Norbin is an endogenous regulator of metabotropic glutamate receptor 5 signaling

Operant Training for Highly Palatable Food Alters Translating Messenger RNA in Nucleus Accumbens D2 Neurons and Reveals a Modulatory Role of Ncdn

BACKGROUND

Highly palatable food triggers behavioral responses including strong motivation. These effects involve the reward system and dopamine neurons, which modulate neurons in the nucleus accumbens (NAc). The molecular mechanisms underlying the long-lasting effects of highly palatable food on feeding behavior are poorly understood.

METHODS

We studied the effects of 2-week operant conditioning of mice with standard or isocaloric highly palatable food. We investigated the behavioral responses and dendritic spine modifications in the NAc. We compared the translating messenger RNA in NAc neurons identified by the type of dopamine receptors they express, depending on the kind of food and training. We tested the consequences of invalidation of an abundant downregulated gene, Ncdn.

RESULTS

Operant conditioning for highly palatable food increased motivation for food even in well-fed mice. In wild-type mice, free choice between regular and highly palatable food increased weight compared with access to regular food only. Highly palatable food increased spine density in the NAc. In animals trained for highly palatable food, translating messenger RNAs were modified in NAc neurons expressing dopamine D2 receptors, mostly corresponding to striatal projection neurons, but not in neurons expressing D1 receptors. Knockout of Ncdn, an abundant downregulated gene, opposed the conditioning-induced changes in satiety-sensitive feeding behavior and apparent motivation for highly palatable food, suggesting that downregulation may be a compensatory mechanism.

CONCLUSIONS

Our results emphasize the importance of messenger RNA alterations in D2 striatal projection neurons in the NAc in the behavioral consequences of highly palatable food conditioning and suggest a modulatory contribution of Ncdn downregulation.

Group I and group II metabotropic glutamate receptors are upregulated in the synapses of infant rats prenatally exposed to valproic acid

RATIONALE

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interaction and restricted/stereotyped behavior. Prenatal exposure to valproic acid (VPA) is associated with an increased risk of developing ASD in humans and autistic-like behaviors in rodents. Increasing evidence indicates that dysfunctions of glutamate receptors at synapses are associated with ASD. In the VPA rat model, an involvement of glutamate receptors in autism-like phenotypes has been suggested; however, few studies were carried out on metabotropic glutamate (mGlu) receptors.

OBJECTIVES

We examined the protein expression levels of group I (mGlu1 and mGlu5) and group II (mGlu2/3) mGlu receptors in rats prenatally exposed to VPA and evaluated the effect of mGlu receptor modulation on an early autism-like phenotype in these animals.

METHODS

We used western blotting analysis on synaptosomes obtained from forebrain of control and VPA rats at different ages (postnatal day P13, 35, 90) and carried out ultrasonic vocalization (USV) emission test in infant control and VPA rats.

RESULTS

The expression levels of all these receptors were significantly increased in infant VPA rats. No changes were detected in adolescent and adult rats. An acute treatment with the preferential mGlu2/3 antagonist, LY341495, attenuated the impairment in the USV emission in VPA rats. No effect was observed after a treatment with the mGlu5 selective antagonist, MTEP.

CONCLUSIONS

Our findings demonstrate that the expression of group I and group II mGlu receptors is upregulated at synapses of infant VPA rats and suggest that mGlu2/3 receptor modulation may have a therapeutic potential in ASD.

The GPCR adaptor protein Norbin regulates S1PR1 trafficking and the morphology, cell cycle and survival of PC12 cells

Norbin is an adaptor protein that binds numerous G protein-coupled receptors (GPCRs), is highly expressed in neurons, and is essential for a functioning nervous system in rodent models. Yet, beyond its control of neurite outgrowth and synaptic plasticity, few cellular roles of Norbin have been investigated to date. Furthermore, while Norbin is known to regulate the steady-state cell surface levels of several GPCRs, only in one case has the protein been shown to control the agonist-induced receptor internalisation which serves to attenuate GPCR signalling. Here, we generated a Norbin-deficient PC12 cell line which enabled us to study both the cellular functions of Norbin and its roles in GPCR trafficking and signalling. We show that Norbin limits cell size and spreading, and is required for the growth, viability and cell cycle progression of PC12 cells. We also found that Norbin regulates both the steady-state surface level and agonist-induced internalisation of the GPCR sphingosine-1-phosphate receptor 1 (S1PR1) in these cells, suggesting that its role in agonist-dependent GPCR trafficking is more widespread than previously appreciated. Finally, we show that Norbin limits the S1P-stimulated activation of Akt and p38 Mapk, and is required for the activation of Erk in PC12 cells. Together, our findings provide a better understanding of the cellular functions of Norbin and its control of GPCR trafficking.

Functions and mechanisms of the GPCR adaptor protein Norbin

Norbin (Neurochondrin, NCDN) is a highly conserved 79 kDa adaptor protein that was first identified more than a quarter of a century ago as a gene up-regulated in rat hippocampus upon induction of long-term potentiation. Most research has focussed on the role of Norbin in the nervous system, where the protein is highly expressed. Norbin regulates neuronal morphology and synaptic plasticity, and is essential for normal brain development and homeostasis. Dysregulation of Norbin is linked to a variety of neurological conditions. Recently, Norbin was shown to be expressed in myeloid cells as well as neurons. Myeloid-cell specific deletion revealed an important role of Norbin as a suppressor of neutrophil-derived innate immunity. Norbin limits the ability of neutrophils to clear bacterial infections by curbing the responsiveness of these cells to inflammatory and infectious stimuli. Mechanistically, Norbin regulates cell responses through binding to its interactors, in particular to a wide range of G protein-coupled receptors (GPCRs). Norbin association with GPCRs controls GPCR trafficking and signalling. Other important Norbin interactors are the Rac guanine-nucleotide exchange factor P-Rex1 and protein kinase A. Downstream signalling pathways regulated by Norbin include ERK, Ca2+ and the small GTPase Rac. Here, we review the current understanding of Norbin structure, expression and its roles in health and disease. We also explore Norbin signalling through its interactors, with a particular focus on GPCR trafficking and signalling. Finally, we discuss avenues that could be pursued in the future to increase our understanding of Norbin biology.

Updates on the Physiopathology of Group I Metabotropic Glutamate Receptors (mGluRI)-Dependent Long-Term Depression

Group I metabotropic glutamate receptors (mGluRI), including mGluR1 and mGluR5 subtypes, modulate essential brain functions by affecting neuronal excitability, intracellular calcium dynamics, protein synthesis, dendritic spine formation, and synaptic transmission and plasticity. Nowadays, it is well appreciated that the mGluRI-dependent long-term depression (LTD) of glutamatergic synaptic transmission (mGluRI-LTD) is a key mechanism by which mGluRI shapes connectivity in various cerebral circuitries, directing complex brain functions and behaviors, and that it is deranged in several neurological and psychiatric illnesses, including neurodevelopmental disorders, neurodegenerative diseases, and psychopathologies. Here, we will provide an updated overview of the physiopathology of mGluRI-LTD, by describing mechanisms of induction and regulation by endogenous mGluRI interactors, as well as functional physiological implications and pathological deviations.

Protein Networks Associated with Native Metabotropic Glutamate 1 Receptors (mGlu1) in the Mouse Cerebellum

The metabotropic glutamate receptor 1 (mGlu1) plays a pivotal role in synaptic transmission and neuronal plasticity. Despite the fact that several interacting proteins involved in the mGlu1 subcellular trafficking and intracellular transduction mechanisms have been identified, the protein network associated with this receptor in specific brain areas remains largely unknown. To identify novel mGlu1-associated protein complexes in the mouse cerebellum, we used an unbiased tissue-specific proteomic approach, namely co-immunoprecipitation followed by liquid chromatography/tandem mass spectrometry analysis. Many well-known protein complexes as well as novel interactors were identified, including G-proteins, Homer, δ2 glutamate receptor, 14-3-3 proteins, and Na/K-ATPases. A novel putative interactor, KCTD12, was further investigated. Reverse co-immunoprecipitation with anti-KCTD12 antibodies revealed mGlu1 in wild-type but not in KCTD12-knock-out homogenates. Freeze-fracture replica immunogold labeling co-localization experiments showed that KCTD12 and mGlu1 are present in the same nanodomain in Purkinje cell spines, although at a distance that suggests that this interaction is mediated through interposed proteins. Consistently, mGlu1 could not be co-immunoprecipitated with KCTD12 from a recombinant mammalian cell line co-expressing the two proteins. The possibility that this interaction was mediated via GABAB receptors was excluded by showing that mGlu1 and KCTD12 still co-immunoprecipitated from GABAB receptor knock-out tissue. In conclusion, this study identifies tissue-specific mGlu1-associated protein clusters including KCTD12 at Purkinje cell synapses.

