Metabotropic glutamate receptor 3 (mGlu3; mGluR3; GRM3) in schizophrenia: Antibody characterisation and a semi-quantitative western blot study

The CB2-PKC pathway is involved in esketamine-induced anti-inflammation in BV-2 microglial cells exposed to lipopolysaccharides

OBJECTIVE

Esketamine (ESK), an intravenous anesthetic, exerts antidepressant effects; however, the antidepression mechanism is not clear. The aim of this study was to explore whether microglial cannabinoid type 2 (CB2) receptor and protein kinase C (PKC) are involved in the antidepressant effects of ESK.

METHODS

In this investigation, lipopolysaccharide (LPS) was used to stimulate BV-2 microglia to mimic neuroinflammation. An enzyme-linked immunosorbent assay (ELISA) and Griess reagent kits were used to determine cytokine and nitrite concentrations in the medium. CB2, inducible nitric oxide synthase (iNOS) and nuclear factor (NF)-κB (p65) protein expression were evaluated by immunocytochemistry and western blot analysis.

RESULTS

Compared with the control, LPS enhanced proinflammatory factor and nitrite concentration in the medium, upregulated iNOS and NF-κB (p65) expressions, and coadministration of ESK decreased proinflammatory cytokine and nitrite levels, and downregulated iNOS and NF-κB (p65) expression. Moreover, ESK exposure enhanced CB2 receptor expression; coadministration of the CB2 receptor antagonist AM630 or the PKC inhibitor chelerythrine (Che), however, markedly blocked the anti-inflammatory effect of ESK in reducing cytokine and nitrite concentration, and downregulating iNOS and NF-κB (p65) expression.

CONCLUSIONS

These observations demonstrated that the microglial CB2-PKC pathway mediates ESK-induced anti-inflammation in LPS-stimulated microglial cells.

Effect of antipsychotic drugs on group II metabotropic glutamate receptor expression and epigenetic control in postmortem brains of schizophrenia subjects

Antipsychotic-induced low availability of group II metabotropic glutamate receptors (including mGlu2R and mGlu3R) in brains of schizophrenia patients may explain the limited efficacy of mGlu2/3R ligands in clinical trials. Studies evaluating mGlu2/3R levels in well-designed, large postmortem brain cohorts are needed to address this issue. Postmortem samples from the dorsolateral prefrontal cortex of 96 schizophrenia subjects and matched controls were collected. Toxicological analyses identified cases who were (AP+) or were not (AP-) receiving antipsychotic treatment near the time of death. Protein and mRNA levels of mGlu2R and mGlu3R, as well as GRM2 and GRM3 promoter-attached histone posttranslational modifications, were quantified. Experimental animal models were used to compare with data obtained in human tissues. Compared to matched controls, schizophrenia cortical samples had lower mGlu2R protein amounts, regardless of antipsychotic medication. Downregulation of mGlu3R was observed in AP- schizophrenia subjects only. Greater predicted occupancy values of dopamine D2 and serotonin 5HT2A receptors correlated with higher density of mGlu3R, but not mGlu2R. Clozapine treatment and maternal immune activation in rodents mimicked the mGlu2R, but not mGlu3R regulation observed in schizophrenia brains. mGlu2R and mGlu3R mRNA levels, and the epigenetic control mechanisms did not parallel the alterations at the protein level, and in some groups correlated inversely. Insufficient cortical availability of mGlu2R and mGlu3R may be associated with schizophrenia. Antipsychotic treatment may normalize mGlu3R, but not mGlu2R protein levels. A model in which epigenetic feedback mechanisms controlling mGlu3R expression are activated to counterbalance mGluR loss of function is described.

An emerging role of astrocytes in aging/neuroinflammation and gut-brain axis with consequences on sleep and sleep disorders

Understanding the role of astrocytes in the central nervous system has changed dramatically over the last decade. The accumulating findings indicate that glial cells are involved not only in the maintenance of metabolic and ionic homeostasis and in the implementation of trophic functions but also in cognitive functions and information processing in the brain. Currently, there are some controversies regarding the role of astrocytes in complex processes such as aging of the nervous system and the pathogenesis of age-related neurodegenerative diseases. Many findings confirm the important functional role of astrocytes in age-related brain changes, including sleep disturbance and the development of neurodegenerative diseases and particularly Alzheimer's disease. Until recent years, neurobiological research has focused mainly on neuron-glial interactions, in which individual astrocytes locally modulate neuronal activity and communication between neurons. The review considers the role of astrocytes in the physiology of sleep and as an important "player" in the development of neurodegenerative diseases. In addition, the features of the astrocytic network reorganization during aging are discussed.

