Glutamate receptor expression in multiple sclerosis lesions

How does Nogo-A signalling influence mitochondrial function during multiple sclerosis pathogenesis?

Multiple sclerosis (MS) is a severe neurological disorder that involves inflammation in the brain, spinal cord and optic nerve with key disabling neuropathological outcomes being axonal damage and demyelination. When degeneration of the axo-glial union occurs, a consequence of inflammatory damage to central nervous system (CNS) myelin, dystrophy and death can lead to large membranous structures from dead oligodendrocytes and degenerative myelin deposited in the extracellular milieu. For the first time, this review covers mitochondrial mechanisms that may be operative during MS-related neurodegenerative changes directly activated during accumulating extracellular deposits of myelin associated inhibitory factors (MAIFs), that include the potent inhibitor of neurite outgrowth, Nogo-A. Axonal damage may occur when Nogo-A binds to and signals through its cognate receptor, NgR1, a multimeric complex, to initially stall axonal transport and limit the delivery of important growth-dependent cargo and subcellular organelles such as mitochondria for metabolic efficiency at sites of axo-glial disintegration as a consequence of inflammation. Metabolic efficiency in axons fails during active demyelination and progressive neurodegeneration, preceded by stalled transport of functional mitochondria to fuel axo-glial integrity.

Small-molecule targeting AMPA-mediated excitotoxicity has therapeutic effects in mouse models for multiple sclerosis

While most research and treatments for multiple sclerosis (MS) focus on autoimmune reactions causing demyelination, it is possible that neurodegeneration precedes the autoimmune response. Hence, glutamate receptor antagonists preventing excitotoxicity showed promise in MS animal models, though blocking glutamate signaling prevents critical neuronal functions. This study reports the discovery of a small molecule that prevents AMPA-mediated excitotoxicity by targeting an allosteric binding site. A machine learning approach was used to screen for small molecules targeting the AMPA receptor GluA2 subunit. The lead candidate has potent effects in restoring neurological function and myelination while reducing the immune response in experimental autoimmune encephalitis and cuprizone MS mouse models without affecting basal neurotransmission or learning and memory. These findings facilitate development of a treatment for MS with a different mechanism of action than current immune modulatory drugs and avoids important off-target effects of glutamate receptor antagonists. This class of MS therapeutics could be useful as an alternative or complementary treatment to existing therapies.

Mechanisms and Functions of Activity-Regulated Cytoskeleton-Associated Protein in Synaptic Plasticity

Activity-regulated cytoskeleton-associated protein (Arc) is one of the most important regulators of cognitive functions in the brain regions. As a hub protein, Arc plays different roles in modulating synaptic plasticity. Arc supports the maintenance of long-term potentiation (LTP) by regulating actin cytoskeletal dynamics, while it guides the endocytosis of AMPAR in long-term depression (LTD). Moreover, Arc can self-assemble into capsids, leading to a new way of communicating among neurons. The transcription and translation of the immediate early gene Arc are rigorous procedures guided by numerous factors, and RNA polymerase II (Pol II) is considered to regulate the precise timing dynamics of gene expression. Since astrocytes can secrete brain-derived neurotrophic factor (BDNF) and L-lactate, their unique roles in Arc expression are emphasized. Here, we review the entire process of Arc expression and summarize the factors that can affect Arc expression and function, including noncoding RNAs, transcription factors, and posttranscriptional regulations. We also attempt to review the functional states and mechanisms of Arc in modulating synaptic plasticity. Furthermore, we discuss the recent progress in understanding the roles of Arc in the occurrence of major neurological disorders and provide new thoughts for future research on Arc.

