Phenotypic profiling of mGlu7 knockout mice reveals new implications for neurodevelopmental disorders

GRM7 deficiency, from excitotoxicity and neuroinflammation to neurodegeneration: Systematic review of GRM7 deficient patients

The metabotropic glutamate receptor 7 (mGluR7) is a presynaptic G-protein-coupled glutamate receptor that modulates neurotransmitter release and synaptic plasticity at presynaptic terminals. It is encoded by GRM7, and recently variants have been identified in patients with autism spectrum disorder (ASD), attention deficit hyperactivity disorder (ADHD), developmental delay (DD), intellectual disability (ID), and brain malformations. To gain updated insights into the function of GRM7 and the phenotypic spectrum of genetic variations within this gene, we conducted a systematic review of relevant literature utilizing PubMed, Web of Science, and Scopus databases. Among the 14 articles meeting the inclusion criteria, a total of 42 patients (from 28 families) harboring confirmed mutations in the GRM7 gene have been documented. Specifically, there were 17 patients with heterozygous mutations, 20 patients with homozygous mutations, and 5 patients with compound heterozygous mutations. Common clinical features included intellectual behavioral disability, seizure/epilepsy, microcephaly, developmental delay, peripheral hypertonia and hypomyelination. Genotype-phenotype correlation was not clear and each variant had unique characteristics including gene dosage, mutant protein surface expression, and degradation pathway that result with a spectrum of phenotype manifestations through ASD or ADHD to severe DD/ID with brain malformations. Neuroinflammation may play a role in the development and/or progression of GRM7-related neurodegeneration along with excitotoxicity. The clinical and functional data presented here demonstrate that both autosomal dominant and recessive inheritance of GRM7 mutation can cause disease spectrum phenotypes through ASD or ADHD to severe DD/ID and seizure with brain malformations.

Novel Inhibitory Site Revealed by Mode of Action on the Metabotropic Glutamate 7 Receptor Venus Flytrap Domain

Metabotropic glutamate (mGlu) receptors play a key role in modulating most synapses in the brain. The mGlu7 receptors inhibit presynaptic neurotransmitter release and offer therapeutic possibilities for post-traumatic stress disorders or epilepsy. Screening campaigns provided mGlu7-specific allosteric modulators as the inhibitor XAP044 (Gee et al. J. Biol. Chem. 2014). In contrast to other mGlu receptor allosteric modulators, XAP044 does not bind in the transmembrane domain but to the extracellular domain of the mGlu7 receptor and not at the orthosteric site. Here, we identified the mode of action of XAP044, combining synthesis of derivatives, modeling and docking experiments, and mutagenesis. We propose a unique mode of action of these inhibitors, preventing the closure of the Venus flytrap agonist binding domain. While acting as a noncompetitive antagonist of L-AP4, XAP044 and derivatives act as apparent competitive antagonists of LSP4-2022. These data revealed more potent XAP044 analogues and new possibilities to target mGluRs.

Novel pharmacological targets for GABAergic dysfunction in ADHD

Attention deficit/hyperactivity disorder (ADHD) is a neurodevelopment disorder that affects approximately 5% of the population. The disorder is characterized by impulsivity, hyperactivity, and deficits in attention and cognition, although symptoms vary across patients due to the heterogenous and polygenic nature of the disorder. Stimulant medications are the standard of care treatment for ADHD patients, and their effectiveness has led to the dopaminergic hypothesis of ADHD in which deficits in dopaminergic signaling, especially in cortical brain regions, mechanistically underly ADHD pathophysiology. Despite their effectiveness in many individuals, almost one-third of patients do not respond to stimulant treatments and the long-term negative side effects of these medications remain unclear. Emerging clinical evidence is beginning to highlight an important role of dysregulated excitatory/inhibitory (E/I) balance in ADHD. These deficits in E/I balance are related to functional abnormalities in glutamate and Gamma-Aminobutyric Acid (GABA) signaling in the brain, with increasing emphasis placed on GABAergic interneurons driving specific aspects of ADHD pathophysiology. Recent genome-wide association studies (GWAS) have also highlighted how genes associated with GABA function are mutated in human populations with ADHD, resulting in the generation of several new genetic mouse models of ADHD. This review will discuss how GABAergic dysfunction underlies ADHD pathophysiology, and how specific receptors/proteins related to GABAergic interneuron dysfunction may be pharmacologically targeted to treat ADHD in subpopulations with specific comorbidities and symptom domains. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Therapeutic Potential for Metabotropic Glutamate Receptor 7 Modulators in Cognitive Disorders

