involved in the observed neuroprotection.

Oligomerization of a G protein-coupled receptor in neurons controlled by its structural dynamics

G protein coupled receptors (GPCRs) play essential roles in intercellular communication. Although reported two decades ago, the assembly of GPCRs into dimer and larger oligomers in their native environment is still a matter of intense debate. Here, using number and brightness analysis of fluorescently labeled receptors in cultured hippocampal neurons, we confirm that the metabotropic glutamate receptor type 2 (mGlu₂) is a homodimer at expression levels in the physiological range, while heterodimeric GABA_B receptors form larger complexes. Surprisingly, we observed the formation of larger mGlu₂ oligomers upon both activation and inhibition of the receptor. Stabilizing the receptor in its inactive conformation using biochemical constraints also led to the observation of oligomers. Following our recent observation that mGlu receptors are in constant and rapid equilibrium between several states under basal conditions, we propose that this structural heterogeneity limits receptor oligomerization. Such assemblies are expected to stabilize either the active or the inactive state of the receptor.

Using machine learning tools for protein database biocuration assistance

Biocuration in the omics sciences has become paramount, as research in these fields rapidly evolves towards increasingly data-dependent models. As a result, the management of web-accessible publicly-available databases becomes a central task in biological knowledge dissemination. One relevant challenge for biocurators is the unambiguous identification of biological entities. In this study, we illustrate the adequacy of machine learning methods as biocuration assistance tools using a publicly available protein database as an example. This database contains information on G Protein-Coupled Receptors (GPCRs), which are part of eukaryotic cell membranes and relevant in cell communication as well as major drug targets in pharmacology. These receptors are characterized according to subtype labels. Previous analysis of this database provided evidence that some of the receptor sequences could be affected by a case of label noise, as they appeared to be too consistently misclassified by machine learning methods. Here, we extend our analysis to recent and quite substantially modified new versions of the database and reveal their now extremely accurate labeling using several machine learning models and different transformations of the unaligned sequences. These findings support the adequacy of our proposed method to identify problematic labeling cases as a tool for database biocuration.

Pretreatment with Group II Metabotropic Glutamate Receptor Agonist LY379268 Protects Neonatal Rat Brains from Oxidative Stress in an Experimental Model of Birth Asphyxia

Hypoxia-ischemia (H-I) at the time of birth may cause neonatal death or lead to persistent brain damage. The search for an effective treatment of asphyxiated infants has not resulted in an effective therapy, and hypothermia remains the only available therapeutic strategy. Among possible experimental therapies, the induction of ischemic tolerance is promising. Recent investigations have shown that activation of group II metabotropic glutamate receptors (mGluR2/3) can provide neuroprotection against H-I, but the mechanism of this effect is not clear. The aim of this study was to investigate whether an mGluR2/3 agonist applied before H-I reduces brain damage in an experimental model of birth asphyxia and whether a decrease in oxidative stress plays a role in neuroprotection. Neonatal H-I on seven-day-old rats was used as an experimental model of birth asphyxia. Rats were injected intraperitoneally with the mGluR2/3 agonist LY379268 24 or 1 h before H-I (5 mg/kg). LY379268 reduced the infarct area in the ischemic hemisphere. Application of the agonist at both times also reduced the elevated levels of reactive oxygen species (ROS) in the ipsilateral hemisphere observed after H-I and prevented the increase in antioxidant enzyme activity in the injured hemisphere. The decrease in glutathione (GSH) level was also restored after agonist application. The results suggest that the neuroprotective mechanisms triggered by the activation of mGluR2/3 before H-I act through the decrease of glutamate release and its extracellular concentration resulting in the inhibition of ROS production and reduction of oxidative stress. This, rather than induction of ischemic tolerance, is probably the main mechanism involved in the observed neuroprotection.