Expression of group II metabotropic glutamate receptors in rat superior cervical ganglion

BACKGROUND

The superior cervical ganglion (SCG) plays critical roles in the regulation of blood pressure and cardiac output. Metabotropic glutamate receptors (mGluRs) in the SCG are not clearly elucidated yet. Most studies on the expression and functions of mGluRs in the SCG focused on the cultured SCG neurons, and yet little information has been reported in the SCG tissue. Chronic intermittent hypoxia (CIH), one of the major clinical features of obstructive sleep apnea (OSA) patients, is a critical pathological cause of secondary hypertension in OSA patients, but its impact on the level of mGluRs in the SCG is unknown.

OBJECTIVE

To explore the expression and localization of mGluR2/3 and the effect of CIH on mGluR2/3 level in rat SCG tissue.

METHODS

RT-PCR and immunostaining were conducted to examine the mRNA and protein expression of mGluR2/3 in rat SCG. Immunofluorescence staining was conducted to examine the distribution of mGluR2/3. Rats were divided into control and CIH group which the rats were exposed to CIH for 6 weeks. Western blots were performed to examine the level of mGluR2/3 in rat SCG.

RESULTS

mRNAs of mGluR2/3 expressed in rat SCG. mGluR2 distributed in principal neurons and small intensely fluorescent cells but not in satellite glial cells, nerve fibers, and vascular endothelial cells; mGluR3 was detected in nerve fibers rather than in the cells mentioned above. CIH exposure reduced the protein level of mGluR2/3 in rat SCG.

CONCLUSION

mGluR2/3 exists in rat SCG with diverse distribution patterns, and may be involved in CIH-induced hypertension.

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.

Defining Specific Cell States of MPTP-Induced Parkinson's Disease by Single-Nucleus RNA Sequencing

Parkinson's disease (PD) is a neurodegenerative disease with an impairment of movement execution that is related to age and genetic and environmental factors. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin widely used to induce PD models, but the effect of MPTP on the cells and genes of PD has not been fully elucidated. By single-nucleus RNA sequencing, we uncovered the PD-specific cells and revealed the changes in their cellular states, including astrocytosis and endothelial cells' absence, as well as a cluster of medium spiny neuron cells unique to PD. Furthermore, trajectory analysis of astrocyte and endothelial cell populations predicted candidate target gene sets that might be associated with PD. Notably, the detailed regulatory roles of astrocyte-specific transcription factors Dbx2 and Sox13 in PD were revealed in our work. Finally, we characterized the cell-cell communications of PD-specific cells and found that the overall communication strength was enhanced in PD compared with a matched control, especially the signaling pathways of NRXN and NEGR. Our work provides an overview of the changes in cellular states of the MPTP-induced mouse brain.

Annotating functional effects of non-coding variants in neuropsychiatric cell types by deep transfer learning

Genomewide association studies (GWAS) have identified a large number of loci associated with neuropsychiatric traits, however, understanding the molecular mechanisms underlying these loci remains difficult. To help prioritize causal variants and interpret their functions, computational methods have been developed to predict regulatory effects of non-coding variants. An emerging approach to variant annotation is deep learning models that predict regulatory functions from DNA sequences alone. While such models have been trained on large publicly available dataset such as ENCODE, neuropsychiatric trait-related cell types are under-represented in these datasets, thus there is an urgent need of better tools and resources to annotate variant functions in such cellular contexts. To fill this gap, we collected a large collection of neurodevelopment-related cell/tissue types, and trained deep Convolutional Neural Networks (ResNet) using such data. Furthermore, our model, called MetaChrom, borrows information from public epigenomic consortium to improve the accuracy via transfer learning. We show that MetaChrom is substantially better in predicting experimentally determined chromatin accessibility variants than popular variant annotation tools such as CADD and delta-SVM. By combining GWAS data with MetaChrom predictions, we prioritized 31 SNPs for Schizophrenia, suggesting potential risk genes and the biological contexts where they act. In summary, MetaChrom provides functional annotations of any DNA variants in the neuro-development context and the general method of MetaChrom can also be extended to other disease-related cell or tissue types.