Metabotropic glutamate receptor 7 (mGlu7) is the most highly conserved and abundantly expressed mGlu receptor in the human brain. The presynaptic localization of mGlu7, coupled with its low affinity for its endogenous agonist, glutamate, are features that contribute to the receptor's role in modulating neuronal excitation and inhibition patterns, including long-term potentiation, in various brain regions. These characteristics suggest that mGlu7 modulation may serve as a novel therapeutic strategy in disorders of cognitive dysfunction, including neurodevelopmental disorders that cause impairments in learning, memory, and attention. Primary mutations in the GRM7 gene have recently been identified as novel causes of neurodevelopmental disorders, and these patients exhibit profound intellectual and cognitive disability. Pharmacological tools, such as agonists, antagonists, and allosteric modulators, have been the mainstay for targeting mGlu7 in its endogenous homodimeric form to probe effects of its function and modulation in disease models. However, recent research has identified diversity in dimerization, as well as trans-synaptic interacting proteins, that also play a role in mGlu7 signaling and pharmacological properties. These novel findings represent exciting opportunities in the field of mGlu receptor drug discovery and highlight the importance of further understanding the functions of mGlu7 in complex neurologic conditions at both the molecular and physiologic levels. SIGNIFICANCE STATEMENT: Proper expression and function of mGlu7 is essential for learning, attention, and memory formation at the molecular level within neural circuits. The pharmacological targeting of mGlu7 is undergoing a paradigm shift by incorporating an understanding of receptor interaction with other cis- and trans- acting synaptic proteins, as well as various intracellular signaling pathways. Based upon these new findings, mGlu7's potential as a drug target in the treatment of cognitive disorders and learning impairments is primed for exploration.

TrkB-dependent regulation of molecular signaling across septal cell types

The lateral septum (LS), a GABAergic structure located in the basal forebrain, is implicated in social behavior, learning, and memory. We previously demonstrated that expression of tropomyosin kinase receptor B (TrkB) in LS neurons is required for social novelty recognition. To better understand molecular mechanisms by which TrkB signaling controls behavior, we locally knocked down TrkB in LS and used bulk RNA-sequencing to identify changes in gene expression downstream of TrkB. TrkB knockdown induces upregulation of genes associated with inflammation and immune responses, and downregulation of genes associated with synaptic signaling and plasticity. Next, we generated one of the first atlases of molecular profiles for LS cell types using single nucleus RNA-sequencing (snRNA-seq). We identified markers for the septum broadly, and the LS specifically, as well as for all neuronal cell types. We then investigated whether the differentially expressed genes (DEGs) induced by TrkB knockdown map to specific LS cell types. Enrichment testing identified that downregulated DEGs are broadly expressed across neuronal clusters. Enrichment analyses of these DEGs demonstrated that downregulated genes are uniquely expressed in the LS, and associated with either synaptic plasticity or neurodevelopmental disorders. Upregulated genes are enriched in LS microglia, associated with immune response and inflammation, and linked to both neurodegenerative disease and neuropsychiatric disorders. In addition, many of these genes are implicated in regulating social behaviors. In summary, the findings implicate TrkB signaling in the LS as a critical regulator of gene networks associated with psychiatric disorders that display social deficits, including schizophrenia and autism, and with neurodegenerative diseases, including Alzheimer's.

Presynaptic Kainate Receptors onto Somatostatin Interneurons Are Recruited by Activity throughout Development and Contribute to Cortical Sensory Adaptation