Metabolomics Profile of the Secretome of Space-Flown Oligodendrocytes

Intracranial hypertension (ICP) and visual impairment intracranial pressure (VIIP) are some of the sequels of long-term space missions. Here we sought to determine how space microgravity (µG) impacts the metabolomics profile of oligodendrocyte progenitors (OLPs), the myelin-forming cells in the central nervous system. We report increased glutamate and energy metabolism while the OLPs were in space for 26 days. We also show that after space flight, OLPs (SPC OLPs) display significantly increased mitochondrial respiration and glycolysis. These data are in agreement with our previous work using simulated microgravity. In addition, our global metabolomics approach allowed for the discovery of endogenous metabolites secreted by OLPs while in space that are significantly modulated by microgravity. Our results provide, for the first time, relevant information about the energetic state of OLPs while in space and after space flight. The functional and molecular relevance of these specific pathways are promising targets for therapeutic intervention for humans in long-term space missions to the moon, Mars and beyond.

Novel therapeutic for multiple sclerosis protects white matter function in EAE mouse model

Multiple sclerosis (MS) is a chronic demyelinating disease with prominent axon dysfunction. Our previous studies in an MS mouse model, experimental autoimmune encephalomyelitis (EAE), demonstrated that major histocompatibility complex Class II constructs can reverse clinical signs of EAE. These constructs block binding and downstream signaling of macrophage migration inhibitory factors (MIF-1/2) through CD74, thereby inhibiting phosphorylation of extracellular signal-regulated kinase (ERK) activation and tissue inflammation and promoting remyelination. To directly assess the effects of a novel third generation construct, DRhQ, on axon integrity in EAE, we compared axon conduction properties using electrophysiology on corpus callosum slices and optic nerves. By using two distinct white matter (WM) tracts, we aimed to assess the impact of the EAE and the benefit of DRhQ on myelinated and unmyelinated axons as well as to test the clinical value of DRhQ on demyelinating lesions in CC and optic myelitis. Our study found that EAE altered axon excitability, delayed axon conduction and slowed spatiotemporal summation correlated with diffuse astrocyte and microglia activation. Because MS predisposes patients to stroke, we also investigated and showed that vulnerability to WM ischemia is increased in the EAE MS mouse model. Treatment with DRhQ after the onset of EAE drastically inhibited microglial and astrocyte activation, improved functional integrity of the myelinated axons and enhanced recovery after ischemia. These results demonstrate that DRhQ administered after the onset of EAE promotes WM integrity and function, and reduces subsequent vulnerability to ischemic injury, suggesting important therapeutic potential for treatment of progressive MS.

Technical approaches and challenges to study AMPA receptors in oligodendrocyte lineage cells: Past, present, and future

Receptors for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) are ligand-gated ionotropic receptors for glutamate that is a major excitatory neurotransmitter in the central nervous system. AMPARs are located at postsynaptic sites of neuronal synapses where they mediate fast synaptic signaling and synaptic plasticity. Remarkably, AMPARs are also expressed by glial cells. Their expression by the oligodendrocyte (OL) lineage cells is of special interest because AMPARs mediate fast synaptic communication between neurons and oligodendrocyte progenitor cells (OPCs), modulate proliferation and differentiation of OPCs, and may also be involved in regulation of myelination. On the other hand, during pathological conditions, AMPARs may mediate damage of the OL lineage cells. In the present review, we focus on the technical approaches that have been used to study AMPARs in the OL lineage cells, and discuss future perspectives of AMPAR research in these glial cells.

Synapse Dysfunctions in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory disease of the central nervous system (CNS) affecting nearly three million humans worldwide. In MS, cells of an auto-reactive immune system invade the brain and cause neuroinflammation. Neuroinflammation triggers a complex, multi-faceted harmful process not only in the white matter but also in the grey matter of the brain. In the grey matter, neuroinflammation causes synapse dysfunctions. Synapse dysfunctions in MS occur early and independent from white matter demyelination and are likely correlates of cognitive and mental symptoms in MS. Disturbed synapse/glia interactions and elevated neuroinflammatory signals play a central role. Glutamatergic excitotoxic synapse damage emerges as a major mechanism. We review synapse/glia communication under normal conditions and summarize how this communication becomes malfunctional during neuroinflammation in MS. We discuss mechanisms of how disturbed glia/synapse communication can lead to synapse dysfunctions, signaling dysbalance, and neurodegeneration in MS.