Operant training for highly palatable food alters translating mRNA in nucleus accumbens D2 neurons and reveals a modulatory role of Neurochondrin

BACKGROUND

Highly palatable food triggers behavioral alterations reminiscent of those induced by addictive drugs. These effects involve the reward system and dopamine neurons, which modulate neurons in the nucleus accumbens (NAc). The molecular mechanisms underlying the effects of highly palatable food on feeding behavior are poorly understood.

METHODS

We studied the effects of 2-week operant conditioning of mice with standard or isocaloric highly palatable food. We investigated the behavioral effects and dendritic spine modifications in the NAc. We compared the translating mRNA in NAc neurons identified by the type of dopamine receptors they express, depending on the type of food and training. We tested the consequences of invalidation of an abundant downregulated gene, Ncdn (Neurochondrin).

RESULTS

Operant conditioning for highly palatable food increases motivation for food even in well-fed mice. In control mice, free access to regular or highly palatable food results in increased weight as compared to regular food only. Highly palatable food increases spine density in the NAc. In animals trained for highly palatable food, translating mRNAs are modified in NAc dopamine D2-receptor-expressing neurons, mostly corresponding to striatal projection neurons, but not in those expressing D1-receptors. Knock-out of Ncdn, an abundant down-regulated gene, opposes the conditioning-induced changes in satiety-sensitive feeding behavior and apparent motivation for highly palatable food, suggesting down-regulation may be a compensatory mechanism.

CONCLUSIONS

Our results emphasize the importance of mRNA alterations D2 striatal projection neurons in the NAc in the behavioral consequences of highly palatable food conditioning and suggest a modulatory contribution of Ncdn downregulation.

Case Report: Alzheimer's Dementia Associated With Cerebrospinal Fluid Neurochondrin Autoantibodies

Background

Neurochondrin autoimmunity is a rare disorder mainly associated with cerebellar and vestibular syndromes. Our report aims to enlarge its phenotypic spectrum to encompass major cognitive disorder with very late onset never before reported in conjunction with neurochondrin antibodies.

Methods

We describe the case of an 85-year-old woman who presented in our memory clinic. Retrospective analysis of patient records included cerebrospinal fluid (CSF) analysis, magnetic resonance imaging (MRI), and neuropsychological testing using the CERAD-plus.

Results

Because of her unknown onset of progressive cognitive dysfunction in conjunction with speech and language problems, we decided to take an extensive differential diagnostic approach including a search for neural autoantibodies potentially involved in cognitive impairment. Our patient presented serum and CSF neurochondrin autoantibodies. Further CSF analysis revealed elevated tau and ptau 181 protein as well as a reduced Aß42/40 ratio in CSF, thus matching a biomarker profile of Alzheimer's disease (AD). Neuropsychological tests revealed predominant and severe deficits in verbal and visual memory. Her MRI showed reduced parietal and cerebellar brain volume.

Discussion

Taken together, this case reveals the novelty of a patient with a CSF-based and typical clinical and imaging profile of AD. She is also likely to have neurochondrin autoimmunity, as we detected neurochondrin autoantibodies in her CSF; we therefore diagnosed AD dementia associated with neurochondrin antibodies. Our case expands the spectrum of neurochondrin autoimmunity to disorders involving major cognitive disorder such as AD dementia. Furthermore, we speculate that neurochondrin autoimmunity might have triggered an acceleration of AD symptoms as its onset was reported only after a short 6-month interval via a synergistic or negatively additive hybrid mechanism of action between neurodegeneration and autoimmunity.

Changes in mGlu5 Receptor Signaling Are Associated with Associative Learning and Memory Extinction in Mice

Using an in vivo method for the assessment of polyphosphoinositide (PI) hydrolysis, we examine whether spatial learning and memory extinction cause changes in mGlu5 metabotropic glutamate receptor signaling in the hippocampus and prefrontal cortex. We use the following five groups of mice: (i) naive mice; (ii) control mice exposed to the same environment as learner mice; (iii) leaner mice, trained for four days in a water maze; (iv) mice in which memory extinction was induced by six trials without the platform; (v) mice that spontaneously lost memory. The mGlu5 receptor-mediated PI hydrolysis was significantly reduced in the dorsal hippocampus of learner mice as compared to naive and control mice. The mGlu5 receptor signaling was also reduced in the ventral hippocampus and prefrontal cortex of learner mice, but only with respect to naive mice. Memory extinction was associated with a large up-regulation of mGlu5 receptor-mediated PI hydrolysis in the three brain regions and with increases in mGlu5 receptor and phospholipase-Cβ protein levels in the ventral and dorsal hippocampus, respectively. These findings support a role for mGlu5 receptors in mechanisms underlying spatial learning and suggest that mGlu5 receptors are candidate drug targets for disorders in which cognitive functions are impaired or aversive memories are inappropriately retained.

A postzygotic de novo NCDN mutation identified in a sporadic FTLD patient results in neurochondrin haploinsufficiency and altered FUS granule dynamics

Frontotemporal dementia (FTD) is a heterogeneous clinical disorder characterized by progressive abnormalities in behavior, executive functions, personality, language and/or motricity. A neuropathological subtype of FTD, frontotemporal lobar degeneration (FTLD)-FET, is characterized by protein aggregates consisting of the RNA-binding protein fused in sarcoma (FUS). The cause of FTLD-FET is not well understood and there is a lack of genetic evidence to aid in the investigation of mechanisms of the disease. The goal of this study was to identify genetic variants contributing to FTLD-FET and to investigate their effects on FUS pathology. We performed whole-exome sequencing on a 50-year-old FTLD patient with ubiquitin and FUS-positive neuronal inclusions and unaffected parents, and identified a de novo postzygotic nonsense variant in the NCDN gene encoding Neurochondrin (NCDN), NM_014284.3:c.1206G > A, p.(Trp402*). The variant was associated with a ~ 31% reduction in full-length protein levels in the patient’s brain, suggesting that this mutation leads to NCDN haploinsufficiency. We examined the effects of NCDN haploinsufficiency on FUS and found that depleting primary cortical neurons of NCDN causes a reduction in the total number of FUS-positive cytoplasmic granules. Moreover, we found that these granules were significantly larger and more highly enriched with FUS. We then examined the effects of a loss of FUS function on NCDN in neurons and found that depleting cells of FUS leads to a decrease in NCDN protein and mRNA levels. Our study identifies the NCDN protein as a likely contributor of FTLD-FET pathophysiology. Moreover, we provide evidence for a negative feedback loop of toxicity between NCDN and FUS, where loss of NCDN alters FUS cytoplasmic dynamics, which in turn has an impact on NCDN expression.

Membrane trafficking and positioning of mGluRs at presynaptic and postsynaptic sites of excitatory synapses

The plethora of functions of glutamate in the brain are mediated by the complementary actions of ionotropic and metabotropic glutamate receptors (mGluRs). The ionotropic glutamate receptors carry most of the fast excitatory transmission, while mGluRs modulate transmission on longer timescales by triggering multiple intracellular signaling pathways. As such, mGluRs mediate critical aspects of synaptic transmission and plasticity. Interestingly, at synapses, mGluRs operate at both sides of the cleft, and thus bidirectionally exert the effects of glutamate. At postsynaptic sites, group I mGluRs act to modulate excitability and plasticity. At presynaptic sites, group II and III mGluRs act as auto-receptors, modulating release properties in an activity-dependent manner. Thus, synaptic mGluRs are essential signal integrators that functionally couple presynaptic and postsynaptic mechanisms of transmission and plasticity. Understanding how these receptors reach the membrane and are positioned relative to the presynaptic glutamate release site are therefore important aspects of synapse biology. In this review, we will discuss the currently known mechanisms underlying the trafficking and positioning of mGluRs at and around synapses, and how these mechanisms contribute to synaptic functioning. We will highlight outstanding questions and present an outlook on how recent technological developments will move this exciting research field forward.