Somatostatin (SST) interneurons produce delayed inhibition because of the short-term facilitation of their excitatory inputs created by the expression of metabotropic glutamate receptor 7 (mGluR7) and presynaptic GluK2-containing kainate receptors (GluK2-KARs). Using mice of both sexes, we find that as synaptic facilitation at layer (L)2/3 SST cell inputs increases during the first few postnatal weeks, so does GluK2-KAR expression. Removal of sensory input by whisker trimming does not affect mGluR7 but prevents the emergence of presynaptic GluK2-KARs, which can be restored by allowing whisker regrowth or by acute calmodulin activation. Conversely, late trimming or acute inhibition of Ca2+/calmodulin-dependent protein kinase II is sufficient to reduce GluK2-KAR activity. This developmental and activity-dependent regulation also produces a specific reduction of L4 GluK2-KARs that advances in parallel with the maturation of sensory processing in L2/3. Finally, we find that removal of both GluK2-KARs and mGluR7 from the synapse eliminates short-term facilitation and reduces sensory adaptation to repetitive stimuli, first in L4 of somatosensory cortex, then later in development in L2/3. The dynamic regulation of presynaptic GluK2-KARs potentially allows for flexible scaling of late inhibition and sensory adaptation.SIGNIFICANCE STATEMENT Excitatory synapses onto somatostatin (SST) interneurons express presynaptic, calcium-permeable kainate receptors containing the GluK2 subunit (GluK2-KARs), activated by high-frequency activity. In this study we find that their presence on L2/3 SST synapses in the barrel cortex is not based on a hardwired genetic program but instead is regulated by sensory activity, in contrast to that of mGluR7. Thus, in addition to standard synaptic potentiation and depression mechanisms, excitatory synapses onto SST neurons undergo an activity-dependent presynaptic modulation that uses GluK2-KARs. Further, we present evidence that loss of the frequency-dependent synaptic components (both GluK2-KARs and mGluR7 via Elfn1 deletion) contributes to a decrease in the sensory adaptation commonly seen on repetitive stimulus presentation.

Differential Activity of Orthosteric Agonists and Allosteric Modulators at Metabotropic Glutamate Receptor 7

Metabotropic glutamate receptor 7 (mGlu7) is a G protein coupled receptor that has demonstrated promise as a therapeutic target across a number of neurologic and psychiatric diseases. Compounds that modulate the activity of mGlu7, such as positive and negative allosteric modulators, may represent new therapeutic strategies to modulate receptor activity. The endogenous neurotransmitter associated with the mGlu receptor family, glutamate, exhibits low efficacy and potency in activating mGlu7, and surrogate agonists, such as the compound L-(+)-2-Amino-4-phosphonobutyric acid (L-AP4), are often used for receptor activation and compound profiling. To understand the implications of the use of such agonists in the development of positive allosteric modulators (PAMs), we performed a systematic evaluation of receptor activation using a system in which mutations can be made in either protomer of the mGlu7 dimer; we employed mutations that prevent interaction with the orthosteric site as well as the G-protein coupling site of the receptor. We then measured increases in calcium levels downstream of a promiscuous G protein to assess the effects of mutations in one of the two protomers in the presence of two different agonists and three positive allosteric modulators. Our results reveal that distinct PAMs, for example N-[3-Chloro-4-[(5-chloro-2-pyridinyl)oxy]phenyl]-2-pyridinecarboxamide (VU0422288) and 3-(2,3-Difluoro-4-methoxyphenyl)-2,5-dimethyl-7-(trifluoromethyl)pyrazolo[1,5-a]pyrimidine (VU6005649), do exhibit different maximal levels of potentiation with L-AP4 versus glutamate, but there appear to be common stable receptor conformations that are shared among all of the compounds examined here. SIGNIFICANCE STATEMENT: This manuscript describes the systematic evaluation of the mGlu7 agonists glutamate and L-(+)-2-Amino-4-phosphonobutyric acid (L-AP4) in the presence and absence of three distinct potentiators examining possible mechanistic differences. These findings demonstrate that mGlu7 potentiators display subtle variances in response to glutamate versus L-AP4.

GRM7 gene mutations and consequences for neurodevelopment

The metabotropic glutamate receptor 7 (mGlu7), encoded by the GRM7 gene in humans, is a presynaptic, G protein-coupled glutamate receptor that is essential for modulating neurotransmission. Mutations in or reduced expression of GRM7 have been identified in different genetic neurodevelopmental disorders (NDDs), and rare biallelic missense variants have been proposed to underlie a subset of NDDs. Clinical GRM7 variants have been associated with a range of symptoms consistent with neurodevelopmental molecular features, including hypomyelination, brain atrophy and defects in axon outgrowth. Here, we review the newest findings regarding the cellular and molecular defects caused by GRM7 variants in NDD patients.

