Group I metabotropic glutamate receptor signalling and its implication in neurological disease

Distinct calcium sources regulate temporal profiles of NMDAR and mGluR-mediated protein synthesis

Calcium signaling is integral for neuronal activity and synaptic plasticity. We demonstrate that the calcium response generated by different sources modulates neuronal activity-mediated protein synthesis, another process essential for synaptic plasticity. Stimulation of NMDARs generates a protein synthesis response involving three phases-increased translation inhibition, followed by a decrease in translation inhibition, and increased translation activation. We show that these phases are linked to NMDAR-mediated calcium response. Calcium influx through NMDARs elicits increased translation inhibition, which is necessary for the successive phases. Calcium through L-VGCCs acts as a switch from translation inhibition to the activation phase. NMDAR-mediated translation activation requires the contribution of L-VGCCs, RyRs, and SOCE. Furthermore, we show that IP3-mediated calcium release and SOCE are essential for mGluR-mediated translation up-regulation. Finally, we signify the relevance of our findings in the context of Alzheimer's disease. Using neurons derived from human fAD iPSCs and transgenic AD mice, we demonstrate the dysregulation of NMDAR-mediated calcium and translation response. Our study highlights the complex interplay between calcium signaling and protein synthesis, and its implications in neurodegeneration.

A molecular perspective on mGluR5 regulation in the antidepressant effect of ketamine

Ketamine, a non-competitive N-methyl-D-aspartate receptor (NMDAR) antagonist, has received much attention for its rapid antidepressant effects. A single administration of ketamine elicits rapid and sustained antidepressant effects in both humans and animals. Current efforts are focused on uncovering molecular mechanisms responsible for ketamine's antidepressant activity. Ketamine primarily acts via the glutamatergic pathway, and increasing evidence suggests that ketamine induces synaptic and structural plasticity through increased translation and release of neurotrophic factors, activation of mammalian target of rapamycin (mTOR), and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR)-mediated synaptic potentiation. However, the initial events triggering activation of intracellular signaling cascades and the mechanisms responsible for the sustained antidepressant effects of ketamine remain poorly understood. Over the last few years, it has become apparent that in addition to the fast actions of the ligand-gated AMPARs and NMDARs, metabotropic glutamate receptors (mGluRs), and particularly mGluR5, may also play a role in the antidepressant action of ketamine. Although research on mGluR5 in relation to the beneficial actions of ketamine is still in its infancy, a careful evaluation of the existing literature can identify converging trends and provide new interpretations. Here, we review the current literature on mGluR5 regulation in response to ketamine from a molecular perspective and propose a possible mechanism linking NMDAR inhibition to mGluR5 modulation.

Co-activation of NMDAR and mGluRs controls protein nanoparticle-induced osmotic pressure in neurotoxic edema

BACKGROUND

Glutamate stimuli and hyperactivation of its receptor are predominant determinants of ischemia-induced cytotoxic cerebral edema, which is closely associated with protein nanoparticle (PN)-induced increases in osmotic pressure. Herein, we investigated the electrochemical and mechanical mechanisms underlying the neuron swelling induced by PNs via the co-activation of N-methyl-D-aspartate receptor subunit (NMDAR) and excitatory metabotropic glutamate receptors (mGluRs).

RESULTS

We observed that co-activation of ionic glutamate receptor NMDAR and Group I metabotropic mGluRs promoted alteration of PN-induced membrane potential and increased intracellular osmosis, which was closely associated with calcium and voltage-dependent ion channels. In addition, activation of NMDAR-induced calmodulin (CaM) and mGluR downstream diacylglycerol (DAG)/protein kinase C α (PKCα) were observed to play crucial roles in cytotoxic hyperosmosis. The crosstalk between CaM and PKCα could upregulate the sensitivity and sustained opening of sulfonylurea receptor 1 (SUR1)-transient receptor potential cation channel subfamily M member 4 (TRPM4) and transmembrane protein 16 A (TMEM16A) channels, respectively, maintaining the massive Na+/Cl- influx, and the resultant neuron hyperosmosis and swelling. Intracellular PNs and Na+/Cl- influx were found to be as potential targets for cerebral edema treatment, using the neurocyte osmosis system and a cerebral ischemic rat model.

CONCLUSIONS

This study highlights PNs as a key factor in "electrochemistry-tension" signal transduction controlling Na+/Cl- ion channels and increased osmotic pressure in ischemia-induced cytotoxic edema. Moreover, enhanced sensitivity in both Na+ and Cl- ion channels also has a crucial role in cerebral edema.

The Involvement of Neuroinflammation in the Onset and Progression of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease exhibiting the fastest growth in incidence in recent years. As with most neurodegenerative diseases, the pathophysiology is incompletely elucidated, but compelling evidence implicates inflammation, both in the central nervous system and in the periphery, in the initiation and progression of the disease, although it is not yet clear what triggers this inflammatory response and where it begins. Gut dysbiosis seems to be a likely candidate for the initiation of the systemic inflammation. The therapies in current use provide only symptomatic relief, but do not interfere with the disease progression. Nonetheless, animal models have shown promising results with therapies that target various vicious neuroinflammatory cascades. Translating these therapeutic strategies into clinical trials is still in its infancy, and a series of issues, such as the exact timing, identifying biomarkers able to identify Parkinson's disease in early and pre-symptomatic stages, or the proper indications of genetic testing in the population at large, will need to be settled in future guidelines.

Neuromodulation in Parkinson's disease targeting opioid and cannabinoid receptors, understanding the role of NLRP3 pathway: a novel therapeutic approach

Parkinson's disease (PD) is a prevalent neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta, resulting in motor and non-motor symptoms. Although levodopa is the primary medication for PD, its long-term use is associated with complications such as dyskinesia and drug resistance, necessitating novel therapeutic approaches. Recent research has highlighted the potential of targeting opioid and cannabinoid receptors as innovative strategies for PD treatment. Modulating opioid transmission, particularly through activating µ (MOR) and δ (DOR) receptors while inhibiting κ (KOR) receptors, shows promise in preventing motor complications and reducing L-DOPA-induced dyskinesia. Opioids also possess neuroprotective properties and play a role in neuroprotection and seizure control. Similar to this, endocannabinoid signalling via CB1 and CB2 receptors influences the basal ganglia and may contribute to PD pathophysiology, making it a potential therapeutic target. In addition to opioid and cannabinoid receptor targeting, the NLRP3 pathway, implicated in neuroinflammation and neurodegeneration, emerges as another potential therapeutic avenue for PD. Recent studies suggest that targeting this pathway holds promise as a therapeutic strategy for PD management. This comprehensive review focuses on neuromodulation and novel therapeutic approaches for PD, specifically highlighting the targeting of opioid and cannabinoid receptors and the NLRP3 pathway. A better understanding of these mechanisms has the potential to enhance the quality of life for PD patients.

Lipid Raft Facilitated Receptor Organization and Signaling: A Functional Rheostat in Embryonic Development, Stem Cell Biology and Cancer

Molecular views of plasma membrane organization and dynamics are gradually changing over the past fifty years. Dynamics of plasma membrane instigate several signaling nexuses in eukaryotic cells. The striking feature of plasma membrane dynamics is that, it is internally transfigured into various subdomains of clustered macromolecules. Lipid rafts are nanoscale subdomains, enriched with cholesterol and sphingolipids, reside as floating entity mostly on the exoplasmic leaflet of the lipid bilayer. In terms of functionality, lipid rafts are unique among other membrane subdomains. Herein, advances on the roles of lipid rafts in cellular physiology and homeostasis are discussed, precisely, on how rafts dynamically harbor signaling proteins, including GPCRs, catalytic receptors, and ionotropic receptors within it and orchestrate multiple signaling pathways. In the developmental proceedings signaling are designed for patterning of overall organism and they differ from the somatic cell physiology and signaling of fully developed organisms. Some of the developmental signals are characteristic in maintenance of stemness and activated during several types of tumor development and cancer progression. The harmony between extracellular signaling and lineage specific transcriptional programs are extremely important for embryonic development. The roles of plasma membrane lipid rafts mediated signaling in lineage specificity, early embryonic development, stem cell maintenance are emerging. In view of this, we have highlighted and analyzed the roles of lipid rafts in receptor organization, cell signaling, and gene expression during embryonic development; from pre-implantation through the post-implantation phase, in stem cell and cancer biology.