CCCH-zinc finger antiviral protein relieves immunosuppression of T cell induced by avian leukosis virus subgroup J via NLP-PKC-δ-NFAT pathway

CCCH-zinc finger antiviral protein (ZAP) can recognize and induce the degradation of mRNAs and proteins of certain viruses, as well as exert its antiviral activity by activating T cell. However, the mechanism of ZAP mediating T cell activation during virus infection remains unclear. Here, we found a potential function of ZAP that relieves immunosuppression of T cell induced by avian leukosis virus subgroup J (ALV-J) via a novel signaling pathway that involves norbin like protein (NLP), protein kinase C delta (PKC-δ) and nuclear factor of activated T cell (NFAT). Specifically, ZAP expression activated T cells by promoting the dephosphorylation and nuclear translocation of NFAT. Furthermore, knockdown of ZAP weakened the reactivity and antiviral response of T cells. Mechanistically, ZAP reduced PKC-δ activity by up-regulating and reactivating NLP through competitively binding with viral protein. Knockdown of NLP decreased the dephosphorylation of PKC-δ by ZAP expression. Moreover, we showed that knockdown of PKC-δ reduced the phosphorylation levels of NFAT and enhanced its nuclear translocation. Taken together, these data revealed that ZAP relieves immunosuppression caused by ALV-J and mediates T cell activation through NLP-PKC-δ-NFAT pathway. Importance The evolution of host defense system is driven synchronously in the process of resisting virus invasion. Accordingly, host innate defense factors exert effectively work in suppressing virus replication. However, it remains unclear that whether the host innate defense factors are involved in antiviral immune response against the invasion of immunosuppressive viruses. Here, we found that CCCH-type zinc finger antiviral protein (ZAP) effectively worked in resistance on immunosuppression caused by avian leukosis virus subgroup J (ALV-J), a classic immunosuppressive virus. Evidence showed that ZAP released the phosphatase activity of NLP inhibited by ALV-J and further activated NFAT by inactivating PKC-δ. This novel molecular mechanism that ZAP regulates antiviral immune response by mediating NLP-PKC-δ-NFAT pathway has greatly enriched the understanding of the functions of host innate defense factors and provided important scientific ideas and theoretical basis for the research of immunosuppressive virus and antiviral immunity.

Metabotropic Glutamate Receptor Trafficking and its Role in Drug-Induced Neurobehavioral Plasticity

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system that guides developmental and experience-dependent changes in many cellular substrates and brain circuits, through the process collectively referred to as neurobehavioral plasticity. Regulation of cell surface expression and membrane trafficking of glutamate receptors represents an important mechanism that assures optimal excitatory transmission, and at the same time, also allows for fine-tuning neuronal responses to glutamate. On the other hand, there is growing evidence implicating dysregulated glutamate receptor trafficking in the pathophysiology of several neuropsychiatric disorders. This review provides up-to-date information on the molecular determinants regulating trafficking and surface expression of metabotropic glutamate (mGlu) receptors in the rodent and human brain and discusses the role of mGluR trafficking in maladaptive synaptic plasticity produced by addictive drugs. As substantial evidence links glutamatergic dysfunction to the progression and the severity of drug addiction, advances in our understanding of mGluR trafficking may provide opportunities for the development of novel pharmacotherapies of addiction and other neuropsychiatric disorders.

The GPCR adaptor protein norbin suppresses the neutrophil-mediated immunity of mice to pneumococcal infection

Streptococcal pneumonia is a worldwide health problem that kills ∼2 million people each year, particularly young children, the elderly, and immunosuppressed individuals. Alveolar macrophages and neutrophils provide the early innate immune response to clear pneumococcus from infected lungs. However, the level of neutrophil involvement is context dependent, both in humans and in mouse models of the disease, influenced by factors such as bacterial load, age, and coinfections. Here, we show that the G protein-coupled receptor (GPCR) adaptor protein norbin (neurochondrin, NCDN), which was hitherto known as a regulator of neuronal function, is a suppressor of neutrophil-mediated innate immunity. Myeloid norbin deficiency improved the immunity of mice to pneumococcal infection by increasing the involvement of neutrophils in clearing the bacteria, without affecting neutrophil recruitment or causing autoinflammation. It also improved immunity during Escherichia coli-induced septic peritonitis. It increased the responsiveness of neutrophils to a range of stimuli, promoting their ability to kill bacteria in a reactive oxygen species-dependent manner, enhancing degranulation, phagocytosis, and the production of reactive oxygen species and neutrophil extracellular traps, raising the cell surface levels of selected GPCRs, and increasing GPCR-dependent Rac and Erk signaling. The Rac guanine-nucleotide exchange factor Prex1, a known effector of norbin, was dispensable for most of these effects, which suggested that norbin controls additional downstream targets. We identified the Rac guanine-nucleotide exchange factor Vav as one of these effectors. In summary, our study presents the GPCR adaptor protein norbin as an immune suppressor that limits the ability of neutrophils to clear bacterial infections.

Cytohesin-2 mediates group I metabotropic glutamate receptor-dependent mechanical allodynia through the activation of ADP ribosylation factor 6 in the spinal cord

Group I metabotropic glutamate receptors (mGluRs), mGluR1 and mGluR5, in the spinal cord are implicated in nociceptive transmission and plasticity through G protein-mediated second messenger cascades leading to the activation of various protein kinases such as extracellular signal-regulated kinase (ERK). In this study, we demonstrated that cytohesin-2, a guanine nucleotide exchange factor for ADP ribosylation factors (Arfs), is abundantly expressed in subsets of excitatory interneurons and projection neurons in the superficial dorsal horn. Cytohesin-2 is enriched in the perisynapse on the postsynaptic membrane of dorsal horn neurons and forms a protein complex with mGluR5 in the spinal cord. Central nervous system-specific cytohesin-2 conditional knockout mice exhibited reduced mechanical allodynia in inflammatory and neuropathic pain models. Pharmacological blockade of cytohesin catalytic activity with SecinH3 similarly reduced mechanical allodynia and inhibited the spinal activation of Arf6, but not Arf1, in both pain models. Furthermore, cytohesin-2 conditional knockout mice exhibited reduced mechanical allodynia and ERK1/2 activation following the pharmacological activation of spinal mGluR1/5 with 3,5-dihydroxylphenylglycine (DHPG). The present study suggests that cytothesin-2 is functionally associated with mGluR5 during the development of mechanical allodynia through the activation of Arf6 in spinal dorsal horn neurons.

Monoallelic and bi-allelic variants in NCDN cause neurodevelopmental delay, intellectual disability, and epilepsy

Neurochondrin (NCDN) is a cytoplasmatic neural protein of importance for neural growth, glutamate receptor (mGluR) signaling, and synaptic plasticity. Conditional loss of Ncdn in mice neural tissue causes depressive-like behaviors, impaired spatial learning, and epileptic seizures. We report on NCDN missense variants in six affected individuals with variable degrees of developmental delay, intellectual disability (ID), and seizures. Three siblings were found homozygous for a NCDN missense variant, whereas another three unrelated individuals carried different de novo missense variants in NCDN. We assayed the missense variants for their capability to rescue impaired neurite formation in human neuroblastoma (SH-SY5Y) cells depleted of NCDN. Overexpression of wild-type NCDN rescued the neurite-phenotype in contrast to expression of NCDN containing the variants of affected individuals. Two missense variants, associated with severe neurodevelopmental features and epilepsy, were unable to restore mGluR5-induced ERK phosphorylation. Electrophysiological analysis of SH-SY5Y cells depleted of NCDN exhibited altered membrane potential and impaired action potentials at repolarization, suggesting NCDN to be required for normal biophysical properties. Using available transcriptome data from human fetal cortex, we show that NCDN is highly expressed in maturing excitatory neurons. In combination, our data provide evidence that bi-allelic and de novo variants in NCDN cause a clinically variable form of neurodevelopmental delay and epilepsy, highlighting a critical role for NCDN in human brain development.