Metabotropic glutamate receptors as drug targets for the treatment of absence epilepsy

Metabotropic glutamate (mGlu) receptors are expressed in key regions of the cortex and the thalamus and are known to regulate spike and wave discharges (SWDs), the electroclinical hallmarks of absence seizures. Recent preclinical studies have highlighted the therapeutic potential of selective group I and III mGlu receptor subtype allosteric modulators, which can suppress pathological SWDs. Of particular interest are positive allosteric modulators (PAMs) for mGlu5 receptors, as they currently show the most promise as novel anti-absence epilepsy drugs. The rational design of novel selective positive and negative allosteric mGlu modulators, especially for the mGlu5 receptor, has been made possible following the recent crystallographic structure determination of group I mGlu receptors. Our current knowledge of the role of different mGlu receptor subtypes in absence epilepsy is outlined in this article.

Mutual activation of glutamatergic mGlu4 and muscarinic M4 receptors reverses schizophrenia-related changes in rodents

RATIONALE

Metabotropic glutamate receptors and muscarinic M₄ receptors have been proposed as novel targets for various brain disorders, including schizophrenia. Both receptors are coupled to G_o/i proteins and are expressed in brain circuits that are important in schizophrenia. Therefore, their mutual activation may be an effective treatment and allow minimizing the doses of ligands required for optimal activity.

OBJECTIVES

In the present studies, subactive doses of mGlu₄ and M₄ activators (LSP4-2022 and VU152100, respectively) were administered to investigate the mutual interaction between mGlu₄ and M₄ receptors in animal models of schizophrenia.

METHODS

The behavioral tests used were MK-801-induced hyperactivity, (±)-2.5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)-induced head twitches, the modified forced swim test, and MK-801-induced disruptions of social interactions and novel object recognition. DOI-induced spontaneous excitatory postsynaptic currents (sEPSCs) in brain slices and positron emission tomography (PET) in were used to establish the ability of these compounds to modulate the glutamatergic and dopaminergic systems. Rotarod was used to assess putative adverse effects.

RESULTS

The mutual administration of subactive doses of LSP4-2022 and VU152100 exerted similar antipsychotic-like efficacy in animals as observed for active doses of both compounds, indicating their additive actions. VU152100 inhibited the DOI-induced frequency (but not amplitude) of sEPSCs in the frontal cortex, confirming presynaptic regulation of glutamate release. Both compounds reversed amphetamine-induced decrease in D₂ receptor levels in the striatum, as measured with [¹⁸F]fallypride. The compounds did not induce any motor impartments when measured in rotarod test.

CONCLUSIONS

Based on our results, the simultaneous activation of M₄ and mGlu₄ receptors is beneficial in reversing MK-801- and amphetamine-induced schizophrenia-related changes in animals.

Trends in GPCR drug discovery: new agents, targets and indications

G protein-coupled receptors (GPCRs) are the most intensively studied drug targets, mostly due to their substantial involvement in human pathophysiology and their pharmacological tractability. Here, we report an up-to-date analysis of all GPCR drugs and agents in clinical trials, which reveals current trends across molecule types, drug targets and therapeutic indications, including showing that 475 drugs (~34% of all drugs approved by the US Food and Drug Administration (FDA)) act at 108 unique GPCRs. Approximately 321 agents are currently in clinical trials, of which ~20% target 66 potentially novel GPCR targets without an approved drug, and the number of biological drugs, allosteric modulators and biased agonists has increased. The major disease indications for GPCR modulators show a shift towards diabetes, obesity and Alzheimer disease, although several central nervous system disorders are also highly represented. The 224 (56%) non-olfactory GPCRs that have not yet been explored in clinical trials have broad untapped therapeutic potential, particularly in genetic and immune system disorders. Finally, we provide an interactive online resource to analyse and infer trends in GPCR drug discovery.

Immuno-pharmacological characterization of group II metabotropic glutamate receptors controlling glutamate exocytosis in mouse cortex and spinal cord

BACKGROUND AND PURPOSE

We recently proposed the existence of mGlu3 -preferring autoreceptors in spinal cord terminals and of mGlu2 -preferring autoreceptors in cortical terminals. This study aims to verify our previous conclusions and to extend their pharmacological characterization.