Possible Effects of Acupuncture in Poststroke Aphasia

Neural plasticity promotes the reorganization of language networks and is an essential recovery mechanism for poststroke aphasia (PSA). Neuroplasticity may be a pivotal bridge to elucidate the potential recovery mechanisms of acupuncture for aphasia. Therefore, understanding the neuroplasticity mechanism of acupuncture in PSA is crucial. However, the underlying therapeutic mechanism of neuroplasticity in PSA after acupuncture needs to be explored. Excitotoxicity after brain injury affects the activity of neurotransmitters and disrupts the transmission of normal neuron information. Thus, a helpful strategy of acupuncture might be to improve PSA by affecting the availability of these neurotransmitters and glutamate receptors at synapses. In addition, the regulation of neuroplasticity by acupuncture may also be related to the regulation of astrocytes. Considering the guiding significance of acupuncture for clinical treatment, it is necessary to carry out further study about the influence of acupuncture on the recovery of aphasia after stroke. This study summarizes the current research on the neural mechanism of acupuncture in treating PSA. It seeks to elucidate the potential effect of acupuncture on the recovery of PSA from the perspective of synaptic plasticity and integrity of gray and white matter.

Blocked metabotropic glutamate receptor 5 enhances chemosensitivity in hepatocellular carcinoma and attenuates chemotoxicity in the normal liver by regulating DNA damage

DNA damaging agents are used as chemotherapeutics in many cancers, including hepatocellular carcinoma (HCC). However, they are associated with problems such as low sensitivity to chemotherapy and the induction of liver injury, underscoring the need to identify new therapies. Here, we investigated the differential regulatory effect of metabotropic glutamate receptor 5 (mGlu5) on chemosensitivity in HCC and chemotoxicity to the normal liver. The expression of mGlu5 was higher in HCC than in the normal liver, and correlated with poor prognosis according to The Cancer Genome Atlas database and Integrative Molecular Database of Hepatocellular Carcinoma. Cisplatin, oxaliplatin or methyl methanesulfonate (MMS) caused cell death by decreasing mGlu5 expression in HCC cells and increased mGlu5 expression in hepatic cells. In HCC cells, inhibition of mGlu5 aggravated MMS-induced DNA damage by increasing intracellular Ca2+ overload and mitogen-activated protein kinase (MAPK) activation, thereby promoting cell death, and activation of mGlu5 rescued the effect of MMS. However, in hepatic cells, mGlu5 inhibition alleviated MMS-induced DNA damage by downregulating Ca2+-derived MAPK pathways to advance hepatic cell survival. The opposite effects of mGlu5 overexpression or knockdown on MMS-induced DNA damage supported that cell death is a result of the differential regulation of mGlu5 expression. Inhibition of mGlu5 increased chemosensitivity and decreased chemotoxicity in a rat tumor model. This study suggests that mGlu5 inhibition could act synergistically with HCC chemotherapeutics with minimal side effects, which may improve the treatment of patients with HCC in the future.

Autism-Risk Gene necab2 Regulates Psychomotor and Social Behavior as a Neuronal Modulator of mGluR1 Signaling

Background

De novo deletion of the neuronal calcium-binding protein 2 (NECAB2) locus is associated with idiopathic autism spectrum disorders (ASDs). The in vivo function of NECAB2 in the brain remains largely elusive.

Methods

We investigated the morphological and behavioral profiles of both necab2 knock-out and overexpression zebrafish models. The expression pattern and molecular role of necab2 were probed through a combination of in vitro and in vivo assays.

Results

We show that Necab2 is a neuronal specific, cytoplasmic, and membrane-associated protein, abundantly expressed in the telencephalon, habenula, and cerebellum. Necab2 is distributed peri-synaptically in subsets of glutamatergic and GABAergic neurons. CRISPR/Cas9-generated necab2 knock-out zebrafish display normal morphology but exhibit a decrease in locomotor activity and thigmotaxis with impaired social interaction only in males. Conversely, necab2 overexpression yields behavioral phenotypes opposite to the loss-of-function. Proteomic profiling uncovers a role of Necab2 in modulating signal transduction of G-protein coupled receptors. Specifically, co-immunoprecipitation, immunofluorescence, and confocal live-cell imaging suggest a complex containing NECAB2 and the metabotropic glutamate receptor 1 (mGluR1). In vivo measurement of phosphatidylinositol 4,5-bisphosphate further substantiates that Necab2 promotes mGluR1 signaling.

Conclusions

Necab2 regulates psychomotor and social behavior via modulating a signaling cascade downstream of mGluR1.

mGluR5 ablation leads to age-related synaptic plasticity impairments and does not improve Huntington’s disease phenotype

Glutamate receptors, including mGluR5, are involved in learning and memory impairments triggered by aging and neurological diseases. However, each condition involves distinct molecular mechanisms. It is still unclear whether the mGluR5 cell signaling pathways involved in normal brain aging differ from those altered due to neurodegenerative disorders. Here, we employed wild type (WT), mGluR5^−/−, BACHD, which is a mouse model of Huntington’s Disease (HD), and mGluR5^−/−/BACHD mice, at the ages of 2, 6 and 12 months, to distinguish the mGluR5-dependent cell signaling pathways involved in aging and neurodegenerative diseases. We demonstrated that the memory impairment exhibited by mGluR5^−/− mice is accompanied by massive neuronal loss and decreased dendritic spine density in the hippocampus, similarly to BACHD and BACHD/mGluR5^−/− mice. Moreover, mGluR5 ablation worsens some of the HD-related alterations. We also show that mGluR5^−/− and BACHD/mGluR5^−/− mice have decreased levels of PSD95, BDNF, and Arc/Arg3.1, whereas BACHD mice are mostly spared. PSD95 expression was affected exclusively by mGluR5 ablation in the aging context, making it a potential target to treat age-related alterations. Taken together, we reaffirm the relevance of mGluR5 for memory and distinguish the mGluR5 cell signaling pathways involved in normal brain aging from those implicated in HD.

Ion channels and neuronal excitability in polyglutamine neurodegenerative diseases

Polyglutamine (polyQ) diseases are a family composed of nine neurodegenerative inherited disorders (NDDs) caused by pathological expansions of cytosine-adenine-guanine (CAG) trinucleotide repeats which encode a polyQ tract in the corresponding proteins. CAG polyQ repeat expansions produce neurodegeneration via multiple downstream mechanisms; among those the neuronal activity underlying the ion channels is affected directly by specific channelopathies or indirectly by secondary dysregulation. In both cases, the altered excitability underlies to gain- or loss-of-function pathological effects. Here we summarize the repertoire of ion channels in polyQ NDDs emphasizing the biophysical features of neuronal excitability and their pathogenic role. The aim of this review is to point out the value of a deeper understanding of those functional mechanisms and processes as crucial elements for the designing and targeting of novel therapeutic avenues.

Insights into Human-Induced Pluripotent Stem Cell-Derived Astrocytes in Neurodegenerative Disorders

Most neurodegenerative disorders have complex and still unresolved pathology characterized by progressive neuronal damage and death. Astrocytes, the most-abundant non-neuronal cell population in the central nervous system, play a vital role in these processes. They are involved in various functions in the brain, such as the regulation of synapse formation, neuroinflammation, and lactate and glutamate levels. The development of human-induced pluripotent stem cells (iPSCs) reformed the research in neurodegenerative disorders allowing for the generation of disease-relevant neuronal and non-neuronal cell types that can help in disease modeling, drug screening, and, possibly, cell transplantation strategies. In the last 14 years, the differentiation of human iPSCs into astrocytes allowed for the opportunity to explore the contribution of astrocytes to neurodegenerative diseases. This review discusses the development protocols and applications of human iPSC-derived astrocytes in the most common neurodegenerative conditions.