A metabotropic glutamate receptor 3 (mGlu3R) isoform playing neurodegenerative roles in astrocytes is prematurely up-regulated in an Alzheimer's model

Subtype 3 metabotropic glutamate receptor (mGlu3R) displays a broad range of neuroprotective effects. We previously demonstrated that mGlu3R activation in astrocytes protects hippocampal neurons from Aβ neurotoxicity through stimulation of both neurotrophin release and Aβ uptake. Alternative-spliced variants of mGlu3R were found in human brains. The most prevalent variant, mGlu3Δ4, lacks exon 4 encoding the transmembrane domain and can inhibit ligand binding to mGlu3R. To date, neither its role in neurodegenerative disorders nor its endogenous expression in CNS cells has been addressed. The present paper describes for the first time an association between altered hippocampal expression of mGlu3Δ4 and Alzheimer's disease (AD) in the preclinical murine model PDAPP-J20, as well as a deleterious effect of mGlu3Δ4 in astrocytes. As assessed by western blot, hippocampal mGlu3R levels progressively decreased with age in PDAPP-J20 mice. On the contrary, mGlu3Δ4 levels were drastically increased with aging in nontransgenic mice, but prematurely over-expressed in 5-month-old PDAPP-J20-derived hippocampi, prior to massive senile plaque deposition. Also, we found that mGlu3Δ4 co-precipitated with mGlu3R mainly in 5-month-old PDAPP-J20 mice. We further showed by western blot that primary cultured astrocytes and neurons expressed mGlu3Δ4, whose levels were reduced by Aβ, thereby discouraging a causal effect of Aβ on mGlu3Δ4 induction. However, heterologous expression of mGlu3Δ4 in astrocytes induced cell death, inhibited mGlu3R expression, and prevented mGlu3R-dependent Aβ glial uptake. Indeed, mGlu3Δ4 promoted neurodegeneration in neuron-glia co-cultures. These results provide evidence of an inhibitory role of mGlu3Δ4 in mGlu3R-mediated glial neuroprotective pathways, which may lie behind AD onset.

SOCS1 Mediates Berberine-Induced Amelioration of Microglial Activated States in N9 Microglia Exposed to β Amyloid

Attenuating β amyloid- (Aβ-) induced microglial activation is considered to be effective in treating Alzheimer's disease (AD). Berberine (BBR) can reduce microglial activation in Aβ-treated microglial cells; the mechanism, however, is still illusive. Silencing of cytokine signaling factor 1 (SOCS1) is the primary regulator of many cytokines involved in immune reactions, whose upregulation can reverse the activation of microglial cells. Microglia could be activated into two different statuses, classic activated state (M1 state) and alternative activated state (M2 state), and M1 state is harmful, but M2 is beneficial. In the present study, N9 microglial cells were exposed to Aβ to imitate microglial activation in AD. And Western blot and immunocytochemistry were taken to observe inducible nitric oxide synthase (iNOS), Arginase-1 (Arg-1), and SOCS1 expressions, and the enzyme-linked immunosorbent assay (ELISA) was used to measure inflammatory and neurotrophic factor release. Compared with the normal cultured control cells, Aβ exposure markedly increased the level of microglial M1 state markers (P < 0.05), including iNOS protein expression, tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and IL-6 releases, and BBR administration upregulated SOSC1 expression and the level of microglial M2 state markers (P < 0.05), such as Arg-1 expression, brain-derived neurotrophic factor (BDNF), and glial cell-derived neurotrophic factor (GDNF) releases, downregulating the SOCS1 expression by using siRNA, however, significantly reversed the BBR-induced effects on microglial M1 and M2 state markers and SOCS1 expression (P < 0.05). These findings indicated that BBR can inhibit Aβ-induced microglial activation via modulating the microglial M1/M2 activated state, and SOCS1 mediates the process.

Studies of aging nonhuman primates illuminate the etiology of early-stage Alzheimer's-like neuropathology: An evolutionary perspective