EXPERIMENTAL APPROACH

We studied the effect of LY566332, an mGlu2 receptor positive allosteric modulator (PAM), and of LY2389575, a selective mGlu3 receptor negative allosteric (NAM) modulator, on the mGlu2/3 agonist LY379268-mediated inhibition of glutamate exocytosis [measured as KCl-evoked release of preloaded [3 H]-D-aspartate]. The mGlu2 PAM BINA and the mGlu3 NAM ML337, as well as selective antibodies recognizing the N-terminal of the receptor proteins, were used to confirm the pharmacological characterization of the native receptors.

KEY RESULTS

Cortical synaptosomes possess LY566332-sensitive autoreceptors that are slightly, although significantly, susceptible to LY2389575. In contrast, LY566332-insensitive and LY2389575-sensitive autoreceptors are present in spinal cord terminals. BINA and ML337 mimicked LY566332 and LY2389575, respectively, in controlling LY379268-mediated inhibition of glutamate exocytosis from both cortical and spinal cord synaptosomes. Incubation of cortical synaptosomes with anti-mGlu2 antibody prevented the LY379268-induced inhibition of glutamate exocytosis, and this response was partially reduced by the anti-mGlu3 antibody. Incubation of spinal cord synaptosomes with the anti-mGlu3 antibody abolished LY379268-mediated reduction of glutamate exocytosis from these terminals, while the anti-mGlu2 antibody was inactive. Western blot analysis and confocal microscopy data were largely consistent with these functional observations.

CONCLUSIONS AND IMPLICATIONS

We confirmed that mGlu3 -preferring autoreceptors exist in spinal cord terminals. Differently, cortical glutamatergic terminals possess mGlu2 /mGlu3 heterodimers, whose inhibitory effect is largely mediated by mGlu2 receptors.

Functional partnership between mGlu3 and mGlu5 metabotropic glutamate receptors in the central nervous system

mGlu5 receptors are involved in mechanisms of activity-dependent synaptic plasticity, and are targeted by drugs developed for the treatment of CNS disorders. We report that mGlu3 receptors, which are traditionally linked to the control of neurotransmitter release, support mGlu5 receptor signaling in neurons and largely contribute to the robust mGlu5 receptor-mediated polyphosphoinositide hydrolysis in the early postnatal life. In cortical pyramidal neurons, mGlu3 receptor activation potentiated mGlu5 receptor-mediated somatic Ca²⁺ mobilization, and mGlu3 receptor-mediated long-term depression in the prefrontal cortex required the endogenous activation of mGlu5 receptors. The interaction between mGlu3 and mGlu5 receptors was also relevant to mechanisms of neuronal toxicity, with mGlu3 receptors shaping the influence of mGlu5 receptors on excitotoxic neuronal death. These findings shed new light into the complex role played by mGlu receptors in physiology and pathology, and suggest reconsideration of some of the current dogmas in the mGlu receptor field.

Role of the Astroglial Glutamate Exchanger xCT in Ventral Hippocampus in Resilience to Stress

We demonstrate that stress differentially regulates glutamate homeostasis in the dorsal and ventral hippocampus and identify a role for the astroglial xCT in ventral dentate gyrus (vDG) in stress and antidepressant responses. We provide an RNA-seq roadmap for the stress-sensitive vDG. The transcription factor REST binds to xCT promoter in co-occupancy with the epigenetic marker H3K27ac to regulate expression of xCT, which is also reduced in a genetic mouse model of inherent susceptibility to depressive-like behavior. Pharmacologically, modulating histone acetylation with acetyl-L-carnitine (LAC) or acetyl-N-cysteine (NAC) rapidly increases xCT and activates a network with mGlu2 receptors to prime an enhanced glutamate homeostasis that promotes both pro-resilient and antidepressant-like responses. Pharmacological xCT blockage counteracts NAC prophylactic effects. GFAP⁺-Cre-dependent overexpression of xCT in vDG mimics pharmacological actions in promoting resilience. This work establishes a mechanism by which vDG protection leads to stress resilience and antidepressant responses via epigenetic programming of an xCT-mGlu2 network.