Preconditioning by ultra-low dose of tramadol reduces the severity of tramadol-induced seizure: Contribution of glutamate receptors

Tramadol, a weak agonist of mu-opioid receptors, causes seizure via several mechanisms. Preconditioning has been purposed to reduce the epileptic seizures in animal models of epilepsy. The preconditioning effect of tramadol on seizure is not studied yet. This study was designed to evaluate the preconditioning effect of ultra-low dose of tramadol on the seizures induced by tramadol at high dose. Furthermore, regarding the critical role of glutamate signaling in the pathogenesis of epilepsy, the effect of preconditioning on some glutamate signaling elements was also examined. Male Wistar rats received tramadol (2 mg/kg, i.p) or normal saline (1 mL/kg, i.p) in preconditioning and control groups, respectively. After 4 days, the challenging tramadol dose (150 mg/kg) was injected to all rats. Epileptic behaviors were recorded during 50 min. The expression of Norbin (as a regulator of metabotropic glutamate receptor 5), Calponin3 (as a regulator of excitatory synaptic markers), NR1 (NMDA receptor subunit 1) and GluR1 (AMPA receptor subunit 1) was measured in hippocampus, prefrontal cortex (PFC) and amygdala. Preconditioning decreased the number and duration of tremors and tonic-clonic seizures. Norbin, Calponin3, NR1 and GluR1 expression were decreased in hippocampus, and preconditioning had no effect on them. In contrast, it increased Norbin expression in PFC and amygdala, and attenuated NR1 and GluR1 upregulation following tramadol at high dose. These findings indicated that preconditioning by ultra-low dose of tramadol protected the animals against seizures following high dose of tramadol mediated, at least in part, by Norbin up regulation, and NR1 and GluR1 down regulation.

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.

Paul Greengard: A persistent desire to comprehend the brain, and also to fix it

Paul Greengard's name is and will remain profoundly associated with Neuroscience, with brain signaling and chemical transmission, with Parkinson's and Alzheimer's diseases, with fundamental discoveries and solving paradoxes, but much less perhaps with drug discovery. This should not be mistaken as disdain. Paul in fact did contemplate developing therapeutic avenues to actually treat brain diseases much more than it is known, perhaps during his entire career, and certainly over the last two decades. As a matter of fact, he did more than contemplate it, he directly and indirectly contributed in the development of treatments for neurological diseases and disorders. Paul's impact on fundamental aspects of the brain has been so gargantuan that any other aspect of Paul's life will have difficulty to shine. It is precisely this less known aspect of Paul's career that will be covered in this review. We will discover how Paul very early on moved away from biophysics to avoid working on nuclear weapons and instead started his career in the pharmacological spheres of a large pharmaceutical company.

The post-synaptic scaffolding protein tamalin regulates ligand-mediated trafficking of metabotropic glutamate receptors

Group I metabotropic glutamate receptors (mGluRs) play important roles in various neuronal functions and have also been implicated in multiple neuropsychiatric disorders like fragile X syndrome, autism, and others. mGluR trafficking not only plays important roles in controlling the spatiotemporal localization of these receptors in the cell but also regulates the activity of these receptors. Despite this obvious significance, the cellular machineries that control the trafficking of group I metabotropic glutamate receptors in the central nervous system have not been studied in detail. The post-synaptic scaffolding protein tamalin has been shown to interact with group I mGluRs and also with many other proteins involved in protein trafficking in neurons. Using a molecular replacement approach in mouse hippocampal neurons, we show here that tamalin plays a critical role in the ligand-dependent internalization of mGluR1 and mGluR5, members of the group I mGluR family. Specifically, knockdown of endogenous tamalin inhibited the ligand-dependent internalization of these two receptors. Both N-terminal and C-terminal regions of tamalin played critical roles in mGluR1 endocytosis. Furthermore, we found that tamalin regulates mGluR1 internalization by interacting with S-SCAM, a protein that has been implicated in vesicular trafficking. Finally, we demonstrate that tamalin plays a critical role in mGluR-mediated internalization of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, a process believed to be the cellular correlate for mGluR-dependent synaptic plasticity. Taken together, these findings reveal a mechanistic role of tamalin in the trafficking of group I mGluRs and suggest its physiological implications in the brain.

Proteomic approach underlying the hippocampal neurodegeneration caused by low doses of methylmercury after long-term exposure in adult rats

Methylmercury (MeHg) is an important toxicant that causes cognitive dysfunctions in humans. This study aimed to investigate the proteomic and biochemical alterations of the hippocampus associated with behavioural consequences of low doses of MeHg in a long-term exposure model, and to realistically mimic in vivo the result of human exposure to this toxicant. Adult Wistar male rats were exposed to a dose of MeHg at 0.04 mg kg-1 day-1 by gavage for 60 days. Total mercury (Hg) content was significantly increased in the hippocampal parenchyma. The increase in the Hg levels was capable of reducing neuron and astrocyte cell density in the CA1, CA3, hilus and dentate gyrus regions, increasing both malondialdehyde and nitrite levels and decreasing antioxidant capacity against peroxyl radicals. The proteomic analysis detected 1041 proteins with altered expression due to MeHg exposure, including 364 proteins with no expression, 295 proteins with de novo expression and 382 proteins with up- or down-regulated expression. This proteomic approach revealed alterations in pathways related to chemical synapses, metabolism, amino acid transport, cell energy, neurodegenerative processes and myelin maintenance. Therefore, even at low doses of MeHg exposure, it is possible to cause hippocampal damage in adult rats at many organisational levels, triggering oxidative stress and proteome misbalance, featuring a neurodegenerative process and culminating in long- and short-term memory and learning deficits.

Monosomes actively translate synaptic mRNAs in neuronal processes

To accommodate their complex morphology, neurons localize messenger RNAs (mRNAs) and ribosomes near synapses to produce proteins locally. However, a relative paucity of polysomes (considered the active sites of translation) detected in electron micrographs of neuronal processes has suggested a limited capacity for local protein synthesis. In this study, we used polysome profiling together with ribosome footprinting of microdissected rodent synaptic regions to reveal a surprisingly high number of dendritic and/or axonal transcripts preferentially associated with monosomes (single ribosomes). Furthermore, the neuronal monosomes were in the process of active protein synthesis. Most mRNAs showed a similar translational status in the cell bodies and neurites, but some transcripts exhibited differential ribosome occupancy in the compartments. Monosome-preferring transcripts often encoded high-abundance synaptic proteins. Thus, monosome translation contributes to the local neuronal proteome.

mGluR5 hypofunction is integral to glutamatergic dysregulation in schizophrenia

Multiple lines of evidence point to glutamatergic signaling in the postsynaptic density (PSD) as a pathophysiologic mechanism in schizophrenia. Integral to PSD glutamatergic signaling is reciprocal interplay between GluN and mGluR5 signaling. We examined agonist-induced mGluR5 signaling in the postmortem dorsolateral prefrontal cortex (DLPFC) derived from 17 patients and age-matched and sex-matched controls. The patient group showed a striking reduction in mGluR5 signaling, manifested by decreases in Gq/11 coupling and association with PI3K and Homer compared to controls (p < 0.01 for all). This was accompanied by increases in serine and tyrosine phosphorylation of mGluR5, which can decrease mGluR5 activity via desensitization (p < 0.01). In addition, we find altered protein-protein interaction (PPI) of mGluR5 with RGS4, norbin, Preso 1 and tamalin, which can also attenuate mGluR5 activity. We previously reported molecular underpinnings of GluN hypofunction (decreased GluN2 phosphorylation) and here we show those of reduced mGluR5 signaling in schizophrenia. We find that reduced GluN2 phosphorylation can be precipitated by attenuated mGluR5 activity and that increased mGluR5 phosphorylation can result from decreased GluN function, suggesting a reciprocal interplay between the two pathways in schizophrenia. Interestingly, the patient group showed decreased mGluR5-GluN association (p < 0.01), a mechanistic basis for the reciprocal facilitation. In sum, we present the first direct evidence for mGluR5 hypoactivity, propose a reciprocal interplay between GluN and mGluR5 pathways as integral to glutamatergic dysregulation and suggest protein-protein interactions in mGluR5-GluN complexes as potential targets for intervention in schizophrenia.