M1/M4-Preferring Muscarinic Cholinergic Receptor Agonist Xanomeline Reverses Wake and Arousal Deficits in Nonpathologically Aged Mice

Degeneration of the cholinergic basal forebrain is implicated in the development of cognitive deficits and sleep/wake architecture disturbances in mild cognitive impairment (MCI) and Alzheimer's disease (AD). Indirect-acting muscarinic cholinergic receptor agonists, such as acetylcholinesterase inhibitors (AChEIs), remain the only FDA-approved treatments for the cognitive impairments observed in AD that target the cholinergic system. Novel direct-acting muscarinic cholinergic receptor agonists also improve cognitive performance in young and aged preclinical species and are currently under clinical development for AD. However, little is known about the effects of direct-acting muscarinic cholinergic receptor agonists on disruptions of sleep/wake architecture and arousal observed in nonpathologically aged rodents, nonhuman primates, and clinical populations. The purpose of the present study was to provide the first assessment of the effects of the direct-acting M1/M4-preferring muscarinic cholinergic receptor agonist xanomeline on sleep/wake architecture and arousal in young and nonpathologically aged mice, in comparison with the AChEI donepezil, when dosed in either the active or inactive phase of the circadian cycle. Xanomeline produced a robust reversal of both wake fragmentation and disruptions in arousal when dosed in the active phase of nonpathologically aged mice. In contrast, donepezil had no effect on either age-related wake fragmentation or arousal deficits when dosed during the active phase. When dosed in the inactive phase, both xanomeline and donepezil produced increases in wake and arousal and decreases in nonrapid eye movement sleep quality and quantity in nonpathologically aged mice. Collectively, these novel findings suggest that direct-acting muscarinic cholinergic agonists such as xanomeline may provide enhanced wakefulness and arousal in nonpathological aging, MCI, and AD patient populations.

Metabotropic glutamate receptor function and regulation of sleep-wake cycles

Metabotropic glutamate (mGlu) receptors are the most abundant family of G-protein coupled receptors and are widely expressed throughout the central nervous system (CNS). Alterations in glutamate homeostasis, including dysregulations in mGlu receptor function, have been indicated as key contributors to multiple CNS disorders. Fluctuations in mGlu receptor expression and function also occur across diurnal sleep-wake cycles. Sleep disturbances including insomnia are frequently comorbid with neuropsychiatric, neurodevelopmental, and neurodegenerative conditions. These often precede behavioral symptoms and/or correlate with symptom severity and relapse. Chronic sleep disturbances may also be a consequence of primary symptom progression and can exacerbate neurodegeneration in disorders including Alzheimer's disease (AD). Thus, there is a bidirectional relationship between sleep disturbances and CNS disorders; disrupted sleep may serve as both a cause and a consequence of the disorder. Importantly, comorbid sleep disturbances are rarely a direct target of primary pharmacological treatments for neuropsychiatric disorders even though improving sleep can positively impact other symptom clusters. This chapter details known roles of mGlu receptor subtypes in both sleep-wake regulation and CNS disorders focusing on schizophrenia, major depressive disorder, post-traumatic stress disorder, AD, and substance use disorder (cocaine and opioid). In this chapter, preclinical electrophysiological, genetic, and pharmacological studies are described, and, when possible, human genetic, imaging, and post-mortem studies are also discussed. In addition to reviewing the important relationships between sleep, mGlu receptors, and CNS disorders, this chapter highlights the development of selective mGlu receptor ligands that hold promise for improving both primary symptoms and sleep disturbances.

Metabotropic glutamate receptors and cognition: From underlying plasticity and neuroprotection to cognitive disorders and therapeutic targets

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors that play pivotal roles in mediating the activity of neurons and other cell types within the brain, communication between cell types, synaptic plasticity, and gene expression. As such, these receptors play an important role in a number of cognitive processes. In this chapter, we discuss the role of mGlu receptors in various forms of cognition and their underlying physiology, with an emphasis on cognitive dysfunction. Specifically, we highlight evidence that links mGlu physiology to cognitive dysfunction across brain disorders including Parkinson's disease, Alzheimer's disease, Fragile X syndrome, post-traumatic stress disorder, and schizophrenia. We also provide recent evidence demonstrating that mGlu receptors may elicit neuroprotective effects in particular disease states. Lastly, we discuss how mGlu receptors can be targeted utilizing positive and negative allosteric modulators as well as subtype specific agonists and antagonist to restore cognitive function across these disorders.

Pathophysiology and Current Drug Treatments for Post-Stroke Depression: A Review

Post-stroke depression (PSD) is a biopsychosocial disorder that affects individuals who have suffered a stroke at any point. PSD has a 20 to 60 percent reported prevalence among stroke survivors. Its effects are usually adverse, can lead to disability, and may increase mortality if not managed or treated early. PSD is linked to several other medical conditions, including anxiety, hyper-locomotor activity, and poor functional recovery. Despite significant awareness of its adverse impacts, understanding the pathogenesis of PSD has proved challenging. The exact pathophysiology of PSD is unknown, yet its complexity has been definitively shown, involving mechanisms such as dysfunction of monoamine, the glutamatergic systems, the gut-brain axis, and neuroinflammation. The current effectiveness of PSD treatment is about 30-40 percent of all cases. In this review, we examined different pathophysiological mechanisms and current pharmacological and non-pharmacological approaches for the treatment of PSD.

