The Metabotropic Glutamate Receptor 4 Positive Allosteric Modulator ADX88178 Inhibits Inflammatory Responses in Primary Microglia

Foliglurax, a positive allosteric modulator of the metabotrophic glutamate receptor 4, protects dopaminergic neurons in MPTP-lesioned male mice

Overactivity of the corticostriatal glutamatergic pathway is documented in Parkinson's disease (PD) and stimulation of presynaptic metabotropic glutamate (mGlu) receptors 4 on these striatal afferents inhibits glutamate release normalizing neuronal activity in the basal ganglia. Moreover, mGlu4 receptors are also expressed in glial cells and are able to modulate glial function making this receptor a potential target for neuroprotection. Hence, we investigated whether foliglurax, a positive allosteric modulator of mGlu4 receptors with high brain exposure after oral administration, has neuroprotective effects in MPTP mice to model early PD. Male mice were treated daily from day 1 to 10 with 1, 3 or 10 mg/kg of foliglurax and administered MPTP on the 5th day then euthanized on the 11th day. Dopamine neuron integrity was assessed with measures of striatal dopamine and its metabolites levels, striatal and nigral dopamine transporter (DAT) binding and inflammation with markers of striatal astrocytes (GFAP) and microglia (Iba1). MPTP lesion produced a decrease in dopamine, its metabolites and striatal DAT specific binding that was prevented by treatment with 3 mg/kg of foliglurax, whereas 1 and 10 mg/kg had no beneficial effect. MPTP mice had increased levels of GFAP; foliglurax treatment (3 mg/kg) prevented this increase. Iba1 levels were unchanged in MPTP mice compared to control mice. There was a negative correlation between dopamine content and GFAP levels. Our results show that positive allosteric modulation of mGlu4 receptors with foliglurax provided neuroprotective effects in the MPTP mouse model of PD.

Localization and expression of the Mas-related G-protein coupled receptor member D (MrgD) in the mouse brain

Numerous studies in the last decades have provided evidence for the existence of a local renin-angiotensin system (RAS) in the central nervous system (CNS). Widespread distribution of the different RAS components in the brain demonstrates the pleiotropic role of this system in the structure and function of CNS. With the advent of new molecular techniques, a novel receptor has been identified within the beneficial arm of the RAS, the Mas-related G-protein coupled receptor D (MrgD), which can be stimulated by two heptapeptides, Ala¹-(Ang-(1-7), also named alamandine, and Ang-(1-7). However, the biological and physiological relevance of this interaction remains obscure. Since several recent studies hinted at a role of MrgD in the CNS, we determined the distribution pattern of MrgD receptors in the adult mouse brain by using a genetic mouse model with tracers of MrgD expression. MrgD-positive cells could be identified in some forebrain areas, including cortex, hippocampus, amygdala, hypothalamus, habenular nuclei, striatum and pallidum, as well as in some mid-brain nuclei in a region-specific manner. The specific localization of MrgD in the reward- and limbic-related areas can hint at a role of MrgD in processes such as pain perception/modulation, synaptic plasticity, learning, memory and cognition.

Glia: A major player in glutamate-GABA dysregulation-mediated neurodegeneration

The imbalance between glutamate and γ-aminobutyric acid (GABA) results in the loss of synaptic strength leading to neurodegeneration. The dogma on the field considered neurons as the main players in this excitation-inhibition (E/I) balance. However, current strategies focusing only on neurons have failed to completely understand this condition, bringing up the importance of glia as an alternative modulator for neuroinflammation as glia alter the activity of neurons and is a source of both neurotrophic and neurotoxic factors. This review's primary goal is to illustrate the role of glia over E/I balance in the central nervous system and its interaction with neurons. Rather than focusing only on the neuronal targets, we take a deeper look at glial receptors and proteins that could also be explored as drug targets, as they are early responders to neurotoxic insults. This review summarizes the neuron-glia interaction concerning GABA and glutamate, possible targets, and its involvement in the E/I imbalance in neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis.