Some Twist of Molecular Circuitry Fast Forwards Overnight Sleep Hours: A Systematic Review of Natural Short Sleepers' Genes

This systematic review focuses on different genetic mutations identified in studies on natural short sleepers, who would not be ill-defined as one type of sleep-related disorder. The reviewed literature is from databases such as PubMed, PMC, Scopus, and ResearchGate. Due to the rare prevalence, the number of studies conducted on natural short sleepers is limited. Hence, searching the search of databases was done without any date restriction and included animal studies, since mouse and fly models share similarities with human sleep behaviors. Of the 12 articles analyzed, four conducted two types of studies, animal and human (cross-sectional or randomized-controlled studies), to testify the effects of human mutant genes in familial natural short sleepers via transgenic mouse or fly models. The remaining eight articles mainly focused on one type of study each: animal study (four articles), cross-sectional study (two articles), review (one article), and case report (one article). Hence, those articles brought different perspectives on the natural short sleep phenomenon by identifying intrinsic factors like DEC2, NPSR1, mGluR1, and β1-AR mutant genes. Natural short sleep traits in either point-mutations or single null mutations in those genes have been examined and confirmed its intrinsic nature in affected individuals without any related health concerns. Finally, this review added a potential limitation in these studies, mainly highlighting intrinsic causes since one case study reported an extrinsically triggered short sleep behavior in an older man without any family history. The overall result of the review study suggests that the molecular mechanisms tuned by identified sleep genes can give some potential points of therapeutic intervention in future studies.

Noncanonical Metabotropic Glutamate Receptor 5 Signaling in Alzheimer's Disease

Metabotropic glutamate receptor 5 (mGluR5) is ubiquitously expressed in brain regions responsible for memory and learning. It plays a key role in modulating rapid changes in synaptic transmission and plasticity. mGluR5 supports long-term changes in synaptic strength by regulating the transcription and translation of essential synaptic proteins. β-Amyloid 42 (Aβ42) oligomers interact with a mGluR5/cellular prion protein (PrP^C) complex to disrupt physiological mGluR5 signal transduction. Aberrant mGluR5 signaling and associated synaptic failure are considered an emerging pathophysiological mechanism of Alzheimer's disease (AD). Therefore, mGluR5 represents an attractive therapeutic target for AD, and recent studies continue to validate the efficacy of various mGluR5 allosteric modulators in improving memory deficits and mitigating disease pathology. However, sex-specific differences in the pharmacology of mGluR5 and activation of noncanonical signaling downstream of the receptor suggest that its utility as a therapeutic target in female AD patients needs to be reconsidered. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

In vitro and In vivo Assessment of Suitable Reference Region and Kinetic Modelling for the mGluR1 Radioligand [11C]ITDM in Mice

PURPOSE

This study aimed at investigating binding specificity, suitability of reference region-based kinetic modelling, and pharmacokinetics of the metabotropic glutamate receptor 1 (mGluR1) radioligand [¹¹C]ITDM in mice.

PROCEDURES

We performed in vivo blocking as well as displacement of [¹¹C]ITDM during positron emission tomography (PET) imaging using the specific mGluR1 antagonist YM-202074. Additionally, we assessed in vitro blocking of [³H]ITDM at two different doses of YM-202074. As an alternative to reference region models, we validated the use of a noninvasive image-derived input function (IDIF) compared to an arterial input function measured with an invasive arteriovenous (AV) shunt using a population-based curve for radiometabolite correction and characterized the pharmacokinetic modelling of [¹¹C]ITDM in the mouse brain. Finally, we also assessed semi-quantitative approaches.

RESULTS

In vivo blocking with YM-202074 resulted in a decreased [¹¹C]ITDM binding, ranging from - 35.8 ± 8.0 % in pons to - 65.8 ± 3.0 % in thalamus. Displacement was also markedly observed in all tested regions. In addition, in vitro [³H]ITDM binding could be blocked in a dose-dependent manner. The volume of distribution (V_T) based on the noninvasive IDIF (V_{T (IDIF)}) showed excellent agreement with the V_T values based on the metabolite-corrected plasma input function regardless of the metabolite correction (r² > 0.943, p < 0.0001). Two-tissue compartmental model (2TCM) was found to be the preferred model and showed optimal agreement with Logan plot (r² > 0.960, p < 0.0001). A minimum scan duration of 80 min was required for proper parameter estimation. SUV was not reliable (r² = 0.379, p = 0.0011), unlike the SUV ratio to the SUV of the input function, which showed to be a valid approach.

CONCLUSIONS

No suitable reference region could be identified for [¹¹C]ITDM as strongly supported by in vivo and in vitro evidence of specific binding in all brain regions. However, by applying appropriate kinetic models, [¹¹C]ITDM PET imaging represents a promising tool to visualize mGluR1 in the mouse brain.

Group 1 Metabotropic Glutamate Receptors in Neurological and Psychiatric Diseases: Mechanisms and Prospective

Metabotropic glutamate receptors (mGluRs) are G-protein coupled receptors that are activated by glutamate in the central nervous system (CNS). Basically, mGluRs contribute to fine-tuning of synaptic efficacy and control the accuracy and sharpness of neurotransmission. Among eight subtypes, mGluR1 and mGluR5 belong to group 1 (Gp1) family, and are implicated in multiple CNS disorders, such as Alzheimer’s disease, autism, Parkinson’s disease, and so on. In the present review, we systematically discussed underlying mechanisms and prospective of Gp1 mGluRs in a group of neurological and psychiatric diseases, including Alzheimer’s disease, Parkinson’s disease, autism spectrum disorder, epilepsy, Huntington’s disease, intellectual disability, Down’s syndrome, Rett syndrome, attention-deficit hyperactivity disorder, addiction, anxiety, nociception, schizophrenia, and depression, in order to provide more insights into the therapeutic potential of Gp1 mGluRs.

The Density of Group I mGlu5 Receptors Is Reduced along the Neuronal Surface of Hippocampal Cells in a Mouse Model of Alzheimer's Disease

Metabotropic glutamate receptor subtype 5 (mGlu₅) is implicated in the pathophysiology of Alzheimer’s disease (AD). However, its alteration at the subcellular level in neurons is still unexplored. Here, we provide a quantitative description on the expression and localisation patterns of mGlu₅ in the APP/PS1 model of AD at 12 months of age, combining immunoblots, histoblots and high-resolution immunoelectron microscopic approaches. Immunoblots revealed that the total amount of mGlu₅ protein in the hippocampus, in addition to downstream molecules, i.e., G_q/11 and PLCβ₁, was similar in both APP/PS1 mice and age-matched wild type mice. Histoblots revealed that mGlu₅ expression in the brain and its laminar expression in the hippocampus was also unaltered. However, the ultrastructural techniques of SDS-FRL and pre-embedding immunogold demonstrated that the subcellular localisation of mGlu₅ was significantly reduced along the neuronal surface of hippocampal principal cells, including CA1 pyramidal cells and DG granule cells, in APP/PS1 mice at 12 months of age. The decrease in the surface localisation of mGlu₅ was accompanied by an increase in its frequency at intracellular sites in the two neuronal populations. Together, these data demonstrate, for the first time, a loss of mGlu₅ at the plasma membrane and accumulation at intracellular sites in different principal cells of the hippocampus in APP/PS1 mice, suggesting an alteration of the excitability and synaptic transmission that could contribute to the cognitive dysfunctions in this AD animal model. Further studies are required to elucidate the specificity of mGlu₅-associated molecules and downstream signalling pathways in the progression of the pathology.

Towards an improved pain assessment in castrated horses using facial expressions (HGS) and circulating miRNAs

BACKGROUND

Pain in horses is an emergent welfare concern, and its assessment represents a challenge for equine clinicians. This study aimed at improving pain assessment in horses through a convergent validation of existing tools: we investigated whether an effective analgesic treatment influences the horse grimace scale (HGS) and the concentration of specific circulating microRNAs (miRNAs).

METHODS

Eleven stallions underwent routine surgical castration under general anaesthesia. They were divided into two analgesic treatment groups: castration with the administration of preoperative flunixin and castration with preoperative flunixin plus a local injection of mepivacaine into the spermatic cords. HGS and levels of seven circulating miRNAs were evaluated pre-, 8 and 20 hours post-procedure.