Sex differences and hormonal regulation of metabotropic glutamate receptor synaptic plasticity

Striking sex differences exist in presentation and incidence of several psychiatric disorders. For example, major depressive disorder is more prevalent in women than men, and women who develop alcohol use disorder progress through drinking milestones more rapidly than men. With regards to psychiatric treatment responses, women respond more favorably to selective serotonin reuptake inhibitors than men, whereas men have better outcomes when prescribed tricyclic antidepressants. Despite such well-documented biases in incidence, presentation, and treatment response, sex as a biological variable has long been neglected in preclinical and clinical research. An emerging family of druggable targets for psychiatric diseases, metabotropic glutamate (mGlu) receptors are G-protein coupled receptors broadly distributed throughout the central nervous system. mGlu receptors confer diverse neuromodulatory actions of glutamate at the levels of synaptic plasticity, neuronal excitability, and gene transcription. In this chapter, we summarize the current preclinical and clinical evidence for sex differences in mGlu receptor function. We first highlight basal sex differences in mGlu receptor expression and function and proceed to describe how gonadal hormones, notably estradiol, regulate mGlu receptor signaling. We then describe sex-specific mechanisms by which mGlu receptors differentially modulate synaptic plasticity and behavior in basal states and models relevant for disease. Finally, we discuss human research findings and highlight areas in need of further research. Taken together, this review emphasizes how mGlu receptor function and expression can differ across sex. Gaining a more complete understanding of how sex differences in mGlu receptor function contribute to psychiatric diseases will be critical in the development of novel therapeutics that are effective in all individuals.

G protein-coupled receptors in cochlea: Potential therapeutic targets for hearing loss

The prevalence of hearing loss-related diseases caused by different factors is increasing worldwide year by year. Currently, however, the patient’s hearing loss has not been effectively improved. Therefore, there is an urgent need to adopt new treatment measures and treatment techniques to help improve the therapeutic effect of hearing loss. G protein-coupled receptors (GPCRs), as crucial cell surface receptors, can widely participate in different physiological and pathological processes, particularly play an essential role in many disease occurrences and be served as promising therapeutic targets. However, no specific drugs on the market have been found to target the GPCRs of the cochlea. Interestingly, many recent studies have demonstrated that GPCRs can participate in various pathogenic process related to hearing loss in the cochlea including heredity, noise, ototoxic drugs, cochlear structure, and so on. In this review, we comprehensively summarize the functions of 53 GPCRs known in the cochlea and their relationships with hearing loss, and highlight the recent advances of new techniques used in cochlear study including cryo-EM, AI, GPCR drug screening, gene therapy vectors, and CRISPR editing technology, as well as discuss in depth the future direction of novel GPCR-based drug development and gene therapy for cochlear hearing loss. Collectively, this review is to facilitate basic and (pre-) clinical research in this area, and provide beneficial help for emerging GPCR-based cochlear therapies.

Differential activity of mGlu7 allosteric modulators provides evidence for mGlu7/8 heterodimers at hippocampal Schaffer collateral-CA1 synapses

Glutamate acts at eight metabotropic glutamate (mGlu) receptor subtypes expressed in a partially overlapping fashion in distinct brain circuits. Recent evidence indicates that specific mGlu receptor protomers can heterodimerize and that these heterodimers can exhibit different pharmacology when compared to their homodimeric counterparts. Group III mGlu agonist-induced suppression of evoked excitatory potentials and induction of long-term potentiation at Schaffer collateral-CA1 (SC-CA1) synapses in the rodent hippocampus can be blocked by the selective mGlu₇ negative allosteric modulator (NAM), ADX71743. Curiously, a different mGlu₇ NAM, 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazonolo[4,5-c]pyridin-4(5H)-one, failed to block these responses in brain slices despite its robust activity at mGlu₇ homodimers in vitro. We hypothesized that this might result from heterodimerization of mGlu₇ with another mGlu receptor protomer and focused on mGlu₈ as a candidate given the reported effects of mGlu₈-targeted compounds in the hippocampus. Here, we used complemented donor acceptor-resonance energy transfer to study mGlu_7/8 heterodimer activation in vitro and observed that ADX71743 blocked responses of both mGlu_7/7 homodimers and mGlu_7/8 heterodimers, whereas 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazonolo[4,5-c]pyridin-4(5H)-one only antagonized responses of mGlu_7/7 homodimers. Taken together with our electrophysiology observations, these results suggest that a receptor with pharmacology consistent with an mGlu_7/8 heterodimer modulates the activity of SC-CA1 synapses. Building on this hypothesis, we identified two additional structurally related mGlu₇ NAMs that also differ in their activity at mGlu_7/8 heterodimers, in a manner consistent with their ability to inhibit synaptic transmission and plasticity at SC-CA1. Thus, we propose that mGlu_7/8 heterodimers are a key molecular target for modulating the activity of hippocampal SC-CA1 synapses.