Generation of a human Neurochondrin deficient iPSC line KICRi002-A-3 using CRISPR/Cas9

The role of Neurochondrin (NCDN) in humans is not well understood. Mice with a conditional Ncdn knock-out show epileptic seizures, depressive-like behaviours and impaired spatial learning. Using CRISPR/Cas9, we generated a Neurochondrin deficient human iPSC line KICRi002-A-3 carrying a homozygous 752 bp deletion / 2 bp insertion in the NCDN gene. The iPSC line maintained a normal 46,XY karyotype, expressed pluripotency markers and exhibited capability to differentiate into the three germ layers in vitro. Off-target editing was excluded and Neurochondrin expression was not detectable. The iPSC line offers a valuable resource to study the role of Neurochondrin during human neurogenesis.

Encephalopathy Associated With Neurochondrin Autoantibodies

We determined the prevalence of autoantibodies against an extended number of established and novel neural antigens in children and adolescents with suspected autoimmune encephalitis, epilepsy, single seizures, or marked epileptiform activity in electroencephalography (EEG). Prospectively, 103 patients were recruited aged between 0 and 18 years and 104 controls. A panel of 35 autoantibodies against neural cell-surface and intracellular antigens was screened. Sixteen of 103 patients (15.5%) showed a positive result for 1 or more autoantibodies, compared to 6 of 104 controls (5.8%, P = .02). Neurochondrin was identified as a possible new target of autoantibodies in 3 patients within this cohort, but none in controls. The patients showed severe behavioral disturbances, memory and cognitive impairment, episodes of reduced responsiveness, but no seizures, and normal MRI. Clinical findings, course, and treatment response of these 3 patients are presented.

Neurochondrin neurological autoimmunity

Objectives

To describe the neurologic spectrum and treatment outcomes for neurochondrin-IgG positive cases identified serologically in the Mayo Clinic Neuroimmunology Laboratory.

Methods

Archived serum and CSF specimens previously scored positive for IgGs that stained mouse hippocampal tissue in a nonuniform synaptic pattern by immunofluorescence assay (89 among 616,025 screened, 1993–2019) were reevaluated. Antibody characterization experiments revealed specificity for neurochondrin, confirmed by recombinant protein assays.

Results

IgG in serum (9) or CSF (4) from 8 patients yielded identical neuron-restricted CNS patterns, most pronounced in hippocampus (stratum lucidum in particular), cerebellum (Purkinje cells and molecular layer), and amygdala. All were neurochondrin-IgG positive. Five were women; median symptom onset age was 43 years (range, 30–69). Of 7 with clinical data, 6 presented with rapidly progressive cerebellar ataxia, brainstem signs, or both; 1 had isolated unexplained psychosis 1 year prior. Five of 6 had cerebellar signs, 4 with additional brainstem symptoms or signs (eye movement abnormalities, 3; dysphagia, 2; nausea and vomiting, 1). One patient with brainstem signs (vocal cord paralysis and VII nerve palsy) had accompanying myelopathy (longitudinally extensive abnormality on MRI; aquaporin-4-IgG and myelin oligodendrocyte glycoprotein-IgG negative). The 7th patient had small fiber neuropathy only. Just 1 of 7 had contemporaneous cancer (uterine). Six patients with ataxia or brainstem signs received immunotherapy, but just 1 remained ambulatory. At last follow-up, 5 had MRI evidence of severe cerebellar atrophy.

Conclusion

In our series, neurochondrin autoimmunity was usually accompanied by a nonparaneoplastic rapidly progressive rhombencephalitis with poor neurologic outcomes. Other phenotypes and occasional paraneoplastic causes may occur.

Cholinergic Neurons of the Medial Septum Are Crucial for Sensorimotor Gating

Hypofunction of NMDA receptors has been considered a possible cause for the pathophysiology of schizophrenia. More recently, indirect ways to regulate NMDA that would be less disruptive have been proposed and metabotropic glutamate receptor subtype 5 (mGluR5) represents one such candidate. To characterize the cell populations involved, we demonstrated here that knock-out (KO) of mGluR5 in cholinergic, but not glutamatergic or parvalbumin (PV)-positive GABAergic, neurons reduced prepulse inhibition of the startle response (PPI) and enhanced sensitivity to MK801-induced locomotor activity. Inhibition of cholinergic neurons in the medial septum by DREADD (designer receptors exclusively activated by designer drugs) resulted in reduced PPI further demonstrating the importance of these neurons in sensorimotor gating. Volume imaging and quantification were used to compare PV and cholinergic cell distribution, density, and total cell counts in the different cell-type-specific KO lines. Electrophysiological studies showed reduced NMDA receptor-mediated currents in cholinergic neurons of the medial septum in mGluR5 KO mice. These results obtained from male and female mice indicate that cholinergic neurons in the medial septum represent a key cell type involved in sensorimotor gating and are relevant to pathologies associated with disrupted sensorimotor gating such as schizophrenia.SIGNIFICANCE STATEMENT The mechanistic complexity underlying psychiatric disorders remains a major challenge that is hindering the drug discovery process. Here, we generated genetically modified mouse lines to better characterize the involvement of the receptor mGluR5 in the fine-tuning of NMDA receptors, specifically in the context of sensorimotor gating. We evaluated the importance of knocking-out mGluR5 in three different cell types in two brain regions and performed different sets of experiments including behavioral testing and electrophysiological recordings. We demonstrated that cholinergic neurons in the medial septum represent a key cell-type involved in sensorimotor gating. We are proposing that pathologies associated with disrupted sensorimotor gating, such as with schizophrenia, could benefit from further evaluating strategies to modulate specifically cholinergic neurons in the medial septum.

New Features on the Expression and Trafficking of mGluR1 Splice Variants Exposed by Two Novel Mutant Mouse Lines

Metabotropic glutamate receptors (mGluRs) couple to G-proteins to modulate slow synaptic transmission via intracellular second messengers. The first cloned mGluR, mGluR1, regulates motor coordination, synaptic plasticity and synapse elimination. mGluR1 undergoes alternative splicing giving rise to four translated variants that differ in their intracellular C-terminal domains. Our current knowledge about mGluR1 relates almost entirely to the long mGluR1α isoform, whereas little is known about the other shorter variants. To study the expression of mGluR1γ, we have generated by means of the CRISPR/Cas9 system a new knock-in (KI) mouse line in which the C-terminus of this variant carries two short tags. Using this mouse line, we could establish that mGluR1γ is either untranslated or in amounts that are undetectable in the mouse cerebellum, indicating that only mGluR1α and mGluR1β are present and active at cerebellar synapses. The trafficking and function of mGluR1 appear strongly influenced by adaptor proteins such as long Homers that bind to the C-terminus of mGluR1α. We generated a second transgenic (Tg) mouse line in which mGluR1α carries a point mutation in its Homer binding domain and studied whether disruption of this interaction influenced mGluR1 subcellular localization at cerebellar parallel fiber (PF)-Purkinje cell (PC) synapses by means of the freeze-fracture replica immunolabeling technique. These Tg animals did not show any overt behavioral phenotype, and despite the typical mGluR1 perisynaptic distribution was not significantly changed, we observed a higher probability of intrasynaptic diffusion suggesting that long Homers regulate the lateral mobility of mGluR1. We extended our ultrastructural analysis to other mouse lines in which only one mGluR1 variant was reintroduced in PC of mGluR1-knock out (KO) mice. This work revealed that mGluR1α preferentially accumulates closer to the edge of the postsynaptic density (PSD), whereas mGluR1β has a less pronounced perijunctional distribution and, in the absence of mGluR1α, its trafficking to the plasma membrane is impaired with an accumulation in intracellular organelles. In conclusion, our study sets several firm points on largely disputed matters, namely expression of mGluR1γ and role of the C-terminal domain of mGluR1 splice variants on their perisynaptic clustering.