Regulation and functional consequences of mGlu4 RNA editing

Metabotropic glutamate receptor 4 (mGlu₄) is one of eight mGlu receptors within the Class C G protein-coupled receptor superfamily. mGlu₄ is primarily localized to the presynaptic membrane of neurons where it functions as an auto and heteroreceptor controlling synaptic release of neurotransmitter. mGlu₄ is implicated in numerous disorders and is a promising drug target; however, more remains to be understood about its regulation and pharmacology. Using high-throughput sequencing, we have validated and quantified an adenosine-to-inosine (A-to-I) RNA editing event that converts glutamine 124 to arginine in mGlu₄; additionally, we have identified a rare but novel K129R site. Using an in vitro editing assay, we then validated the pre-mRNA duplex that allows for editing by ADAR enzymes and predicted its conservation across the mammalian species. Structural modeling of the mGlu₄ protein predicts the Q124R substitution to occur in the B helix of the receptor that is critical for receptor dimerization and activation. Interestingly, editing of a receptor homodimer does not disrupt G protein activation in response to the endogenous agonist, glutamate. Using an assay designed to specifically measure heterodimer populations at the surface, however, we found that Q124R substitution decreased the propensity of mGlu₄ to heterodimerize with mGlu₂ and mGlu₇ Our study is the first to extensively describe the extent and regulatory factors of RNA editing of mGlu₄ mRNA transcripts. In addition, we have proposed a novel functional consequence of this editing event that provides insights regarding its effects in vivo and expands the regulatory capacity for mGlu receptors.

Targeting the dysfunction of glutamate receptors for the development of novel antidepressants

Increasing evidence indicates that dysfunction of glutamate receptors is involved in the pathophysiology of major depressive disorder (MDD). Although accumulating efforts have been made to elucidate the applications and mechanisms underlying antidepressant-like effects of ketamine, a non-selective antagonist of N-methyl-d-aspartate receptor (NMDAR), the role of specific glutamate receptor subunit in regulating depression is not completely clear. The current review aims to discuss the relationships between glutamate receptor subunits and depressive-like behaviors. Research literatures were searched from inception to July 2020. We summarized the alterations of glutamate receptor subunits in patients with MDD and animal models of depression. Animal behaviors in response to dysfunction of glutamate receptor subunits were also surveyed. To fully understand mechanisms underlying antidepressant-like effects of modulators targeting glutamate receptors, we discussed effects of each glutamate receptor subunit on serotonin system, synaptic plasticity, neurogenesis and neuroinflammation. Finally, we collected most recent clinical applications of glutamate receptor modulators and pointed out the limitations of these candidates in the treatment of MDD.

Receptors, Channel Proteins, and Enzymes Involved in Microglia-mediated Neuroinflammation and Treatments by Targeting Microglia in Ischemic Stroke

Stroke is the largest contributor to global neurological disability-adjusted life-years, posing a huge economic and social burden to the world. Though pharmacological recanalization with recombinant tissue plasminogen activator and mechanical thrombectomy have greatly improved the prognosis of patients with ischemic stroke, clinically, there is still no effective treatment for the secondary injury caused by cerebral ischemia. In recent years, more and more evidences show that neuroinflammation plays a pivotal role in the pathogenesis and progression of ischemic cerebral injury. Microglia are brain resident innate immune cells and act the role peripheral macrophages. They play critical roles in mediating neuroinflammation after ischemic stroke. Microglia-mediated neuroinflammation is not an isolated process and has complex relationships with other pathophysiological processes as oxidative/nitrative stress, excitotoxicity, necrosis, apoptosis, pyroptosis, autophagy, and adaptive immune response. Upon activation, microglia differentially express various receptors, channel proteins, and enzymes involved in promoting or inhibiting the inflammatory processes, making them the targets of intervention for ischemic stroke. To inhibit microglia-related neuroinflammation and promote neurological recovery after ischemic stroke, numerous biochemical agents, cellular therapies, and physical methods have been demonstrated to have therapeutic potentials. Though accumulating experimental evidences have demonstrated that targeting microglia is a promising approach in the treatment of ischemic stroke, the clinical progress is slow. Till now, no clinical study could provide convincing evidence that any biochemical or physical therapies could exert neuroprotective effect by specifically targeting microglia following ischemic stroke.