Metabotropic Glutamate Receptors at Ribbon Synapses in the Retina and Cochlea

Our senses define our view of the world. They allow us to adapt to environmental stimuli and are essential for communication and social behaviour. For most humans, seeing and hearing are central senses for their daily life. Our eyes and ears respond to an extraordinary broad range of stimuli covering about 12 log units of light intensity or acoustic power, respectively. The cellular basis is represented by sensory cells (photoreceptors in the retina and inner hair cells in the cochlea) that convert sensory inputs into electrical signals. Photoreceptors and inner hair cells have developed a specific pre-synaptic structure, termed synaptic ribbon, that is decorated with numerous vesicles filled with the excitatory neurotransmitter glutamate. At these ribbon synapses, glutamatergic signal transduction is guided by distinct sets of metabotropic glutamate receptors (mGluRs). MGluRs belong to group II and III of the receptor classification can inhibit neuronal activity, thus protecting neurons from overstimulation and subsequent degeneration. Consequently, dysfunction of mGluRs is associated with vision and hearing disorders. In this review, we introduce the principle characteristics of ribbon synapses and describe group II and III mGluRs in these fascinating structures in the retina and cochlea.

MGluR7 is a presynaptic metabotropic glutamate receptor at ribbon synapses of inner hair cells

Glutamate is the most pivotal excitatory neurotransmitter in the central nervous system. Metabotropic glutamate receptors (mGluRs) dimerize and can couple to inhibitory intracellular signal cascades, thereby protecting glutamatergic neurons from excessive excitation and cell death. MGluR7 is correlated with age-related hearing deficits and noise-induced hearing loss; however its exact localization in the cochlea is unknown. Here, we analyzed the expression and localization of mGluR7a and mGluR7b in mouse cochlear wholemounts in detail, using confocal microscopy and 3D reconstructions. We observed a presynaptic localization of mGluR7a at inner hair cells (IHCs), close to the synaptic ribbon. To detect mGluR7b, newly generated antibodies were characterized and showed co-localization with mGluR7a at IHC ribbon synapses. Compared to the number of synaptic ribbons, the numbers of mGluR7a and mGluR7b puncta were reduced at higher frequencies (48 to 64 kHz) and in older animals (6 and 12 months). Previously, we reported a presynaptic localization of mGluR4 and mGluR8b at this synapse type. This enables the possibility for the formation of homo- and/or heterodimeric receptors composed of mGluR4, mGluR7a, mGluR7b and mGluR8b at IHC ribbon synapses. These receptor complexes might represent new molecular targets suited for pharmacological concepts to protect the cochlea against noxious stimuli and excitotoxicity.

Overexpression of mGluR7 in the Prefrontal Cortex Attenuates Autistic Behaviors in Mice

Autism spectrum disorder (ASD) is associated with a range of abnormalities pertaining to socialization, communication, repetitive behaviors, and restricted interests. Owing to its complexity, the etiology of ASD remains incompletely understood. The presynaptic G protein-coupled glutamate receptor metabotropic glutamate receptor 7 (mGluR7) is known to be essential for synaptic transmission and is also tightly linked with ASD incidence. Herein, we report that prefrontal cortex (PFC) mGluR7 protein levels were decreased in C57BL/6J mice exposed to valproic acid (VPA) and BTBR T⁺ Itpr3^tf/J mice. The overexpression of mGluR7 in the PFC of these mice using a lentiviral vector was sufficient to reduce the severity of ASD-like behavioral patterns such that animals exhibited decreases in abnormal social interactions and communication, anxiety-like, and stereotyped/repetitive behaviors. Intriguingly, patch-clamp recordings revealed that the overexpression of mGluR7 suppressed neuronal excitability by inhibiting action potential discharge frequencies, together with enhanced action potential threshold and increased rheobase. These data offer a scientific basis for the additional study of mGluR7 as a promising therapeutic target in ASD and related neurodevelopmental disorders.