Functional organization of postsynaptic glutamate receptors

Glutamate receptors are the most abundant excitatory neurotransmitter receptors in the brain, responsible for mediating the vast majority of excitatory transmission in neuronal networks. The AMPA- and NMDA-type ionotropic glutamate receptors (iGluRs) are ligand-gated ion channels that mediate the fast synaptic responses, while metabotropic glutamate receptors (mGluRs) are coupled to downstream signaling cascades that act on much slower timescales. These functionally distinct receptor sub-types are co-expressed at individual synapses, allowing for the precise temporal modulation of postsynaptic excitability and plasticity. Intriguingly, these receptors are differentially distributed with respect to the presynaptic release site. While iGluRs are enriched in the core of the synapse directly opposing the release site, mGluRs reside preferentially at the border of the synapse. As such, to understand the differential contribution of these receptors to synaptic transmission, it is important to not only consider their signaling properties, but also the mechanisms that control the spatial segregation of these receptor types within synapses. In this review, we will focus on the mechanisms that control the organization of glutamate receptors at the postsynaptic membrane with respect to the release site, and discuss how this organization could regulate synapse physiology.

Chronic Adolescent CDPPB Treatment Alters Short-Term, but not Long-Term, Glutamatergic Receptor Expression

Dysfunction of the glutamatergic system is believed to underlie many neurodevelopmental disorders including autism, Rett syndrome and schizophrenia. Metabotropic glutamate receptor (mGluR5) positive allosteric modulators (PAM) potentiate glutamatergic signaling, particularly indirectly via the NMDA receptor. Preclinical studies report mGluR5 PAMs can improve schizophrenia-relevant behaviours. Furthermore, adolescent administration has shown to prevent cognitive induced deficits in adult rodents. However, there is limited understanding of the short- and long-term neurochemical effects of mGluR5 PAMs, which may underlie their therapeutic effects. We examined the effect of 7-day adolescent (PN28-34) treatment with the mGluR5 PAM, CDDPB (30 mg/kg), on glutamatergic receptor expression at adolescence (PN35) and adulthood (PN96). Immunoblot analysis revealed that 7-day adolescent CDPPB treatment increased protein expression of glutamatergic receptors including the NMDA receptor subunits, NR1 and NR2A and the AMPA subunits (GluA1 and GluA2) in the adolescent hippocampus, changes that did not extend to adulthood. In contrast, there were no changes in the adolescent frontal cortex, however elevated mGluR5 protein expression was observed at adulthood following adolescent CDPPB treatment. The present study indicates adolescent CDPPB treatment may cause brain region dependent effects on the glutamatergic system, which do not persist into adulthood. These findings may have implications for the preclinical development of mGluR5 PAMs for the treatment of neurodevelopmental disorders.

Neurochondrin interacts with the SMN protein suggesting a novel mechanism for spinal muscular atrophy pathology

Spinal muscular atrophy (SMA) is an inherited neurodegenerative condition caused by a reduction in the amount of functional survival motor neuron (SMN) protein. SMN has been implicated in transport of mRNA in neural cells for local translation. We previously identified microtubule-dependent mobile vesicles rich in SMN and SNRPB, a member of the Sm family of small nuclear ribonucleoprotein (snRNP)-associated proteins, in neural cells. By comparing the interactomes of SNRPB and SNRPN, a neural-specific Sm protein, we now show that the essential neural protein neurochondrin (NCDN) interacts with Sm proteins and SMN in the context of mobile vesicles in neurites. NCDN has roles in protein localisation in neural cells and in maintenance of cell polarity. NCDN is required for the correct localisation of SMN, suggesting they may both be required for formation and transport of trafficking vesicles. NCDN may have potential as a therapeutic target for SMA together with, or in place of the targeting of SMN expression.

This article has an associated First Person interview with the first author of the paper.

Highlighted Article: The essential neural protein neurochondrin interacts with the spinal muscular atrophy (SMA) protein SMN in cell lines and in mice. This might be relevant to the molecular pathology of SMA.

Metabotropic glutamate receptor trafficking

The metabotropic glutamate receptors (mGlu receptors) are G protein-coupled receptors that bind to the excitatory neurotransmitter glutamate and are important in the modulation of neuronal excitability, synaptic transmission, and plasticity in the central nervous system. Trafficking of mGlu receptors in and out of the synaptic plasma membrane is a fundamental mechanism modulating excitatory synaptic function through regulation of receptor abundance, desensitization, and signaling profiles. In this review, we cover the regulatory mechanisms determining surface expression and endocytosis of mGlu receptors, with particular focus on post-translational modifications and receptor-protein interactions. The literature we review broadens our insight into the precise events defining the expression of functional mGlu receptors at synapses, and will likely contribute to the successful development of novel therapeutic targets for a variety of developmental, neurological, and psychiatric disorders.

A postmortem analysis of NMDA ionotropic and group 1 metabotropic glutamate receptors in the nucleus accumbens in schizophrenia

BACKGROUND

The nucleus accumbens (NAcc) has been implicated in the pathology and treatment of schizophrenia. Recent postmortem evidence suggests a hyperglutamatergic state in the NAcc. With the present study we aimed to explore possible glutamatergic alterations in the NAcc of a large schizophrenia cohort.

METHODS

We performed immunoblots on postmortem NAcc samples from 30 individuals who had schizophrenia and 30 matched controls. We examined the protein expression of primary glutamatergic receptors, including the N-methyl-D-aspartate (NMDA) receptor (NR1, NR2A and NR2B subunits) and the group 1 metabotropic glutamate receptor (mGluR1 and mGluR5; dimeric and monomeric forms). In addition, we measured the group 1 mGluR endogenous regulators, neurochondrin and Homer1b/c, which have recently been implicated in the pathophysiology of schizophrenia.

RESULTS

Protein levels of glutamatergic receptors and endogenous regulators were not significantly different between the controls and individuals who had schizophrenia. Furthermore, mGluR5, but not mGluR1, showed a positive association with NMDA receptor subunits, suggesting differential interactions between these receptors in this brain region.

LIMITATIONS

Investigation of these proteins in antipsychotic-naive individuals, in addition to the subregions of the NAcc and subcellular fractions, will strengthen future studies.

CONCLUSION

The present study does not provide evidence for glutamatergic abnormalities within the NAcc of individuals with schizophrenia. Taken together with the results of previous studies, these findings suggest NMDA receptors and group 1 mGluRs are altered in a brain region-dependent manner in individuals with schizophrenia. The differential associations between mGluR1, mGluR5 and NMDA receptors observed in this study warrant further research into the interactions of these proteins and the implications for the therapeutic and adverse effect profile of glutamatergic-based novel therapeutics.

Effects of short- and long-term aripiprazole treatment on Group I mGluRs in the nucleus accumbens: Comparison with haloperidol

The D2 receptor partial agonist, aripiprazole, has shown increased therapeutic efficacy for schizophrenia, autism and Tourette's syndrome compared to traditional antipsychotics such as the D2 receptor antagonist, haloperidol. Recent evidence suggests this superior profile may be associated with downstream effects on glutamatergic synapses. Group 1 metabotropic glutamate receptors (mGluRs) and their endogenous modulators, Norbin and Homer1, are regulated by D2 receptor activity, particularly within the nucleus accumbens (NAc), a target region of aripiprazole and haloperidol. This study sought to evaluate the effects of aripiprazole on Group 1 mGluRs, Norbin and Homer1 in the NAc, in comparison to haloperidol. Sprague-Dawley rats were orally administered daily doses of aripiprazole (2.25mg/kg), haloperidol (0.3mg/kg) or vehicle for 1 or 10-weeks. Immunoblot analyses revealed Group 1 mGluR protein levels were not altered following 1-week and 10-week aripiprazole or haloperidol treatment, compared to vehicle treated rodents. However, 1-week aripiprazole and haloperidol treatment significantly elevated Homer1a and Norbin protein expression, respectively. After 10 weeks of treatment, aripiprazole, but not haloperidol, significantly increased Norbin expression. These findings indicate the antipsychotics, aripiprazole and haloperidol, exert differential temporal effects on Norbin and Homer1 expression that may have consequences on synaptic glutamatergic transmission underlying their therapeutic profile.