Targeting N-type calcium channels in young-onset of some neurological diseases

Calcium (Ca ²⁺) is an important second messenger in charge of many critical processes in the central nervous system (CNS), including membrane excitability, neurotransmission, learning, memory, cell proliferation, and apoptosis. In this way, the voltage-gated calcium channels (VGCCs) act as a key supply for Ca²⁺ entry into the cytoplasm and organelles. Importantly, the dysregulation of these channels has been reported in many neurological diseases of young-onset, with associated genetic factors, such as migraine, multiple sclerosis, and Huntington's disease. Notably, the literature has pointed to the role of N-type Ca²⁺ channels (NTCCs) in controlling a variety of processes, including pain, inflammation, and excitotoxicity. Moreover, several Ca²⁺ channel blockers that are used for therapeutic purposes have been shown to act on the N-type channels. Therefore, this review provides an overview of the NTCCs in neurological disorders focusing mainly on Huntington's disease, multiple sclerosis, and migraine. It will discuss possible strategies to generate novel therapeutic strategies.

Tamalin Function Is Required for the Survival of Neurons and Oligodendrocytes in the CNS

Tamalin is a post-synaptic scaffolding protein that interacts with group 1 metabotropic glutamate receptors (mGluRs) and several other proteins involved in protein trafficking and cytoskeletal events, including neuronal growth and actin reorganization. It plays an important role in synaptic plasticity in vitro by controlling the ligand-dependent trafficking of group 1 mGluRs. Abnormal regulation of mGluRs in the central nervous system (CNS) is associated with glutamate-mediated neurodegenerative disorders. However, the pathological consequences of tamalin deficiency in the CNS are unclear. In this study, tamalin knockout (KO) zebrafish and mice exhibited neurodegeneration along with oligodendrocyte degeneration in the post-embryonic CNS to adulthood without any developmental defects, thus suggesting the function of tamalin is more important in the postnatal stage to adulthood than that in CNS development. Interestingly, hypomyelination was independent of axonal defects in the CNS of tamalin knockout zebrafish and mice. In addition, the loss of Arf6, a downstream signal of tamalin scaffolding protein, synergistically induced neurodegeneration in tamalin KO zebrafish even in the developing CNS. Furthermore, tamalin KO zebrafish displayed increased mGluR5 expression. Taken together, tamalin played an important role in neuronal and oligodendrocyte survival and myelination through the regulation of mGluR5 in the CNS.

Targeting the Type 5 Metabotropic Glutamate Receptor: A Potential Therapeutic Strategy for Neurodegenerative Diseases?

The type 5 metabotropic glutamate receptor, mGlu₅, has been proposed as a potential therapeutic target for the treatment of several neurodegenerative diseases. In preclinical neurodegenerative disease models, novel allosteric modulators have been shown to improve cognitive performance and reduce disease-related pathology. A common pathological hallmark of neurodegenerative diseases is a chronic neuroinflammatory response, involving glial cells such as astrocytes and microglia. Since mGlu₅ is expressed in astrocytes, targeting this receptor could provide a potential mechanism by which neuroinflammatory processes in neurodegenerative disease may be modulated. This review will discuss current evidence that highlights the potential of mGlu₅ allosteric modulators to treat neurodegenerative diseases, including Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Furthermore, this review will explore the role of mGlu₅ in neuroinflammatory responses, and the potential for this G protein-coupled receptor to modulate neuroinflammation.

Abuse and dependence potential of sphingosine-1-phosphate (S1P) receptor modulators used in the treatment of multiple sclerosis: a review of literature and public data

Abuse and misuse of prescription drugs remains an ongoing concern in the USA and worldwide; thus, all centrally active new drugs must be assessed for abuse and dependence potential. Sphingosine-1-phosphate (S1P) receptor modulators are used primarily in the treatment of multiple sclerosis. Among the new S1P receptor modulators, siponimod, ozanimod, and ponesimod have recently been approved in the USA, European Union (EU), and other countries. This review of literature and other public data has been undertaken to assess the potential for abuse of S1P receptor modulators, including ozanimod, siponimod, ponesimod, and fingolimod, as well as several similar compounds in development. The S1P receptor modulators have not shown chemical or pharmacological similarity to known drugs of abuse; have not shown abuse or dependence potential in animal models for subjective effects, reinforcement, or physical dependence; and do not have adverse event profiles demonstrating effects of interest to individuals who abuse drugs (such as sedative, stimulant, mood-elevating, or hallucinogenic effects). In addition, no reports of actual abuse, misuse, or dependence were identified in the scientific literature for fingolimod, which has been on the market since 2010 (USA) and 2011 (EU). Overall, the data suggest that S1P receptor modulators are not associated with significant potential for abuse or dependence, consistent with their unscheduled status in the USA and internationally.