Trans-Synaptic Regulation of Metabotropic Glutamate Receptors by Elfn Proteins in Health and Disease

Trans-regulation of G protein-coupled receptors (GPCRs) by leucine-rich repeat (LRR) transmembrane proteins has emerged as a novel type of synaptic molecular interaction in the last decade. Several studies on LRR–GPCR interactions have revealed their critical role in synapse formation and in establishing synaptic properties. Among them, LRR–GPCR interactions between extracellular LRR fibronectin domain-containing family proteins (Elfn1 and Elfn2) and metabotropic glutamate receptors (mGluRs) are particularly interesting as they can affect a broad range of synapses through the modulation of signaling by glutamate, the principal excitatory transmitter in the mammalian central nervous system (CNS). Elfn–mGluR interactions have been investigated in hippocampal, cortical, and retinal synapses. Postsynaptic Elfn1 in the hippocampus and cerebral cortex mediates the tonic regulation of excitatory input onto somatostatin-positive interneurons (INs) through recruitment of presynaptic mGluR7. In the retina, presynaptic Elfn1 binds to mGluR6 and is necessary for synapse formation between rod photoreceptor cells and rod-bipolar cells. The repertoire of binding partners for Elfn1 and Elfn2 includes all group III mGluRs (mGluR4, mGluR6, mGluR7, and mGluR8), and both Elfn1 and Elfn2 can alter mGluR-mediated signaling through trans-interaction. Importantly, both preclinical and clinical studies have provided support for the involvement of the Elfn1–mGluR7 interaction in attention-deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), and epilepsy. In fact, Elfn1–mGluR7-associated disorders may reflect the altered function of somatostatin-positive interneuron inhibitory neural circuits, the mesolimbic and nigrostriatal dopaminergic pathway, and habenular circuits, highlighting the need for further investigation into this interaction.

A GRM7 mutation associated with developmental delay reduces mGlu7 expression and produces neurological phenotypes

The metabotropic glutamate receptor 7 (mGlu₇) is a G protein–coupled receptor that has been recently linked to neurodevelopmental disorders. This association is supported by the identification of GRM7 variants in patients with autism spectrum disorder, attention deficit hyperactivity disorder, and severe developmental delay. One GRM7 mutation previously reported in 2 patients results in a single amino acid change, I154T, within the mGlu₇ ligand-binding domain. Here, we report 2 new patients with this mutation who present with severe developmental delay and epilepsy. Functional studies of the mGlu₇-I154T mutant reveal that this substitution resulted in significant loss of mGlu₇ protein expression in HEK293A cells and in mice. We show that this occurred posttranscriptionally at the level of protein expression and trafficking. Similar to mGlu₇–global KO mice, mGlu₇-I154T animals exhibited reduced motor coordination, deficits in contextual fear learning, and seizures. This provides functional evidence that a disease-associated mutation affecting the mGlu₇ receptor was sufficient to cause neurological dysfunction in mice and further validates GRM7 as a disease-causing gene in the human population.

Pathogenic GRM7 Mutations Associated with Neurodevelopmental Disorders Impair Axon Outgrowth and Presynaptic Terminal Development