Microglia reprogram metabolic profiles for phenotype and function changes in central nervous system

In response to various types of environmental and cellular stress, microglia rapidly activate and exhibit either pro- or anti-inflammatory phenotypes to maintain tissue homeostasis. Activation of microglia can result in changes in morphology, phagocytosis capacity, and secretion of cytokines. Furthermore, microglial activation also induces changes to cellular energy demand, which is dependent on the metabolism of various metabolic substrates including glucose, fatty acids, and amino acids. Accumulating evidence demonstrates metabolic reprogramming acts as a key driver of microglial immune response. For instance, microglia in pro-inflammatory states preferentially use glycolysis for energy production, whereas, cells in anti-inflammatory states are mainly powered by oxidative phosphorylation and fatty acid oxidation. In this review, we summarize recent findings regarding microglial metabolic pathways under physiological and pathological circumtances. We will then discuss how metabolic reprogramming can orchestrate microglial response to a variety of central nervous system pathologies. Finally, we highlight how manipulating metabolic pathways can reprogram microglia towards beneficial functions, and illustrate the therapeutic potential for inflammation-related neurological diseases.

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.

Identification of Compounds With Glucocorticoid Sparing Effects on Suppression of Chemokine and Cytokine Production by Rheumatoid Arthritis Fibroblast-Like Synoviocytes

In recent years target based drug discovery has expanded our therapeutic armamentarium in the treatment of inflammatory and autoimmune diseases. Despite these advances and adverse effects, glucocorticoids remain reliable agents that are used in many of these diseases. The anti-inflammatory mechanisms of glucocorticoids include the suppression of transcription factor activity like nuclear factor kappa B (NF-κB). By reanalyzing data from two prior high throughput screens (HTS) that utilized a NF-κB reporter construct in THP-1 cells, we identified 1824 small molecule synthetic compounds that demonstrated NF-κB suppressive activities similar to the glucocorticoids included in the original >134,000 compound libraries. These 1824 compounds were then rescreened for attenuating NF-κB activity at 5 and 16 h after LPS stimuli in the NF-κB THP-1 reporter cells. After a “Top X” selection approach 122 hit compounds were further tested for toxicity and suppression of LPS induced CXCL8 release in THP-1 cells. Excluding cytotoxic compounds, the remaining active compounds were grouped into chemotype families using Tanimoto based clustering. Promising representatives from clustered chemotype groups were commercially purchased for further testing. Amongst these index compounds a lead chemotype: 1H-pyrazolo [3,4 d] pyrimidin-4-amine, effectively suppressed CXCL8, and TNF production by THP-1 cells when stimulated with LPS, TNF or IL-1ß. Extending these studies to primary cells, these lead compounds also reduced IL-6 and CXCL8 production by TNF stimulated fibroblast-like synoviocytes (FLS) from rheumatoid arthritis (RA) patients. Importantly a lead 1H-pyrazolo [3,4 d] pyrimidin-4-amine compound demonstrated synergistic effects with dexamethasone when co-administered to TNF stimulated THP-1 cells and RA FLS in suppressing chemokine production. In summary, a cell based HTS approach identified lead compounds that reduced NF-κB activity and chemokine secretion induced by potent immunologic stimuli, and one lead compound that acted synergistically with dexamethasone as an anti-inflammatory agent showing a dose-sparing effect.