Tau pathology in Alzheimer's disease (AD) preferentially afflicts the limbic and recently enlarged association cortices, causing a progression of mnemonic and cognitive deficits. Although genetic mouse models have helped reveal mechanisms underlying the rare, autosomal-dominant forms of AD, the etiology of the more common, sporadic form of AD remains unknown, and is challenging to study in mice due to their limited association cortex and lifespan. It is also difficult to study in human brains, as early-stage tau phosphorylation can degrade postmortem. In contrast, rhesus monkeys have extensive association cortices, are long-lived, and can undergo perfusion fixation to capture early-stage tau phosphorylation in situ. Most importantly, rhesus monkeys naturally develop amyloid plaques, neurofibrillary tangles comprised of hyperphosphorylated tau, synaptic loss, and cognitive deficits with advancing age, and thus can be used to identify the early molecular events that initiate and propel neuropathology in the aging association cortices. Studies to date suggest that the particular molecular signaling events needed for higher cognition-for example, high levels of calcium to maintain persistent neuronal firing- lead to tau phosphorylation and inflammation when dysregulated with advancing age. The expression of NMDAR-NR2B (GluN2B)-the subunit that fluxes high levels of calcium-increases over the cortical hierarchy and with the expansion of association cortex in primate evolution, consistent with patterns of tau pathology. In the rhesus monkey dorsolateral prefrontal cortex, spines contain NMDAR-NR2B and the molecular machinery to magnify internal calcium release near the synapse, as well as phosphodiesterases, mGluR3, and calbindin to regulate calcium signaling. Loss of regulation with inflammation and/or aging appears to be a key factor in initiating tau pathology. The vast expansion in the numbers of these synapses over primate evolution is consistent with the degree of tau pathology seen across species: marmoset < rhesus monkey < chimpanzee < human, culminating in the vast neurodegeneration seen in humans with AD.

Physiological and Pathological Ageing of Astrocytes in the Human Brain

Ageing is the greatest risk factor for dementia, although physiological ageing by itself does not lead to cognitive decline. In addition to ageing, APOE ε4 is genetically the strongest risk factor for Alzheimer's disease and is highly expressed in astrocytes. There are indications that human astrocytes change with age and upon expression of APOE4. As these glial cells maintain water and ion homeostasis in the brain and regulate neuronal transmission, it is likely that age- and APOE4-related changes in astrocytes have a major impact on brain functioning and play a role in age-related diseases. In this review, we will discuss the molecular and morphological changes of human astrocytes in ageing and the contribution of APOE4. We conclude this review with a discussion on technical issues, innovations, and future perspectives on how to gain more knowledge on astrocytes in the human ageing brain.

Antioxidant and neuroprotective effects of mGlu3 receptor activation on astrocytes aged in vitro

Astrocytes play a key role by providing antioxidant support to nearby neurons under oxidative stress. We have previously demonstrated that in vitro astroglial subtype 3 metabotropic glutamate receptor (mGlu3R) is neuroprotective. However, its role during aging has been poorly explored. Our study aimed to determine whether LY379268, an mGlu3R agonist, exerts an antioxidant effect on aged cultured rat astrocytes. Aged cultured astrocytes obtained after 9-weeks (9w) in vitro were positive for β-galactosidase stain, showed decreased mGlu3R and glutathione (GSH) levels and superoxide dismutase (SOD) activity, while nuclear erythroid factor 2 (Nrf2) protein levels, reactive oxygen species (ROS) production and apoptosis were increased. Treatment of 9w astrocytes with LY379268 resulted in an increase in mGlu3R and Nrf2 protein levels and SOD activity, and decreased mitochondrial ROS levels and apoptosis. mGlu3R activation in aged astrocytes also prevented hippocampal neuronal death induced by Aβ_1-42 in co-culture assays. We conclude that activation of mGlu3R in aged astrocytes had an anti-oxidant effect and protected hippocampal neurons against Aβ-induced neurotoxicity. The present study suggests mGlu3R activation in aging astrocytes as a therapeutic strategy to slow down age-associated neurodegeneration.

Changes in levels of cortical metabotropic glutamate 2 receptors with gender and suicide but not psychiatric diagnoses

BACKGROUND

We previously reported that, compared to controls, there are lower levels of [³H]LY341495 binding to metabotropic 2/3 receptors (GRM2/3) in Brodmann's area (BA) 24, but not 17 or 46, from subjects with major depressive disorders (MDD) but not bipolar disorders (BD) or schizophrenia. To be able to better interpret these data we have now measured levels of GRM2 in two of these cortical regions.

METHODS

Using a rabbit anti-metabotropic GRM2 monoclonal antibody with Western blotting we measured levels of GRM2 in BA 24 and 46 from subjects with MDD, BD, schizophrenia and controls (n = 15 per group).

RESULTS

Compared to controls, levels of GRM2, normalised to β-actin, did not differ in BA 24 or 46 from subjects with MDD, BD or schizophrenia (p from 0.36 to 0.79). Levels of GRM2 in BA 46, but not BA 24, were significantly higher in males compared to females (p < 0.01) and in suicide completers (p < 0.01) compare to death by other causes.

LIMITATIONS

Our cohort sizes, whilst being comparable to many postmortem CNS studies, are relatively low.