Dominant Mutations in GRM1 Cause Spinocerebellar Ataxia Type 44

The metabotropic glutamate receptor 1 (mGluR1) is abundantly expressed in the mammalian central nervous system, where it regulates intracellular calcium homeostasis in response to excitatory signaling. Here, we describe heterozygous dominant mutations in GRM1, which encodes mGluR1, that are associated with distinct disease phenotypes: gain-of-function missense mutations, linked in two different families to adult-onset cerebellar ataxia, and a de novo truncation mutation resulting in a dominant-negative effect that is associated with juvenile-onset ataxia and intellectual disability. Crucially, the gain-of-function mutations could be pharmacologically modulated in vitro using an existing FDA-approved drug, Nitazoxanide, suggesting a possible avenue for treatment, which is currently unavailable for ataxias.

Allosteric Modulation of GPCRs: New Insights and Potential Utility for Treatment of Schizophrenia and Other CNS Disorders

G-protein-coupled receptors (GPCRs) play critical roles in regulating brain function. Recent advances have greatly expanded our understanding of these receptors as complex signaling machines that can adopt numerous conformations and modulate multiple downstream signaling pathways. While agonists and antagonists have traditionally been pursued to target GPCRs, allosteric modulators provide several mechanistic advantages, including the ability to distinguish between closely related receptor subtypes. Recently, the discovery of allosteric ligands that confer bias and modulate some, but not all, of a given receptor's downstream signaling pathways can provide pharmacological modulation of brain circuitry with remarkable precision. In addition, allosteric modulators with unprecedented specificity have been developed that can differentiate between subpopulations of a given receptor subtype based on the receptor's dimerization state. These advances are not only providing insight into the biological roles of specific receptor populations, but hold great promise for treating numerous CNS disorders.

How do antidepressants work? New perspectives for refining future treatment approaches

Most currently available antidepressants target monoamine neurotransmitter function. However, a purely neurotransmitter-based explanation for antidepressant drug action is challenged by the delayed clinical onset of most agents and the need to explain how neurochemical changes reverse the many different symptoms of depression. Novel approaches to understanding of antidepressant drug action include a focus on early changes in emotional and social processing and the role of neural plasticity. In this Review, we discuss the ways in which these two different theories reflect different or complementary approaches, and how they might be integrated to offer novel solutions for people with depression. We consider the predictions made by these mechanistic approaches for the stratification and development of new therapeutics for depression, and the next steps that need to be made to facilitate this translation of science to the clinic.

Epilepsy, Antiepileptic Drugs, and Aggression: An Evidence-Based Review

Antiepileptic drugs (AEDs) have many benefits but also many side effects, including aggression, agitation, and irritability, in some patients with epilepsy. This article offers a comprehensive summary of current understanding of aggressive behaviors in patients with epilepsy, including an evidence-based review of aggression during AED treatment. Aggression is seen in a minority of people with epilepsy. It is rarely seizure related but is interictal, sometimes occurring as part of complex psychiatric and behavioral comorbidities, and it is sometimes associated with AED treatment. We review the common neurotransmitter systems and brain regions implicated in both epilepsy and aggression, including the GABA, glutamate, serotonin, dopamine, and noradrenaline systems and the hippocampus, amygdala, prefrontal cortex, anterior cingulate cortex, and temporal lobes. Few controlled clinical studies have used behavioral measures to specifically examine aggression with AEDs, and most evidence comes from adverse event reporting from clinical and observational studies. A systematic approach was used to identify relevant publications, and we present a comprehensive, evidence-based summary of available data surrounding aggression-related behaviors with each of the currently available AEDs in both adults and in children/adolescents with epilepsy. A psychiatric history and history of a propensity toward aggression/anger should routinely be sought from patients, family members, and carers; its presence does not preclude the use of any specific AEDs, but those most likely to be implicated in these behaviors should be used with caution in such cases.