Glutamate-induced excitotoxicity in Parkinson's disease: The role of glial cells

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamate transmission efficiency depends on the correct functionality and expression of a plethora of receptors and transporters, located both on neurons and glial cells. Of note, glutamate reuptake by dedicated transporters prevents its accumulation at the synapse as well as non-physiological spillover. Indeed, extracellular glutamate increase causes aberrant synaptic signaling leading to neuronal excitotoxicity and death. Moreover, extrasynaptic glutamate diffusion is strongly associated with glia reaction and neuroinflammation. Glutamate-induced excitotoxicity is mainly linked to an impaired ability of glial cells to reuptake and respond to glutamate, then this is considered a common hallmark in many neurodegenerative diseases, including Parkinson's disease (PD). In this review, we discuss the function of astrocytes and microglia in glutamate homeostasis, focusing on how glial dysfunction causes glutamate-induced excitotoxicity leading to neurodegeneration in PD.

Putative mGluR4 positive allosteric modulators activate Gi-independent anti-inflammatory mechanisms in microglia

Chronic dysregulated microglial activation may lead to persistent inflammation and progressive neurodegeneration. A previous study reported that ADX88178, a putative metabotropic glutamate receptor 4 (mGluR4) positive allosteric modulator (PAM), exerts anti-inflammatory effects in microglia by activating mGluR4. We employed in vitro models of immortalized microglia cell lines and primary microglia to elucidate the molecular mechanisms responsible for the regulation of inflammatory pathways by ADX88178 and other mGluR4 PAMs. ADX88178 downregulated lipopolysaccharide (LPS)-induced expression of pro-inflammatory mediators, including TNF-α, IL-1β, CCL-2, IL-6, NOS2, and miR-155, as well as NO levels, in BV2 cells and primary microglia. Other mGluR4 modulators had divergent activities; VU0361737 (PAM) showed anti-inflammatory effects, whereas the orthosteric group III agonist, L-AP4, and VU0155041 (PAM) displayed no anti-inflammatory actions. In contrast to the earlier report, ADX88178 anti-inflammatory effects appeared to be mGluR4-independent as mGluR4 expression in our in vitro models was very low and its actions were not altered by pharmacological or molecular inhibition of mGluR4. Moreover, we showed that ADX88178 activated G_i-independent, alternative signaling pathways as indicated by the absence of pertussis toxin-mediated inhibition and by increased phosphorylation of cAMP-response element binding protein (CREB), an inhibitor of the NFkB pro-inflammatory pathway. ADX88178 also attenuated NFkB activation by reducing the degradation of IkB and the associated translocation of NFkB-p65 to the nucleus. ADX88178 did not exert its anti-inflammatory effects through adenosine receptors, reported as mGluR4 heteromerization partners. Thus, our results indicate that in microglia, putative mGluR4 PAMs activate mGluR4/G_i-independent mechanisms to attenuate pro-inflammatory pathways.

Regulation of microglia by glutamate and its signal pathway in neurodegenerative diseases

Microglia are an essential component of the central nervous system (CNS) and are involved in the primary response to microorganisms, neuroinflammation, homeostasis, and tissue regeneration, as well as contributing to the pathogenesis of neurodegenerative diseases. Research has shown that microglial diversity, multifunctionality, and their relationship with glutamate are crucial to determining their roles in these diseases. In this review, we focus on recent progress in determining microglial characteristics and the role of glutamate and its receptors in microglia regulation, which could be a novel therapeutic strategy for neurodegenerative diseases.

Synthesis and Characterization of Fluorine-18-Labeled N-(4-Chloro-3-((fluoromethyl-d2)thio)phenyl)picolinamide for Imaging of mGluR4 in Brain