RESULTS

Compared to pre-castration, HGS, miR-126-5p, miR-145 and miR-let7e increased significantly in horses receiving flunixin at 8 hours post-castration (Friedman test, p < 0.05). Both behavioural and molecular changes occurred in horses receiving flunixin only, confirming that the addition of local mepivacaine is an effective analgesic treatment.

CONCLUSIONS

Combining the use of HGS and circulating miRNAs, particularly miR-145, could be meaningful to monitor acute pain conditions in horses. Our results further validate the HGS as a method to assess acute pain in horses and point out miR-145 as a promising biomarker to identify pain.

Metabotropic glutamate receptor 5 inhibits α-synuclein-induced microglia inflammation to protect from neurotoxicity in Parkinson's disease

Background

Microglia activation induced by α-synuclein (α-syn) is one of the most important factors in Parkinson’s disease (PD) pathogenesis. However, the molecular mechanisms by which α-syn exerts neuroinflammation and neurotoxicity remain largely elusive. Targeting metabotropic glutamate receptor 5 (mGluR5) has been an attractive strategy to mediate microglia activation for neuroprotection, which might be an essential regulator to modulate α-syn-induced neuroinflammation for the treatment of PD. Here, we showed that mGluR5 inhibited α-syn-induced microglia inflammation to protect from neurotoxicity in vitro and in vivo.

Methods

Co-immunoprecipitation assays were utilized to detect the interaction between mGluR5 and α-syn in microglia. Griess, ELISA, real-time PCR, western blotting, and immunofluorescence assays were used to detect the regulation of α-syn-induced inflammatory signaling, cytokine secretion, and lysosome-dependent degradation.

Results

α-syn selectively interacted with mGluR5 but not mGluR3, and α-syn N terminal deletion region was essential for binding to mGluR5 in co-transfected HEK293T cells. The interaction between these two proteins was further detected in BV2 microglia, which was inhibited by the mGluR5 specific agonist CHPG without effect by its selective antagonist MTEP. Moreover, in both BV2 cells and primary microglia, activation of mGluR5 by CHPG partially inhibited α-syn-induced inflammatory signaling and cytokine secretion and also inhibited the microglia activation to protect from neurotoxicity. We further found that α-syn overexpression decreased mGluR5 expression via a lysosomal pathway, as evidenced by the lysosomal inhibitor, NH₄Cl, by blocking mGluR5 degradation, which was not evident with the proteasome inhibitor, MG132. Additionally, co-localization of mGluR5 with α-syn was detected in lysosomes as merging with its marker, LAMP-1. Consistently, in vivo experiments with LPS- or AAV-α-syn-induced rat PD model also confirmed that α-syn accelerated lysosome-dependent degradation of mGluR5 involving a complex, to regulate neuroinflammation. Importantly, the binding is strengthened with LPS or α-syn overexpression but alleviated by urate, a potential clinical biomarker for PD.

Conclusions

These findings provided evidence for a novel mechanism by which the association of α-syn with mGluR5 was attributed to α-syn-induced microglia activation via modulation of mGluR5 degradation and its intracellular signaling. This may be a new molecular target for an effective therapeutic strategy for PD pathology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02079-1.

Prenatal Hypoxia Induces Premature Aging Accompanied by Impaired Function of the Glutamatergic System in Rat Hippocampus

Prenatal hypoxia is among leading causes of progressive brain pathologies in postnatal life. This study aimed to analyze the characteristics of the hippocampal glutamatergic system and behavior of rats in early (2 weeks), adult (3 months) and advanced (18 months) postnatal ontogenesis after exposure to prenatal severe hypoxia (PSH, 180 Torr, 5% O2, 3 h) during the critical period in the formation of the hippocampus (days 14-16 of gestation). We have shown an age-dependent progressive decrease in the hippocampal glutamate levels, a decrease of the neuronal cell number in the CA1 hippocampal region, as well as impairment of spatial long-term memory in the Morris water navigation task. The gradual decrease of glutamate was accompanied by decreased expression of the genes that mediate glutamate metabolism and recycling in the hippocampus. That deficiency apparently correlated with an increase of the metabotropic glutamate receptor type 1 (mGluR1) and synaptophysin expression. Generation of the lipid peroxidation products in the hippocampus of adult rats subjected to prenatal severe hypoxia (PSH rats) was not increased compared to the control animals when tested in a model of glutamate excitotoxicity induced by severe hypoxia. This demonstrates that excessive glutamate sensitivity in PSH rats does not compensate for glutamate deficiency. Our results show a significant contribution of the glutamate system dysfunction to age-associated decrease of this mediator, cognitive decline, and early neuronal loss in PSH rats.

SUMOylation of synaptic and synapse-associated proteins: An update

SUMOylation is a post-translational modification that regulates protein signalling and complex formation by adjusting the conformation or protein-protein interactions of the substrate protein. There is a compelling and rapidly expanding body of evidence that, in addition to SUMOylation of nuclear proteins, SUMOylation of extranuclear proteins contributes to the control of neuronal development, neuronal stress responses and synaptic transmission and plasticity. In this brief review we provide an update of recent developments in the identification of synaptic and synapse-associated SUMO target proteins and discuss the cell biological and functional implications of these discoveries.

New Evidence on the Role of D-Aspartate Metabolism in Regulating Brain and Endocrine System Physiology: From Preclinical Observations to Clinical Applications

The endogenous amino acids serine and aspartate occur at high concentrations in free D-form in mammalian organs, including the central nervous system and endocrine glands. D-serine (D-Ser) is largely localized in the forebrain structures throughout pre and postnatal life. Pharmacologically, D-Ser plays a functional role by acting as an endogenous coagonist at N-methyl-D-aspartate receptors (NMDARs). Less is known about the role of free D-aspartate (D-Asp) in mammals. Notably, D-Asp has a specific temporal pattern of occurrence. In fact, free D-Asp is abundant during prenatal life and decreases greatly after birth in concomitance with the postnatal onset of D-Asp oxidase expression, which is the only enzyme known to control endogenous levels of this molecule. Conversely, in the endocrine system, D-Asp concentrations enhance after birth during its functional development, thereby suggesting an involvement of the amino acid in the regulation of hormone biosynthesis. The substantial binding affinity for the NMDAR glutamate site has led us to investigate the in vivo implications of D-Asp on NMDAR-mediated responses. Herein we review the physiological function of free D-Asp and of its metabolizing enzyme in regulating the functions of the brain and of the neuroendocrine system based on recent genetic and pharmacological human and animal studies.

Mutations in Metabotropic Glutamate Receptor 1 Contribute to Natural Short Sleep Trait

Sufficient and efficient sleep is crucial for our health. Natural short sleepers can sleep significantly shorter than the average population without a desire for more sleep and without any obvious negative health consequences. In searching for genetic variants underlying the short sleep trait, we found two different mutations in the same gene (metabotropic glutamate receptor 1) from two independent natural short sleep families. In vitro, both of the mutations exhibited loss of function in receptor-mediated signaling. In vivo, the mice carrying the individual mutations both demonstrated short sleep behavior. In brain slices, both of the mutations changed the electrical properties and increased excitatory synaptic transmission. These results highlight the important role of metabotropic glutamate receptor 1 in modulating sleep duration.

New insights on the influence of free d-aspartate metabolism in the mammalian brain during prenatal and postnatal life

Free d-aspartate is abundant in the mammalian embryonic brain. However, following the postnatal onset of the catabolic d-aspartate oxidase (DDO) activity, cerebral d-aspartate levels drastically decrease, remaining constantly low throughout life. d-Aspartate stimulates both glutamatergic NMDA receptors (NMDARs) and metabotropic Glu5 receptors. In rodents, short-term d-aspartate exposure increases spine density and synaptic plasticity, and improves cognition. Conversely, persistently high d-Asp levels produce NMDAR-dependent neurotoxic effects, leading to precocious neuroinflammation and cell death. These pieces of evidence highlight the dichotomous impact of d-aspartate signaling on NMDAR-dependent processes and, in turn, unveil a neuroprotective role for DDO in preventing the detrimental effects of excessive d-aspartate stimulation during aging. Here, we will focus on the in vivo influence of altered d-aspartate metabolism on the modulation of glutamatergic functions and its involvement in translational studies. Finally, preliminary data on the role of embryonic d-aspartate in the mouse brain will also be reviewed.