Altered Norbin Expression in Patients with Epilepsy and a Rat Model

Norbin is widely distributed in neuronal tissues, is a regulator of Ca2⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation. Norbin is also an important endogenous modulator of metabotropic glutamate receptor 5 (mGluR5) signaling, and nervous system-specific homozygous gene disruptions, result in epileptic seizures. In this study, we aimed to investigate norbin expression patterns in epilepsy and to elucidate the relationships between norbin and mGluR5 and p-CaMKII in epilepsy. Double-immunolabeling, immunohistochemistry and immunoblotting studies showed that norbin was downregulated in the temporal neocortex of patients with temporal lobe epilepsy (TLE) compared with control subjects. Moreover, in a rat model of lithium chloride-pilocarpine-induced epilepsy, norbin expression began to decrease at 6 h after the onset of status epilepticus and remained at a low level until 60 days. In addition, p-CaMKII expression was significantly increased in both patients with TLE and in animal model. Norbin and mGluR5 were found to be co-expressed in neurons of epileptic tissues. Finally, norbin over-expression facilitated by injections of adeno-associated viral vector into the rat hippocampus increased latency and survival in the lithium chloride-pilocarpine model. Thus, our results indicate norbin participates in the pathogenesis of epilepsy, perhaps by modulating mGluR5 signaling, regulating CaMKII phosphorylation, and may exert antiepileptic effects.

APP Deletion Accounts for Age-Dependent Changes in the Bioenergetic Metabolism and in Hyperphosphorylated CaMKII at Stimulated Hippocampal Presynaptic Active Zones

Synaptic release sites are characterized by exocytosis-competent synaptic vesicles tightly anchored to the presynaptic active zone (PAZ) whose proteome orchestrates the fast signaling events involved in synaptic vesicle cycle and plasticity. Allocation of the amyloid precursor protein (APP) to the PAZ proteome implicated a functional impact of APP in neuronal communication. In this study, we combined state-of-the-art proteomics, electrophysiology and bioinformatics to address protein abundance and functional changes at the native hippocampal PAZ in young and old APP-KO mice. We evaluated if APP deletion has an impact on the metabolic activity of presynaptic mitochondria. Furthermore, we quantified differences in the phosphorylation status after long-term-potentiation (LTP) induction at the purified native PAZ. We observed an increase in the phosphorylation of the signaling enzyme calmodulin-dependent kinase II (CaMKII) only in old APP-KO mice. During aging APP deletion is accompanied by a severe decrease in metabolic activity and hyperphosphorylation of CaMKII. This attributes an essential functional role to APP at hippocampal PAZ and putative molecular mechanisms underlying the age-dependent impairments in learning and memory in APP-KO mice.

Group I Metabotropic Glutamate Receptor Interacting Proteins: Fine-Tuning Receptor Functions in Health and Disease

Group I metabotropic glutamate receptors mediate slow excitatory neurotransmission in the central nervous system and are critical to activity-dependent synaptic plasticity, a cellular substrate of learning and memory. Dysregulated receptor signaling is implicated in neuropsychiatric conditions ranging from neurodevelopmental to neurodegenerative disorders. Importantly, group I metabotropic glutamate receptor signaling functions can be modulated by interacting proteins that mediate receptor trafficking, expression and coupling efficiency to signaling effectors. These interactions afford cell- or pathway-specific modulation to fine-tune receptor function, thus representing a potential target for pharmacological interventions in pathological conditions.

Sequences within the C Terminus of the Metabotropic Glutamate Receptor 5 (mGluR5) Are Responsible for Inner Nuclear Membrane Localization

Traditionally, G-protein-coupled receptors (GPCR) are thought to be located on the cell surface where they transmit extracellular signals to the cytoplasm. However, recent studies indicate that some GPCRs are also localized to various subcellular compartments such as the nucleus where they appear required for various biological functions. For example, the metabotropic glutamate receptor 5 (mGluR5) is concentrated at the inner nuclear membrane (INM) where it mediates Ca²⁺ changes in the nucleoplasm by coupling with G_q/11 Here, we identified a region within the C-terminal domain (amino acids 852-876) that is necessary and sufficient for INM localization of the receptor. Because these sequences do not correspond to known nuclear localization signal motifs, they represent a new motif for INM trafficking. mGluR5 is also trafficked to the plasma membrane where it undergoes re-cycling/degradation in a separate receptor pool, one that does not interact with the nuclear mGluR5 pool. Finally, our data suggest that once at the INM, mGluR5 is stably retained via interactions with chromatin. Thus, mGluR5 is perfectly positioned to regulate nucleoplasmic Ca²⁺in situ.

Metabotropic Glutamate Receptors and Interacting Proteins in Epileptogenesis

Neurotransmitter and receptor systems are involved in different neurological and neuropsychological disorders such as Parkinson's disease, depression, Alzheimer’s disease and epilepsy. Recent advances in studies of signal transduction pathways or interacting proteins of neurotransmitter receptor systems suggest that different receptor systems may share the common signal transduction pathways or interacting proteins which may be better therapeutic targets for development of drugs to effectively control brain diseases. In this paper, we reviewed metabotropic glutamate receptors (mGluRs) and their related signal transduction pathways or interacting proteins in status epilepticus and temporal lobe epilepsy, and proposed some novel therapeutical drug targets for controlling epilepsy and epileptogenesis.

Neurochondrin is a neuronal target antigen in autoimmune cerebellar degeneration

OBJECTIVE

To report on a novel neuronal target antigen in 3 patients with autoimmune cerebellar degeneration.

METHODS

Three patients with subacute to chronic cerebellar ataxia and controls underwent detailed clinical and neuropsychological assessment together with quantitative high-resolution structural MRI. Sera and CSF were subjected to comprehensive autoantibody screening by indirect immunofluorescence assay (IFA) and immunoblot. Immunoprecipitation with lysates of hippocampus and cerebellum combined with mass spectrometric analysis was used to identify the autoantigen, which was verified by recombinant expression in HEK293 cells and use in several immunoassays. Multiparameter flow cytometry was performed on peripheral blood and CSF, and peripheral blood was subjected to T-cell receptor spectratyping.

RESULTS

Patients presented with a subacute to chronic cerebellar and brainstem syndrome. MRI was consistent with cortical and cerebellar gray matter atrophy associated with subsequent neuroaxonal degeneration. IFA screening revealed strong immunoglobulin G1 reactivity in sera and CSF with hippocampal and cerebellar molecular and granular layers, but not with a panel of 30 recombinantly expressed established neural autoantigens. Neurochondrin was subsequently identified as the target antigen, verified by IFA and immunoblot with HEK293 cells expressing human neurochondrin as well as the ability of recombinant neurochondrin to neutralize the autoantibodies' tissue reaction. Immune phenotyping revealed intrathecal accumulation and activation of B and T cells during the acute but not chronic phase of the disease. T-cell receptor spectratyping suggested an antigen-specific T-cell response accompanying the formation of antineurochondrin autoantibodies. No such neurochondrin reactivity was found in control cohorts of various neural autoantibody-associated neurologic syndromes, relapsing-remitting multiple sclerosis, cerebellar type of multiple system atrophy, hereditary cerebellar ataxias, other neurologic disorders, or healthy donors.

CONCLUSION

Neurochondrin is a neuronal target antigen in autoimmune cerebellar degeneration.