We have synthesized and characterized [18F]-N-(4-chloro-3-((fluoromethyl-d2)thio)phenyl)-picolinamide ([18F]15) as a potential ligand for the positron emission tomography (PET) imaging of mGluR4 in the brain. Radioligand [18F]15 displays central nervous system drug-like properties, including mGluR4 affinity, potent mGluR4 PAM activity, and selectivity against other mGluRs, as well as sufficient metabolic stability. Radiosynthesis was carried out in two steps. The radiochemical yield of [18F]15 was 11.6 ± 2.9% (n = 7, decay corrected) with a purity of 99% and a molar activity of 84.1 ± 11.8 GBq/μmol. Ex vivo biodistribution studies showed reversible binding of [18F]15 in all investigated tissues including the brain, liver, heart, lungs, and kidneys. PET imaging studies in male Sprague Dawley rats showed that [18F]15 accumulates in the brain regions known to express mGluR4. Pretreatment with the unlabeled mGluR4 PAM compounds 13 (methylthio analogue) and 15 showed significant dose-dependent blocking effects. These results suggest that [18F]15 is a promising radioligand for PET imaging mGluR4 in the brain.

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.

Insights into the Structural Aspects of the mGlu Receptor Orthosteric Binding Site

The amino terminal domain (ATD) of the metabotropic glutamate (mGlu) receptors contains the orthosteric glutamate recognition site, which is highly conserved across the eight mGlu receptor subtypes. In total, 29 X-ray crystal structures of the mGlu ATD proteins have been reported to date. These structures span across 3 subgroups and 6 subtypes, and include apo, agonist- and antagonist-bound structures. We will discuss the insights gained from the analysis of these structures with the focus on the interactions contributing to the observed group and subtype selectivity for select agonists. Furthermore, we will define the full expanded orthosteric ligand binding pocket (LBP) of the mGlu receptors, and discuss the macroscopic features of the mGlu ATD proteins.

Metal Chelation Therapy and Parkinson's Disease: A Critical Review on the Thermodynamics of Complex Formation between Relevant Metal Ions and Promising or Established Drugs

The present review reports a list of approximately 800 compounds which have been used, tested or proposed for Parkinson’s disease (PD) therapy in the year range 2014–2019 (April): name(s), chemical structure and references are given. Among these compounds, approximately 250 have possible or established metal-chelating properties towards Cu(II), Cu(I), Fe(III), Fe(II), Mn(II), and Zn(II), which are considered to be involved in metal dyshomeostasis during PD. Speciation information regarding the complexes formed by these ions and the 250 compounds has been collected or, if not experimentally available, has been estimated from similar molecules. Stoichiometries and stability constants of the complexes have been reported; values of the cologarithm of the concentration of free metal ion at equilibrium (pM), and of the dissociation constant K_d (both computed at pH = 7.4 and at total metal and ligand concentrations of 10⁻⁶ and 10⁻⁵ mol/L, respectively), charge and stoichiometry of the most abundant metal–ligand complexes existing at physiological conditions, have been obtained. A rigorous definition of the reported amounts is given, the possible usefulness of this data is described, and the need to characterize the metal–ligand speciation of PD drugs is underlined.

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.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Structural Basis for ( S)-3,4-Dicarboxyphenylglycine (DCPG) As a Potent and Subtype Selective Agonist of the mGlu8 Receptor

( S)-3,4-Dicarboxyphenylglycine (DCPG) was first reported in 2001 as a potent orthosteric agonist with high subtype selectivity for the mGlu₈ receptor, but the structural basis for its high selectivity is not well understood. We have solved a cocrystal structure of recombinant human mGlu₈ amino terminal domain (ATD) protein bound to ( S)-DCPG, which possesses the largest lobe opening angle observed to date among known agonist-bound mGlu ATD crystal structures. The binding conformation of ( S)-DCPG observed in the crystal structure is significantly different from that in the homology model built from an l-glutamate-bound rat mGlu₁ ATD crystal structure, which has a smaller lobe opening angle. This highlights the importance of considering various lobe opening angles when modeling mGlu ATD-ligand complex. New homology models of other mGlu receptors based on the ( S)-DCPG-bound mGlu₈ ATD crystal structure were explored to rationalize ( S)-DCPG's high mGlu₈ receptor subtype selectivity.

Metabotropic Glutamate Receptors in Glial Cells: A New Potential Target for Neuroprotection?