Metabotropic glutamate receptor 7 (mGlu7) is an inhibitory heterotrimeric G-protein-coupled receptor that modulates neurotransmitter release and synaptic plasticity at presynaptic terminals in the mammalian central nervous system. Recent studies have shown that rare mutations in glutamate receptors and synaptic scaffold proteins are associated with neurodevelopmental disorders (NDDs). However, the role of presynaptic mGlu7 in the pathogenesis of NDDs remains largely unknown. Recent whole-exome sequencing (WES) studies in families with NDDs have revealed that several missense mutations (c.1865G>A:p.R622Q; c.461T>C:p.I154T; c.1972C>T:p.R658W and c.2024C>A:p.T675K) or a nonsense mutation (c.1757G>A:p.W586X) in the GRM7 gene may be linked to NDDs. In the present study, we investigated the mechanistic links between GRM7 point mutations and NDD pathology. We find that the pathogenic GRM7 I154T and R658W/T675K mutations lead to the degradation of the mGlu7 protein. In particular, the GRM7 R658W/T675K mutation results in a lack of surface mGlu7 expression in heterologous cells and cultured neurons isolated from male and female rat embryos. We demonstrate that the expression of mGlu7 variants or exposure to mGlu7 antagonists impairs axon outgrowth through the mitogen-activated protein kinase (MAPK)-cAMP-protein kinase A (PKA) signaling pathway during early neuronal development, which subsequently leads to a decrease in the number of presynaptic terminals in mature neurons. Treatment with an mGlu7 agonist restores the pathologic phenotypes caused by mGlu7 I154T but not by mGlu7 R658W/T675K because of its lack of neuronal surface expression. These findings provide evidence that stable neuronal surface expression of mGlu7 is essential for neural development and that mGlu7 is a promising therapeutic target for NDDs.SIGNIFICANCE STATEMENT Neurodevelopmental disorders (NDDs) affect brain development and function by multiple etiologies. Metabotropic glutamate receptor 7 (mGlu7) is a receptor that controls excitatory neurotransmission and synaptic plasticity. Since accumulating evidence indicates that the GRM7 gene locus is associated with NDD risk, we analyzed the functional effects of human GRM7 variants identified in patients with NDDs. We demonstrate that stable neuronal surface expression of mGlu7 is essential for axon outgrowth and presynaptic terminal development in neurons. We found that mitogen-activated protein kinase (MAPK)-cAMP-protein kinase A (PKA) signaling and subsequent cytoskeletal dynamics are defective because of the degradation of mGlu7 variants. Finally, we show that the defects caused by mGlu7 I154T can be reversed by agonists, providing the rationale for proposing mGlu7 as a potential therapeutic target for NDDs.

LSP2-9166, an orthosteric mGlu4 and mGlu7 receptor agonist, reduces cocaine self-administration under a progressive ratio schedule in rats

Cocaine addiction is a serious health issue in Western countries. Despite the regular increase in cocaine consumption across the population, there is no specific treatment for cocaine addiction. Critical roles for glutamate neurotransmission in the rewarding effects of psychostimulants as well as relapse have been suggested and accumulating evidence indicates that targeting mGlu group III receptors could represent a promising strategy to develop therapeutic compounds to treat addiction. In this context, the aim of our study was to examine the effect of LSP2-9166, a mGlu4/mGlu7 receptor orthosteric agonist, on the motivation for cocaine intake. We used an intravenous self-administration paradigm in male Wistar rats as a reliable model of voluntary drug intake. We first evaluated the direct impact of cocaine on Grm4 and Grm7 gene expression. Voluntary cocaine intake under a fixed ratio schedule of injections induced an increase of both mGlu4 and mGlu7 receptor transcripts in nucleus accumbens and hippocampus. We then evaluated the ability of LSP2-9166 to affect cocaine self-administration under a progressive ratio schedule of reinforcement. We found that this compound inhibits the motivation to obtain the drug, although it induced a hypolocomotor effect which could biais motivation index. Our findings demonstrate that mGlu group III receptors represent new targets for decreasing motivation to self-administer cocaine.

Modulation of arousal and sleep/wake architecture by M1 PAM VU0453595 across young and aged rodents and nonhuman primates

Degeneration of basal forebrain cholinergic circuitry represents an early event in the development of Alzheimer's disease (AD). These alterations in central cholinergic function are associated with disruptions in arousal, sleep/wake architecture, and cognition. Changes in sleep/wake architecture are also present in normal aging and may represent a significant risk factor for AD. M₁ muscarinic acetylcholine receptor (mAChR) positive allosteric modulators (PAMs) have been reported to enhance cognition across preclinical species and may also provide beneficial effects for age- and/or neurodegenerative disease-related changes in arousal and sleep. In the present study, electroencephalography was conducted in young animals (mice, rats and nonhuman primates [NHPs]) and in aged mice to examine the effects of the selective M₁ PAM VU0453595 in comparison with the acetylcholinesterase inhibitor donepezil, M₁/M₄ agonist xanomeline (in NHPs), and M₁ PAM BQCA (in rats) on sleep/wake architecture and arousal. In young wildtype mice, rats, and NHPs, but not in M₁ mAChR KO mice, VU0453595 produced dose-related increases in high frequency gamma power, a correlate of arousal and cognition enhancement, without altering duration of time across all sleep/wake stages. Effects of VU0453595 in NHPs were observed within a dose range that did not induce cholinergic-mediated adverse effects. In contrast, donepezil and xanomeline increased time awake in rodents and engendered dose-limiting adverse effects in NHPs. Finally, VU0453595 attenuated age-related decreases in REM sleep duration in aged wildtype mice. Development of M₁ PAMs represents a viable strategy for attenuating age-related and dementia-related pathological disturbances of sleep and arousal.