Characterization of Group I Metabotropic Glutamate Receptors in Rat and Human Adrenal Glands

Glutamate and its receptors have been demonstrated to promote both basal and nicotine-evoked catecholamine release in bovine chromaffin cells. Multiple glutamate receptors, including metabotropic glutamate receptors (mGluRs), are found in the adrenal glands of several species, as well as in chromaffin cells. However, there is limited information available regarding the expression of glutamate metabotropic receptor (GRM)1-8 mRNAs and the detailed localization of group I mGluRs (mGluR1 and mGluR5) in the rat and human adrenal cortex and medulla. Therefore, we examined mRNA expression of GRM1-8 subunits using reverse transcription-polymerase chain reaction (RT-PCR) and the distribution of mGluR1 and mGluR5 by immunostaining. The results showed that the GRM1-8 mRNAs were expressed in both the cortex and medulla of rat and human adrenal glands with the exception of GRM1, which was not detectable in the rat adrenal cortex. Immunostaining of mGluR1 revealed that it was localized only in the adrenal medulla of rats but was present in both the adrenal cortex and medulla in humans. In the adrenal medulla, the central part of the adrenal glands, mGluR1 was detected in chromaffin cells but not in nerve fibers and ganglion cells. Immunoactivity of mGluR5 was visible in the capillary wall throughout the adrenal cortex and medulla in rat and human samples. Its immunoactivity was also observed in ganglion cells in the rat adrenal medulla. There was no mGluR5 immunoactivity detected in chromaffin cells and nerve fibers in the rat and human adrenal medulla. Using dissected rat adrenal medulla as a model, we found that treatment with a mGluR1 agonist activated extracellular signal-regulated kinase (ERK) 1/2 and increased the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis. Moreover, these results showed that mGluR1 signaling was involved in hypoxia-induced upregulation of TH in the rat adrenal medulla. This study shows the expression of GRM1-8 mRNAs in rat and human adrenal glands and indicates that glutamate, through the activation of mGluRs, may play various physiological roles in the adrenal gland. Furthermore, mGluR1 may be involved in catecholamine biosynthesis by regulating TH, and mGluR5 may affect cortical and medullar hormone levels by regulating microvascular function.

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

A plethora of neurological disorders shares a final common deadly pathway known as excitotoxicity. Among these disorders, ischemic injury is a prominent cause of death and disability worldwide. Brain ischemia stems from cardiac arrest or stroke, both responsible for insufficient blood supply to the brain parenchyma. Glucose and oxygen deficiency disrupts oxidative phosphorylation, which results in energy depletion and ionic imbalance, followed by cell membrane depolarization, calcium (Ca²⁺) overload, and extracellular accumulation of excitatory amino acid glutamate. If tight physiological regulation fails to clear the surplus of this neurotransmitter, subsequent prolonged activation of glutamate receptors forms a vicious circle between elevated concentrations of intracellular Ca²⁺ ions and aberrant glutamate release, aggravating the effect of this ischemic pathway. The activation of downstream Ca²⁺-dependent enzymes has a catastrophic impact on nervous tissue leading to cell death, accompanied by the formation of free radicals, edema, and inflammation. After decades of “neuron-centric” approaches, recent research has also finally shed some light on the role of glial cells in neurological diseases. It is becoming more and more evident that neurons and glia depend on each other. Neuronal cells, astrocytes, microglia, NG2 glia, and oligodendrocytes all have their roles in what is known as glutamate excitotoxicity. However, who is the main contributor to the ischemic pathway, and who is the unsuspecting victim? In this review article, we summarize the so-far-revealed roles of cells in the central nervous system, with particular attention to glial cells in ischemia-induced glutamate excitotoxicity, its origins, and consequences.

Sex differences in the glutamate system: Implications for addiction

Clinical and preclinical research have identified sex differences in substance use and addiction-related behaviors. Historically, substance use disorders are more prevalent in men than women, though this gap is closing. Despite this difference, women appear to be more susceptible to the effects of many drugs and progress to substance abuse treatment more quickly than men. While the glutamate system is a key regulator of addiction-related behaviors, much of the work implicating glutamate signaling and glutamatergic circuits has been conducted in men and male rodents. An increasing number of studies have identified sex differences in drug-induced glutamate alterations as well as sex and estrous cycle differences in drug seeking behaviors. This review will describe sex differences in the glutamate system with an emphasis on implications for substance use disorders, highlighting the gaps in our current understanding of how innate and drug-induced alterations in the glutamate system may contribute to sex differences in addiction-related behaviors.

Hypergravity induced disruption of cerebellar motor coordination

The cerebellum coordinates voluntary movements for balanced motor activity in a normal gravity condition. It remains unknown how hypergravity is associated with cerebellum-dependent motor behaviors and Purkinje cell’s activities. In order to investigate the relationship between gravity and cerebellar physiology, we measured AMPA-mediated fast currents and mGluR1-mediated slow currents of cerebellar Purkinje cells along with cerebellum-dependent behaviors such as the footprint and irregular ladder under a hypergravity condition. We found abnormal animal behaviors in the footprint and irregular ladder tests under hypergravity. They are correlated with decreased AMPA and mGluR1-mediated synaptic currents of Purkinje cells. These results indicate that gravity regulates the activity of Purkinje cells, thereby modulating cerebellum-dependent motor outputs.

Attenuation of Acute Intracerebral Hemorrhage-Induced Microglial Activation and Neuronal Death Mediated by the Blockade of Metabotropic Glutamate Receptor 5 In Vivo

The activation of microglia in response to intracerebral hemorrhagic stroke is one of the principal components of the progression of this disease. It results in the formation of pro-inflammatory cytokines that lead to neuronal death, a structural deterioration that, in turn interferes with functional recovery. Metabotropic glutamate receptor 5 (mGluR5) is highly expressed in reactive microglia and is involved in the pathological processes of brain disorders, but its role in intracerebral hemorrhage (ICH) remains unknown. We hypothesized that mGluR5 regulates microglial activation and ICH maintenance. In this study, collagenase-induced ICH mice received a single intraperitoneal injection of the mGluR5 antagonist-, MTEP, or vehicle 2 h after injury. We found that acute ICH upregulated mGluR5 and microglial activation. mGluR5 was highly localized in reactive microglia in the peri-hematomal cortex and striatum on days 3 and 7 post-ICH. The MTEP-mediated pharmacological inhibition of mGluR5 in vivo resulted in the substantial attenuation of acute microglial activation and IL-6, and TNF-α release. We also showed that the blockade of mGluR5 markedly reduced cell apoptosis, and neurodegeneration and markedly elevated neuroprotection. Furthermore, the MTEP-mediated inhibition of mGluR5 significantly reduced the lesion volume and improved functional recovery. Taken together, our results demonstrate that ICH injury enhances mGluR5 expression in the acute and subacute stages and that mGluR5 is highly localized in reactive microglia. The blockade of mGluR5 reduces ICH-induced acute microglial activation, provides neuroprotection and promotes neurofunctional recovery after ICH. The inhibition of mGluR5 may be a relevant therapeutic target for intracerebral hemorrhagic stroke.