Neurodegenerative disorders are characterized by excitotoxicity and neuroinflammation that finally lead to slow neuronal degeneration and death. Although neurons are the principal target, glial cells are important players as they contribute by either exacerbating or dampening the events that lead to neuroinflammation and neuronal damage. A dysfunction of the glutamatergic system is a common event in the pathophysiology of these diseases. Metabotropic glutamate (mGlu) receptors belong to a large family of G protein-coupled receptors largely expressed in neurons as well as in glial cells. They often appear overexpressed in areas involved in neurodegeneration, where they can modulate glutamatergic transmission. Of note, mGlu receptor upregulation may involve microglia or, even more frequently, astrocytes, where their activation causes release of factors potentially able to influence neuronal death. The expression of mGlu receptors has been also reported on oligodendrocytes, a glial cell type specifically involved in the development of multiple sclerosis. Here we will provide a general overview on the possible involvement of mGlu receptors expressed on glial cells in the pathogenesis of different neurodegenerative disorders and the potential use of subtype-selective mGlu receptor ligands as candidate drugs for the treatment of neurodegenerative disorders. Negative allosteric modulators (NAM) of mGlu5 receptors might represent a relevant pharmacological tool to develop new neuroprotective strategies in these diseases. Recent evidence suggests that targeting astrocytes and microglia with positive allosteric modulators (PAM) of mGlu3 receptor or oligodendrocytes with mGlu4 PAMS might represent novel pharmacological approaches for the treatment of neurodegenerative disorders.

Discovery, Structure-Activity Relationship, and Biological Characterization of a Novel Series of 6-((1 H-Pyrazolo[4,3- b]pyridin-3-yl)amino)-benzo[ d]isothiazole-3-carboxamides as Positive Allosteric Modulators of the Metabotropic Glutamate Receptor 4 (mGlu4)

This work describes the discovery and characterization of novel 6-(1 H-pyrazolo[4,3- b]pyridin-3-yl)amino-benzo[ d]isothiazole-3-carboxamides as mGlu₄ PAMs. This scaffold provides improved metabolic clearance and CYP1A2 profiles compared to previously discovered mGlu₄ PAMs. From this work, 27o (VU6001376) was identified as a potent (EC₅₀ = 50.1 nM, 50.5% GluMax) and selective mGlu₄ PAM with an excellent rat DMPK profile ( in vivo rat CL_p = 3.1 mL/min/kg, t_1/2 = 445 min, CYP1A2 IC₅₀ > 30 μM). Compound 27o was also active in reversing haloperidol induced catalepsy in a rodent preclinical model of Parkinson's disease.

mGlu4 allosteric modulation for treating Parkinson's disease

2017 is the 200th anniversary of the first published description of Parkinson's disease (PD). Fifty years ago, the clinical benefit of levodopa was first documented, representing the most important advance in the treatment of PD so far. Among the novel targets identified in the last decade, positive allosteric modulators (PAM) of mGlu₄ receptors show great promise, with the potential to change the paradigm of the PD treatment approach. mGlu₄ PAMs have shown consistent efficacy in various preclinical models of PD, and entered clinical trials for the first time in 2017. This review synthesizes the rationale for mGlu₄ PAM development for PD and progress to date, reporting the key achievements from preclinical studies to the first-in-class compound assessment in man.

Targeting metabotropic glutamate receptors for the treatment of neuroinflammation

A large body of evidence suggests that neuroinflammation lies at the core of nearly all CNS disorders, including psychiatric disorders. Invading and local immune cells orchestrate the series of events that lead to either tissue repair or damage in response to neuroinflammation. Both lymphocytes and microglia express metabotropic glutamate (mGlu) receptors, which respond to glutamate or other endogenous activators (e.g. some kynurenine metabolites of tryptophan metabolism) influencing immune phenotype and the balance between pro-inflammatory and anti-inflammatory cytokines. Here, we offer an up-to-date on the role of individual mGlu receptor subtypes in the regulation of innate and adaptive immune response, highlighting the relevance of this information in the development of subtype-selective mGlu receptor ligands for treatment of CNS disorders.