Group II metabotropic glutamate receptor interactions with NHERF scaffold proteins: Implications for receptor localization in brain

The group II metabotropic glutamate receptors mGluR2 and mGluR3 are key modulators of glutamatergic neurotransmission. In order to identify novel Group II metabotropic glutamate receptor (mGluR)-interacting partners, we screened the C-termini of mGluR2 and mGluR3 for interactions with an array of PDZ domains. These screens identified the Na+/H+ exchanger regulatory factors 1 and 2 (NHERF-1 & -2) as candidate interacting partners. Follow-up co-immunoprecipitation studies demonstrated that both mGluR2 and mGluR3 can associate with NHERF-1 and NHERF-2 in a cellular context. Functional studies revealed that disruption of PDZ interactions with mGluR2 enhanced receptor signaling to Akt. However, further studies of mGluR2 and mGluR3 signaling in astrocytes in which NHERF expression was reduced by gene knockout (KO) and/or siRNA knockdown techniques revealed that the observed differences in signaling between WT and mutant mGluR2 were likely not due to disruption of interactions with the NHERF proteins. Electron microscopic analyses revealed that Group II mGluRs were primarily expressed in glia and unmyelinated axons in WT, NHERF-1 and NHERF-2 KO mice, but the relative proportion of labeled axons over glial processes was higher in NHERF-2 KO mice than in controls and NHERF-1 KO mice. Interestingly, our anatomical studies also revealed that loss of either NHERF protein results in ventriculomegaly, which may be related to the high incidence of hydrocephaly that has previously been observed in NHERF-1 KO mice. Together, these studies support a role for NHERF-1 and NHERF-2 in regulating the distribution of Group II mGluRs in the murine brain, while conversely the effects of the mGluR2/3 PDZ-binding motifs on receptor signaling are likely mediated by interactions with other PDZ scaffold proteins beyond the NHERF proteins.

Expression of the K+/Cl- cotransporter, KCC2, in cerebellar Purkinje cells is regulated by group-I metabotropic glutamate receptors

The neuronal K+/Cl- symporter, KCC2, shapes synaptic responses mediated by Cl--permeant GABAA receptors. Moving from the evidence that excitatory neurotransmission drives changes in KCC2 expression in cerebellar neurons, we studied the regulation of KCC2 expression by group-I metabotropic glutamate (mGlu) receptors in the cerebellum of adult mice. Mice lacking mGlu5 receptors showed a large reduction in cerebellar KCC2 protein levels and a loss of KCC2 immunoreactivity in Purkinje cells. Similar changes were seen in mice treated with the mGlu5 receptor antagonist, MPEP, whereas treatment with the mGlu5 receptor positive allosteric modulator (PAM), VU0360172, increased KCC2 expression. In contrast, pharmacological inhibition of mGlu1 receptors with JNJ16259685 enhanced cerebellar KCC2 protein levels and KCC2 immunoreactivity in Purkinje cells, whereas treatment with the mGlu1 receptor PAM, RO0711401, reduced KCC2 expression. To examine whether the reduction in KCC2 expression caused by the absence or the inhibition of mGlu5 receptors could affect GABAergic transmission, we performed electrophysiological and behavioral studies. Recording of extracellular action potentials in Purkinje cells showed that the inhibitory effect of the GABAA receptor agonist, muscimol, was lost in cerebellar slices prepared from mGlu5-/- mice or from mice treated systemically with MPEP, in line with the reduction in KCC2 expression. Similarly, motor impairment caused by the GABAA receptor PAM, diazepam, was attenuated in mice pre-treated with MPEP. These findings disclose a novel function of mGlu5 receptors in the cerebellum and suggest that mGlu5 receptor ligands might influence GABAergic transmission in the cerebellum and affect motor responses to GABA-mimetic drugs. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.