Elevated Type 1 Metabotropic Glutamate Receptor Availability in a Mouse Model of Huntington's Disease: a Longitudinal PET Study

Impairment of group I metabotropic glutamate receptors (mGluRs) results in altered glutamate signalling, which is associated with several neurological disorders including Huntington’s Disease (HD), an autosomal neurodegenerative disease. In this study, we assessed in vivo pathological changes in mGluR1 availability in the Q175DN mouse model of HD using longitudinal positron emission tomography (PET) imaging with the radioligand [¹¹C]ITDM. Ninety-minute dynamic PET imaging scans were performed in 22 heterozygous (HET) Q175DN mice and 22 wild-type (WT) littermates longitudinally at 6, 12, and 16 months of age. Analyses of regional volume of distribution with an image-derived input function (V_{T (IDIF)}) and voxel-wise parametric V_{T (IDIF)} maps were performed to assess differences between genotypes. Post-mortem evaluation at 16 months was done to support in vivo findings. [¹¹C]ITDM V_{T (IDIF)} quantification revealed higher mGluR1 availability in the brain of HET mice compared to WT littermates (e.g. cerebellum: + 15.0%, + 17.9%, and + 17.6% at 6, 12, and 16 months, respectively; p < 0.001). In addition, an age-related decline in [¹¹C]ITDM binding independent of genotype was observed between 6 and 12 months. Voxel-wise analysis of parametric maps and post-mortem quantifications confirmed the elevated mGluR1 availability in HET mice compared to WT littermates. In conclusion, in vivo measurement of mGluR1 availability using longitudinal [¹¹C]ITDM PET imaging demonstrated higher [¹¹C]ITDM binding in extra-striatal brain regions during the course of disease in the Q175DN mouse model.

Modulation of mTOR and CREB pathways following mGluR5 blockade contribute to improved Huntington's pathology in zQ175 mice

Huntington’s disease (HD) is a neurodegenerative disorder caused by a genetic abnormality in the huntingtin gene that leads to a polyglutamine repeat expansion of the huntingtin protein. The cleaved polyglutamine expansion of mutant huntingtin (mHTT) protein can form aggregates strongly correlated with HD progression. We have previously shown that the inhibition of mGluR5 using CTEP, a selective negative allosteric mGluR5 modulator, can delay disease progression and reduce in mHTT aggregates in the zQ175 mouse model of HD. This was paralleled by enhanced catalytic activity of Unc-51-like kinase 1 (ULK1), a kinase modulated by mammalian target of rapamycin (mTOR) and key regulator of autophagy initiation. In the present study, we show that CTEP can correct aberrant phosphoinositide 3-kinase (PI3K)/Akt/mTOR signaling detected in zQ175 mice that may underlie the enhanced ULK1 activity and activation of autophagy. We also show that CTEP can facilitate cAMP response element-binding protein (CREB)-mediated expression of brain-derived neurotrophic factor (BDNF) to foster neuronal survival and reduce apoptosis. Taken together, our findings provide the molecular evidence for how targeting mGluR5 using a well-tolerated selective NAM can mitigate two critical mechanisms of neurodegeneration, autophagy and apoptosis.

The dual face of glutamate: from a neurotoxin to a potential survival factor-metabolic implications in health and disease

Glutamate is the major excitatory neurotransmitter in the central nervous system. Beyond this function, glutamate also plays a key role in intermediary metabolism in all organs and tissues, linking carbohydrate and amino acid metabolism via the tricarboxylic acid cycle. Under both physiological and pathological conditions, we have recently found that the ability of glutamate to fuel cell metabolism selectively relies on the activity of two main transporters: the sodium-calcium exchanger (NCX) and the sodium-dependent excitatory amino-acid transporters (EAATs). In ischemic settings, when glutamate is administered at the onset of the reoxygenation phase, the coordinate activity of EAAT and NCX allows glutamate to improve cell viability by stimulating ATP production. So far, this phenomenon has been observed in both cardiac and neuronal models. In this review, we focus on the most recent findings exploring the unusual activity of glutamate as a potential survival factor in different settings.

The Akt pathway in oncology therapy and beyond (Review)

Protein kinase B (Akt), similar to many other protein kinases, is at the crossroads of cell death and survival, playing a pivotal role in multiple interconnected cell signaling mechanisms implicated in cell metabolism, growth and division, apoptosis suppression and angiogenesis. Akt protein kinase displays important metabolic effects, among which are glucose uptake in muscle and fat cells or the suppression of neuronal cell death. Disruptions in the Akt-regulated pathways are associated with cancer, diabetes, cardiovascular and neurological diseases. The regulation of the Akt signaling pathway renders Akt a valuable therapeutic target. The discovery process of Akt inhibitors using various strategies has led to the identification of inhibitors with great selectivity, low side-effects and toxicity. The usefulness of Akt emerges beyond cancer therapy and extends to other major diseases, such as diabetes, heart diseases, or neurodegeneration. This review presents key features of Akt structure and functions, and presents the progress of Akt inhibitors in regards to drug development, and their preclinical and clinical activity in regards to therapeutic efficacy and safety for patients.

Metabotropic Glutamate Receptor 5 in the Medial Prefrontal Cortex as a Molecular Determinant of Pain and Ensuing Depression

Pain and depression affect one another, and this bidirectional interaction implies the existence of common or interacting neural pathways. Among the neural circuits relevant to negative affection, the medial prefrontal cortex (mPFC) is known to be involved in both pain and depression. Persistent stress from physical pain and mental distress can evoke maladaptive changes in mPFC circuits to induce depression. Conversely, the unpleasant mood condition alters mPFC circuits to distort the appraisal of aversion and make individuals vulnerable to pain. In this article, recent findings regarding mPFC in chronic pain and/or depression are reviewed, with particular focus on the metabotropic glutamate receptor 5 (mGluR5). Although the involvement of mGluR5 within the mPFC in both pain and depressive disorders has been extensively studied, there are controversies regarding changes in the activity of the mPFC during chronic pain and depression, and the functional roles of mGluR5 on altered mPFC activity. We discuss alterations in the availability of mGluR5 in the mPFC in these disorders, its role in behavioral manifestations, and its possible influence on cellular subpopulations that mediate dysfunction in the mPFC. We also propose molecular mechanisms that may cause expressional changes in mGluR5 within the mPFC circuitry.

The mGluR5 positive allosteric modulator VU0409551 improves synaptic plasticity and memory of a mouse model of Huntington's disease

Huntington's Disease (HD) is an autosomal-dominant neurodegenerative disorder, characterized by involuntary body movements, cognitive impairment, and psychiatric disorder. The metabotropic glutamate receptor 5 (mGluR5) plays an important role in HD and we have recently demonstrated that mGluR5-positive allosteric modulators (PAMs) can ameliorate pathology and the phenotypic signs of a mouse model of HD. In this study, we investigated the molecular mechanisms involved in mGluR5 PAMs effect on memory. Our results demonstrate that subchronic treatment with the mGluR5 PAM VU0409551 was effective in reversing the memory deficits exhibited by BACHD mice, a mouse model for HD. Moreover, VU0409551 treatment stabilized mGluR5 at the cellular plasma membrane of BACHD mice, increasing the expression of several genes important for synaptic plasticity, including c-Fos, brain-derived neurotrophic factor, Arc/Arg3.1, syntaxin 1A, and post-synaptic density-95. In addition, VU0409551 treatment also increased dendritic spine density and maturation and augmented the number of pre-synaptic sites. In conclusion, our results demonstrate that VU0409551 triggered the activation of cell signaling pathways important for synaptic plasticity, enhancing the level of dendritic spine maturation and rescuing BACHD memory impairment. OPEN PRACTICES: Open Science: This manuscript was awarded with the Open Materials Badge. For more information see: https://cos.io/our-services/open-science-badges/.