CONCLUSIONS

Our data suggests levels of GRM2 are not altered in two cortical regions from subjects with mood disorders or schizophrenia. Given we have found lower levels of [³H]LY341495 binding to GRM2/3 in BA 24 from subjects with MDD, our new data argues the lower levels of radioligand binding was due to lower levels of GRM3. Our data also suggests that glutamatergic activity through GRM2 in BA 46 may differ with gender and suicide ideation.

Metabotropic Glutamate Receptor 2/3 (mGluR2/3) Activation Suppresses TRPV1 Sensitization in Mouse, But Not Human, Sensory Neurons

The use of human tissue to validate putative analgesic targets identified in rodents is a promising strategy for improving the historically poor translational record of preclinical pain research. We recently demonstrated that in mouse and human sensory neurons, agonists for metabotropic glutamate receptors 2 and 3 (mGluR2/3) reduce membrane hyperexcitability produced by the inflammatory mediator prostaglandin E2 (PGE2). Previous rodent studies indicate that mGluR2/3 can also reduce peripheral sensitization by suppressing inflammation-induced sensitization of TRPV1. Whether this observation similarly translates to human sensory neurons has not yet been tested. We found that activation of mGluR2/3 with the agonist APDC suppressed PGE2-induced sensitization of TRPV1 in mouse, but not human, sensory neurons. We also evaluated sensory neuron expression of the gene transcripts for mGluR2 (Grm2), mGluR3 (Grm3), and TRPV1 (Trpv1). The majority of Trpv1+ mouse and human sensory neurons expressed Grm2 and/or Grm3, and in both mice and humans, Grm2 was expressed in a greater percentage of sensory neurons than Grm3. Although we demonstrated a functional difference in the modulation of TRPV1 sensitization by mGluR2/3 activation between mouse and human, there were no species differences in the gene transcript colocalization of mGluR2 or mGluR3 with TRPV1 that might explain this functional difference. Taken together with our previous work, these results suggest that mGluR2/3 activation suppresses only some aspects of human sensory neuron sensitization caused by PGE2. These differences have implications for potential healthy human voluntary studies or clinical trials evaluating the analgesic efficacy of mGluR2/3 agonists or positive allosteric modulators.

Considerations for optimal use of postmortem human brains for molecular psychiatry: lessons from schizophrenia

Schizophrenia is a disabling disease impacting millions of people around the world, for which there is no known cure. Current antipsychotic treatments for schizophrenia mainly target psychotic symptoms, do little to ameliorate social or cognitive deficits, have side-effects that cause weight gain, and diabetes and 30% of people do not respond. Thus, better therapeutics for schizophrenia aimed at the route biologic changes are needed and discovering the underlying neurobiology is key to this quest. Postmortem brain studies provide the most direct and detailed way to determine the pathophysiology of schizophrenia. This chapter outlines steps that can be taken to ensure the best-quality molecular data from postmortem brain tissue are obtained. In this chapter, we also discuss targeted and high-throughput methods for examining gene and protein expression and some of the strengths and limitations of each method. We briefly consider why gene and protein expression changes may not always concur within brain tissue. We conclude that postmortem brain research that investigates gene and protein expression in well-characterized and matched brain cohorts provides an important foundation to be considered when interpreting data obtained from studies of living schizophrenia patients.

A group II metabotropic glutamate receptor 3 (mGlu3, GRM3) isoform implicated in schizophrenia interacts with canonical mGlu3 and reduces ligand binding

As well as being expressed as a full-length transcript, the group II metabotropic glutamate receptor 3 (GRM3, mGlu3) gene is expressed as an mRNA isoform which lacks exon 4 (GRM3Δ4) and which is predicted to encode a protein with a novel C terminus (called mGlu3Δ4). This variant may contribute to the mechanism by which GRM3 acts as a schizophrenia risk gene. However, little is known about the properties or function of mGlu3Δ4. Here, using transiently transfected HEK293T/17 cells, we confirm that GRM3Δ4 cDNA is translated, with mGlu3Δ4 existing as a homodimer as well as a monomer, and localizing primarily to cell membranes including the plasma membrane. Co-immunoprecipitation shows that mGlu3Δ4 interacts with canonical mGlu3. mGlu3Δ4 does not bind the mGlu2/3 antagonist [³H]LY341495, but the presence of mGlu3Δ4 reduces binding of [³H]LY341495 to mGlu3, paralleled by a decrease in the abundance of membrane-associated mGlu3. These experiments indicate that mGlu3Δ4 may negatively modulate mGlu3, and thereby impact on the roles of GRM3/mGlu3 in schizophrenia and as a therapeutic target.