The effects of proteasomal inhibition on synaptic proteostasis

Synaptic function crucially depends on uninterrupted synthesis and degradation of synaptic proteins. While much has been learned on synaptic protein synthesis, little is known on the routes by which synaptic proteins are degraded. Here we systematically studied how inhibition of the ubiquitin-proteasome system (UPS) affects the degradation rates of thousands of neuronal and synaptic proteins. We identified a group of proteins, including several proteins related to glutamate receptor trafficking, whose degradation rates were significantly slowed by UPS inhibition. Unexpectedly, however, degradation rates of most synaptic proteins were not significantly affected. Interestingly, many of the differential effects of UPS inhibition were readily explained by a quantitative framework that considered known metabolic turnover rates for the same proteins. In contrast to the limited effects on protein degradation, UPS inhibition profoundly and preferentially suppressed the synthesis of a large number of synaptic proteins. Our findings point to the importance of the UPS in the degradation of certain synaptic proteins, yet indicate that under basal conditions most synaptic proteins might be degraded through alternative pathways.

Norbin Stimulates the Catalytic Activity and Plasma Membrane Localization of the Guanine-Nucleotide Exchange Factor P-Rex1

P-Rex1 is a guanine-nucleotide exchange factor (GEF) that activates the small G protein (GTPase) Rac1 to control Rac1-dependent cytoskeletal dynamics, and thus cell morphology. Three mechanisms of P-Rex1 regulation are currently known: (i) binding of the phosphoinositide second messenger PIP₃, (ii) binding of the Gβγ subunits of heterotrimeric G proteins, and (iii) phosphorylation of various serine residues. Using recombinant P-Rex1 protein to search for new binding partners, we isolated the G-protein-coupled receptor (GPCR)-adaptor protein Norbin (Neurochondrin, NCDN) from mouse brain fractions. Coimmunoprecipitation confirmed the interaction between overexpressed P-Rex1 and Norbin in COS-7 cells, as well as between endogenous P-Rex1 and Norbin in HEK-293 cells. Binding assays with purified recombinant proteins showed that their interaction is direct, and mutational analysis revealed that the pleckstrin homology domain of P-Rex1 is required. Rac-GEF activity assays with purified recombinant proteins showed that direct interaction with Norbin increases the basal, PIP₃- and Gβγ-stimulated Rac-GEF activity of P-Rex1. Pak-CRIB pulldown assays demonstrated that Norbin promotes the P-Rex1-mediated activation of endogenous Rac1 upon stimulation of HEK-293 cells with lysophosphatidic acid. Finally, immunofluorescence microscopy and subcellular fractionation showed that coexpression of P-Rex1 and Norbin induces a robust translocation of both proteins from the cytosol to the plasma membrane, as well as promoting cell spreading, lamellipodia formation, and membrane ruffling, cell morphologies generated by active Rac1. In summary, we have identified a novel mechanism of P-Rex1 regulation through the GPCR-adaptor protein Norbin, a direct P-Rex1 interacting protein that promotes the Rac-GEF activity and membrane localization of P-Rex1.

Regulation of Group I Metabotropic Glutamate Receptors by MAPK/ERK in Neurons

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are regulated by protein kinases. A recent focus is mitogen-activated protein kinases (MAPK). A prototypic subclass of MAPKs, extracellular signal-regulated kinases (ERK), is densely expressed in adult brain postmitotic neurons. This kinase resides in not only the cytoplasm around the nucleus, also the neuronal peripheral structures such as synapses. Recombinant ERK2 binds to C terminal tails of mGluR1a in vitro and native ERK1/2 forms complexes with mGluR1/5 in neurons in vivo. Association of ERK with mGluR1/5 enables the kinase to phosphorylate mGluR1/5 at a cluster of serine sites in the distal C terminus, including a serine residue within the Homer binding site. The ERK-mediated phosphorylation of mGluR1/5 promotes surface expression of mGluR1a in cerebellar neurons. ERK also regulates mGluR1/5 signaling and functions. Among different functional outputs surveyed, ERK exerts an output-specific role in either potentiating or inhibiting their activities. In sum, synaptic group I mGluRs are sufficient substrates of MAPK/ERK. Phosphorylation of mGluR1/5 by ERK has a significant impact on subcellular expression and function of phospho-modified receptors.

Alteration by p11 of mGluR5 localization regulates depression-like behaviors

Mood disorders and antidepressant therapy involve alterations of monoaminergic and glutamatergic transmission. The protein S100A10 (p11) was identified as a regulator of serotonin receptors, and it has been implicated in the etiology of depression and in mediating the antidepressant actions of selective serotonin reuptake inhibitors. Here we report that p11 can also regulate depression-like behaviors via regulation of a glutamatergic receptor in mice. p11 directly binds to the cytoplasmic tail of metabotropic glutamate receptor 5 (mGluR5). p11 and mGluR5 mutually facilitate their accumulation at the plasma membrane, and p11 increases cell surface availability of the receptor. Whereas p11 overexpression potentiates mGluR5 agonist-induced calcium responses, overexpression of mGluR5 mutant, which does not interact with p11, diminishes the calcium responses in cultured cells. Knockout of mGluR5 or p11 specifically in glutamatergic neurons in mice causes depression-like behaviors. Conversely, knockout of mGluR5 or p11 in GABAergic neurons causes antidepressant-like behaviors. Inhibition of mGluR5 with an antagonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), induces antidepressant-like behaviors in a p11-dependent manner. Notably, the antidepressant-like action of MPEP is mediated by parvalbumin-positive GABAergic interneurons, resulting in a decrease of inhibitory neuronal firing with a resultant increase of excitatory neuronal firing. These results identify a molecular and cellular basis by which mGluR5 antagonism achieves its antidepressant-like activity.

Metabotropic glutamate receptor subtype 5: molecular pharmacology, allosteric modulation and stimulus bias

The metabotropic glutamate receptor subtype 5 (mGlu5 ) is a family C GPCR that has been implicated in various neuronal processes and, consequently, in several CNS disorders. Over the past few decades, GPCR-based drug discovery, including that for mGlu5 receptors, has turned considerable attention to targeting allosteric binding sites. Modulation of endogenous agonists by allosteric ligands offers the advantages of spatial and temporal fine-tuning of receptor activity, increased selectivity and reduced adverse effects with the potential to elicit improved clinical outcomes. Further, with greater appreciation of the multifaceted nature of the transduction of mGlu5 receptor signalling, it is increasingly apparent that drug discovery must take into consideration unique receptor conformations and the potential for stimulus-bias. This novel paradigm proposes that different ligands may differentially modulate distinct signalling pathways arising from the same receptor. We review our current understanding of the complexities of mGlu5 receptor signalling and regulation, and how these relate to allosteric ligands. Ultimately, a deeper appreciation of these relationships will provide the foundation for targeted drug design of compounds with increased selectivity, not only for the desired receptor but also for the desired signalling outcome from the receptor. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Shifting towards a model of mGluR5 dysregulation in schizophrenia: Consequences for future schizophrenia treatment

Metabotropic glutamate receptor subtype 5 (mGluR5), encoded by the GRM5 gene, represents a compelling novel drug target for the treatment of schizophrenia. mGluR5 is a postsynaptic G-protein coupled glutamate receptor strongly linked with several critical cellular processes that are reported to be disrupted in schizophrenia. Accordingly, mGluR5 positive allosteric modulators show encouraging therapeutic potential in preclinical schizophrenia models, particularly for the treatment of cognitive dysfunctions against which currently available therapeutics are largely ineffective. More work is required to support the progression of mGluR5-targeting drugs into the clinic for schizophrenia treatment, although some obstacles may be overcome by comprehensively understanding how mGluR5 itself is involved in the neurobiology of the disorder. Several processes that are necessary for the regulation of mGluR5 activity have been identified, but not examined, in the context of schizophrenia. These processes include protein-protein interactions, dimerisation, subcellular trafficking, the impact of genetic variability or mutations on protein function, as well as epigenetic, post-transcriptional and post-translational processes. It is essential to understand these aspects of mGluR5 to determine whether they are affected in schizophrenia pathology, and to assess the consequences of mGluR5 dysfunction for the future use of mGluR5-based drugs. Here, we summarise the known processes that regulate mGluR5 and those that have already been studied in schizophrenia, and discuss the consequences of this dysregulation for current mGluR5 pharmacological strategies. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.