Triptolide up-regulates metabotropic glutamate receptor 5 to inhibit microglia activation in the lipopolysaccharide-induced model of Parkinson's disease

Metabotropic glutamate receptor (mGlu)₅ regulates microglia activation, which contributes to inflammation. However, the role of mGlu₅ in neuroinflammation associated with Parkinson's disease (PD) remains unclear. Triptolide (T10) exerts potent immunosuppressive and anti-inflammatory effects and protects neurons by inhibiting microglia activation. In this study, we investigated the role of mGlu₅ in the anti-inflammatory effect of T10 in a lipopolysaccharide (LPS)-induced PD model. In cultured BV2 cells and primary microglia, blocking mGlu₅ activity or knocking down its expression abolished T10-inhibited release of proinflammatory cytokines induced by LPS. Moreover, T10 up-regulated mGlu₅ expression decreased by LPS through enhancing mRNA expression and protein stability. T10 also reversed the reduction in mGlu₅ membrane localization and modulated receptor-mediated mitogen-activated protein kinase activity induced by LPS. Pharmacological inhibition of signaling molecules increased nitric oxide level and inducible nitric oxide synthase (iNOS), tumor necrosis factor-α, and interleukin (IL)-1β and -6 transcript levels that were downregulated by treatment with T10. Consistent with these in vitro findings, blocking mGlu₅ attenuated the anti-inflammatory effects of T10 in an LPS-induced PD model and blocked the decreases in the number and morphology of ionized calcium binding adaptor molecule 1-positive microglia and LPS-induced iNOS protein expression caused by T10 treatment. Besides, mGlu₅ mediated the effect of T10 on microglia-induced astrocyte activation in vitro and in vivo. The findings provide evidence for a novel mechanism by which mGlu₅ regulates T10-inhibited microglia activation via modulating protein expression of the receptor and its intracellular signaling. The study might contribute to the biological effects of Chinese herbs as an approach for protecting against neurotoxicity in PD.

Noninvasive Relative Quantification of [11C]ABP688 PET Imaging in Mice Versus an Input Function Measured Over an Arteriovenous Shunt

Impairment of the metabotropic glutamate receptor 5 (mGluR5) has been implicated with various neurologic disorders. Although mGluR5 density can be quantified with the PET radiotracer [¹¹C]ABP688, the methods for reproducible quantification of [¹¹C]ABP688 PET imaging in mice have not been thoroughly investigated yet. Thus, this study aimed to assess and validate cerebellum as reference region for simplified reference tissue model (SRTM), investigate the feasibility of a noninvasive cardiac image-derived input function (IDIF) for relative quantification, to validate the use of a PET template instead of an MRI template for spatial normalization, and to determine the reproducibility and within-subject variability of [¹¹C]ABP688 PET imaging in mice. Blocking with the mGluR5 antagonist MPEP resulted in a reduction of [¹¹C]ABP688 binding of 41% in striatum (p < 0.0001), while no significant effect could be found in cerebellum (−4.8%, p > 0.99) indicating cerebellum as suitable reference region for mice. DVR-1 calculated using a noninvasive IDIF and an arteriovenous input function correlated significantly when considering the cerebellum as the reference region (striatum: DVR-1, r = 0.978, p < 0.0001). Additionally, strong correlations between binding potential calculated from SRTM (BP_ND) with DVR-1 based on IDIF (striatum: r = 0.980, p < 0.0001) and AV shunt (striatum: r = 0.987, p < 0.0001). BP_ND displayed higher discrimination power than V_T values in determining differences between wild-types and heterozygous Q175 mice, an animal model of Huntington's disease. Furthermore, we showed high agreement between PET- and MRI-based spatial normalization approaches (striatum: r = 0.989, p < 0.0001). Finally, both spatial normalization approaches did not reveal any significant bias between test-retest scans, with a relative difference below 5%. This study indicates that noninvasive quantification of [¹¹C]ABP688 PET imaging is reproducible and cerebellum can be used as reference region in mice.

Silent Allosteric Modulation of mGluR5 Maintains Glutamate Signaling while Rescuing Alzheimer's Mouse Phenotypes

Metabotropic glutamate receptor 5 (mGluR5) has been implicated in Alzheimer's disease (AD) pathology. We sought to understand whether mGluR5's role in AD requires glutamate signaling. We used a potent mGluR5 silent allosteric modulator (SAM, BMS-984923) to separate its well-known physiological role in glutamate signaling from a pathological role in mediating amyloid-β oligomer (Aβo) action. Binding of the SAM to mGluR5 does not change glutamate signaling but strongly reduces mGluR5 interaction with cellular prion protein (PrP^C) bound to Aβo. The SAM compound prevents Aβo-induced signal transduction in brain slices and in an AD transgenic mouse model, the APPswe/PS1ΔE9 strain. Critically, 4 weeks of SAM treatment rescues memory deficits and synaptic depletion in the APPswe/PS1ΔE9 transgenic mouse brain. Our data show that mGluR5's role in Aβo-dependent AD phenotypes is separate from its role in glutamate signaling and silent allosteric modulation of mGluR5 has promise as a disease-modifying AD intervention with a broad therapeutic window.

Ablation of TFR1 in Purkinje Cells Inhibits mGlu1 Trafficking and Impairs Motor Coordination, But Not Autistic-Like Behaviors

Group 1 metabotropic glutamate receptors (mGlu1/5s) are critical to synapse formation and participate in synaptic LTP and LTD in the brain. mGlu1/5 signaling alterations have been documented in cognitive impairment, neurodegenerative disorders, and psychiatric diseases, but underlying mechanisms for its modulation are not clear. Here, we report that transferrin receptor 1 (TFR1), a transmembrane protein of the clathrin complex, modulates the trafficking of mGlu1 in cerebellar Purkinje cells (PCs) from male mice. We show that conditional knock-out of TFR1 in PCs does not affect the cytoarchitecture of PCs, but reduces mGlu1 expression at synapses. This regulation by TFR1 acts in concert with that by Rab8 and Rab11, which modulate the internalization and recycling of mGlu1, respectively. TFR1 can bind to Rab proteins and facilitate their expression at synapses. PC ablation of TFR1 inhibits parallel fiber-PC LTD, whereas parallel fiber-LTP and PC intrinsic excitability are not affected. Finally, we demonstrate that PC ablation of TFR1 impairs motor coordination, but does not affect social behaviors in mice. Together, these findings underscore the importance of TFR1 in regulating mGlu1 trafficking and suggest that mGlu1- and mGlu1-dependent parallel fiber-LTD are associated with regulation of motor coordination, but not autistic behaviors.SIGNIFICANCE STATEMENT Group 1 metabotropic glutamate receptor (mGlu1/5) signaling alterations have been documented in cognitive impairment, neurodegenerative disorders, and psychiatric diseases. Recent work suggests that altered mGlu1 signaling in Purkinje cells (PCs) may be involved in not only motor learning, but also autistic-like behaviors. We find that conditional knock-out of transferrin receptor 1 (TFR1) in PCs reduces synaptic mGlu1 by tethering Rab8 and Rab11 in the cytosol. PC ablation of TFR1 inhibits parallel fiber-PC LTD, whereas parallel fiber-PC LTP and PC intrinsic excitability are intact. Motor coordination is impaired, but social behaviors are normal in TFR1^flox/flox;pCP2-cre mice. Our data reveal a new regulator for trafficking and synaptic expression of mGlu1 and suggest that mGlu1-dependent LTD is associated with motor coordination, but not autistic-like behaviors.

A systems medicine approach for finding target proteins affecting treatment outcomes in patients with non-Hodgkin lymphoma

Autoantibody profiling with a systems medicine approach can help identify critical dysregulated signaling pathways (SPs) in cancers. In this way, immunoglobulins G (IgG) purified from the serum samples of 92 healthy controls, 10 pre-treated (PR) non-Hodgkin lymphoma (NHL) patients, and 20 NHL patients who underwent chemotherapy (PS) were screened with a phage-displayed random peptide library. Protein-protein interaction networks of the PR and PS groups were analyzed and visualized by Gephi. The results indicated AXIN2, SENP2, TOP2A, FZD6, NLK, HDAC2, HDAC1, and EHMT2, in addition to CAMK2A, PLCG1, PLCG2, GRM5, GRIN2B, GRIN2D, CACNA2D3, and SPTAN1 as hubs in 11 and 7 modules of PR and PS networks, respectively. PR- and PS-specific hubs were evaluated in the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Reactome databases. The PR-specific hubs were involved in Wnt SP, signaling by Notch1 in cancer, telomere maintenance, and transcriptional misregulation. In contrast, glutamate receptor SP, Fc receptor-related pathways, growth factors-related SPs, and Wnt SP were statistically significant enriched pathways, based on the pathway analysis of PS hubs. The results revealed that the most PR-specific proteins were associated with events involved in tumor development, while chemotherapy in the PS group was associated with side effects of drugs and/or cancer recurrence. As the findings demonstrated, PR- and PS-specific proteins in this study can be promising therapeutic targets in future studies.

Amyloid beta: structure, biology and structure-based therapeutic development

Amyloid beta peptide (Aβ) is produced through the proteolytic processing of a transmembrane protein, amyloid precursor protein (APP), by β- and γ-secretases. Aβ accumulation in the brain is proposed to be an early toxic event in the pathogenesis of Alzheimer's disease, which is the most common form of dementia associated with plaques and tangles in the brain. Currently, it is unclear what the physiological and pathological forms of Aβ are and by what mechanism Aβ causes dementia. Moreover, there are no efficient drugs to stop or reverse the progression of Alzheimer's disease. In this paper, we review the structures, biological functions, and neurotoxicity role of Aβ. We also discuss the potential receptors that interact with Aβ and mediate Aβ intake, clearance, and metabolism. Additionally, we summarize the therapeutic developments and recent advances of different strategies for treating Alzheimer's disease. Finally, we will report on the progress in searching for novel, potentially effective agents as well as selected promising strategies for the treatment of Alzheimer's disease. These prospects include agents acting on Aβ, its receptors and tau protein, such as small molecules, vaccines and antibodies against Aβ; inhibitors or modulators of β- and γ-secretase; Aβ-degrading proteases; tau protein inhibitors and vaccines; amyloid dyes and microRNAs.

Metabotropic glutamate receptor 5 deficiency inhibits neutrophil infiltration after traumatic brain injury in mice

Both brain native inflammatory cells and infiltrated peripheral white blood cells (WBCs) are primary participants in the brain inflammatory damage post-TBI. Metabotropic glutamate receptor 5 (mGluR5) has been reported to regulate microglias and astrocytes to affect inflammation after TBI, but its effect on modulating infiltrated peripheral WBCs remains unclear. In a mouse moderate TBI model, we found that mGluR5 knockout (KO) significantly reduced neutrophil infiltration and inflammatory cytokine expression in the brain at 24 hours post TBI, which was accompanied by improved neurological dysfunction. Further investigation indicated that mGluR5 KO reduced the permeability of blood-brain barrier (BBB), the entrance for neutrophils to enter brain, and markedly decreased the mRNA levels of neutrophil-associated chemokines in brain tissue, including CXCL1, CXCL2, CCL2, CCL4 and CCL5. Using brain microvascular endothelial cells (BMECs), neutrophils and a BBB model in vitro, we confirmed the inhibitory effect of mGluR5 deficiency on neutrophil infiltration and demonstrated that blockade of protein kinase C (PKC) signaling was involved in it. These results provide insight into the role of mGluR5 in the regulation of inflammation in the acute phase of TBI, which may provide novel clues for TBI therapy.

The prion protein regulates glutamate-mediated Ca2+ entry and mitochondrial Ca2+ accumulation in neurons

The cellular prion protein (PrP^C) whose conformational misfolding leads to the production of deadly prions, has a still-unclarified cellular function despite decades of intensive research. Following our recent finding that PrP^C limits Ca²⁺ entry via store-operated Ca²⁺ channels in neurons, we investigated whether the protein could also control the activity of ionotropic glutamate receptors (iGluRs). To this end, we compared local Ca²⁺ movements in primary cerebellar granule neurons and cortical neurons transduced with genetically encoded Ca²⁺ probes and expressing, or not expressing, PrP^C Our investigation demonstrated that PrP^C downregulates Ca²⁺ entry through each specific agonist-stimulated iGluR and after stimulation by glutamate. We found that, although PrP-knockout (KO) mitochondria were displaced from the plasma membrane, glutamate addition resulted in a higher mitochondrial Ca²⁺ uptake in PrP-KO neurons than in their PrP^C-expressing counterpart. This was because the increased Ca²⁺ entry through iGluRs in PrP-KO neurons led to a parallel increase in Ca²⁺-induced Ca²⁺ release via ryanodine receptor channels. These data thus suggest that PrP^C takes part in the cell apparatus controlling Ca²⁺ homeostasis, and that PrP^C is involved in protecting neurons from toxic Ca²⁺ overloads.

MicroRNA in glutamate receptor-dependent neurological diseases

Glutamate-mediated excitotoxicity is the major neuropathological process contributing to numerous neurological diseases. Recently, emerging evidence indicates that microRNAs (miRNAs) play essential roles in the pathophysiology of a wide range of neurological diseases. Notably, there have been significant developments in understanding the biogenesis of miRNAs, their regulatory mechanisms, and their potential as effective biomarkers and therapies. In the present review, we summarize the recent literature that highlights the versatile roles played by miRNAs in glutamate receptor (GluR)-dependent neurological diseases. Based on the reported studies to date, modulation of miRNAs could emerge as a promising therapeutic target for a variety of neurological diseases that were discussed in this review.

Cytoskeleton as a Target of Quinolinic Acid Neurotoxicity: Insight from Animal Models

Cytoskeletal proteins are increasingly recognized as having important roles as a target of the action of different neurotoxins. In the last years, several works of our group have shown that quinolinic acid (QUIN) was able to disrupt the homeostasis of the cytoskeleton of neural cells and this was associated with cell dysfunction and neurodegeneration. QUIN is an excitotoxic metabolite of tryptophan metabolism and its accumulation is associated with several neurodegenerative diseases. In the present review, we provide a comprehensive view of the actions of QUIN upstream of glutamate receptors, eliciting kinase/phosphatase signaling cascades that disrupt the homeostasis of the phosphorylation system associated with intermediate filament proteins of astrocytes and neurons. We emphasize the critical role of calcium in these actions and the evidence that misregulated cytoskeleton takes part of the cell response to the injury resulting in neurodegeneration in different brain regions, disrupted cell signaling in acute tissue slices, and disorganized cytoskeleton with altered cell morphology in primary cultures. We also discuss the interplay among misregulated cytoskeleton, oxidative stress, and cell-cell contact through gap junctions mediating the quinolinic acid injury in rat brain. The increasing amount of cross talks identified between cytoskeletal proteins and cellular signaling cascades reinforces the exciting possibility that cytoskeleton could be a new target in the neurotoxicity of QUIN and further studies will be necessary to develop strategies to protect the cytoskeleton and counteracts the cytotoxicity of this metabolite.

Activated mGluR5 protects BV2 cells against OGD/R induced cytotoxicity by modulating BDNF-TrkB pathway

Activated Metabotropic glutamate receptors 5(mGluR5) exhibits protective effects against ischemic brain damage, but the underlying mechanisms are not clearly known. Brain-derived neurotrophic factor (BDNF), as a valuable member of neurotrophic factor family, exerts its protection by combining with its high-affinity receptor tyrosine protein kinase B (TrkB). To investigate the role of activated mGluR5 against oxygen-glucose deprivation (OGD)/reoxygenation (R)-mediated cytotoxicity, the cell viability, apoptosis, the release of inflammatory cytokines and accumulation of reactive oxygen species (ROS) were evaluated in BV2 cells (Microglia cell line) with or without OGD/R exposure. Our data show that CHPG (the selective mGluR5 agonist) pretreatment, as an mGluR5 agonist, protected BV2 cells against OGD/R-induced cytotoxicity, apoptosis, the release of inflammatory cytokines, and the accumulation of ROS. However, these effects were significantly reversed by the mGluR5 antagonist MPEP pretreatment. Our data also show that the expressions of BDNF and TrkB were significantly decreased in BV2 cells with OGD/R exposure. CHPG pretreatment significantly enhanced the expressions of BDNF and TrkB in BV2 cells with OGD/R exposure. However, the increased expressions were significantly abrogated by MPEP pretreatment. In addition, inhibition of BDNF/TrKB pathway by K252a also attenuated the protective effects of activated mGluR5 against OGD/R-induced cytotoxicity, apoptosis and the release of inflammatory cytokines. Morever, pretreatment with exogenous BDNF protected BV2 cells against OGD/R induced apoptosis and release of inflammatory cytokines. These data suggested that BDNF/TrKB pathway may be involved in regulating activated mGluR5' protective effects against OGD/R induced cytotoxicity in BV2 cells.