Pathophysiological Ionotropic Glutamate Signalling in Neuroinflammatory Disease as a Therapeutic Target

Glutamate signalling is an essential aspect of neuronal communication involving many different glutamate receptors, and underlies the processes of memory, learning and synaptic plasticity. Despite neuroinflammatory diseases covering a range of maladies with very different biological causes and pathophysiologies, a central role for dysfunctional glutamate signalling is becoming apparent. This is not just restricted to the well-described role of glutamate in mediating neurodegeneration, but also includes a myriad of other influences that glutamate can exert on the vasculature, as well as immune cell and glial regulation, reflecting the ability of neurons to communicate with these compartments in order to couple their activity with neuronal requirements. Here, we discuss the role of pathophysiological glutamate signalling in neuroinflammatory disease, using both multiple sclerosis and Alzheimer's disease as examples, and how current steps are being made to harness our growing understanding of these processes in the development of neuroprotective strategies. This review focuses in particular on N-methyl-D-aspartate (NMDA) and 2-amino-3-(3-hydroxy-5-methylisooxazol-4-yl) propionate (AMPA) type ionotropic glutamate receptors, although metabotropic, G-protein-coupled glutamate receptors may also contribute to neuroinflammatory processes. Given the indispensable roles of glutamate-gated ion channels in synaptic communication, means of pharmacologically distinguishing between physiological and pathophysiological actions of glutamate will be discussed that allow deleterious signalling to be inhibited whilst minimising the disturbance of essential neuronal function.

RNA aptamers for AMPA receptors

RNA aptamers are single-stranded RNA molecules, and they are selected against a target of interest so that they can bind to and modulate the activity of the target, such as inhibiting the target activity, with high potency and selectivity. Antagonists, such as RNA aptamers, acting on AMPA receptors, a major subtype of ionotropic glutamate receptors, are potential drug candidates for treatment of a number of CNS diseases that involve excessive receptor activation and/or elevated receptor expression. Here we review the approach to discover RNA aptamers targeting AMPA receptors from a random sequence library (∼10¹⁴ sequences) through a process called systematic evolution of ligands by exponential enrichment (SELEX). As compared with small-molecule compounds, RNA aptamers are a new class of regulatory agents with interesting and desirable pharmacological properties. Some AMPA receptor aptamers we have developed are presented in this review. The promises and challenges of translating RNA aptamers into potential drugs and treatment options are also discussed. This article is part of the special Issue on 'Glutamate Receptors - AMPA receptors'.

Temporal and spatial evolution of various functional neurons during demyelination induced by cuprizone

Multiple sclerosis (MS) is an inflammatory, demyelinating, and neurodegenerative disease of the central nervous system (CNS). Here we report the temporal and spatial evolution of various functional neurons during demyelination in a cuprizone (CPZ)-induced mouse model. CPZ did not significantly induce the damage of axons and neurons after 2 wk of feeding. However, after 4-6 wk of CPZ feeding, axons and neurons were markedly reduced in the cortex, posterior thalamic nuclear group, and hippocampus. Simultaneously, the expression of TPH⁺ tryptophan neurons and VGLUT1⁺ glutamate neurons was obviously decreased, and the expression of TH⁺ dopaminergic neurons was slightly decreased in the tail part of the substantia nigra striatum, whereas the number of ChAT⁺ cholinergic neurons was not significantly different in the brain. In the second week of feeding, CPZ caused a higher level of glutamate secretion and upregulated the expression of EAAT2 on astrocytes, which should contribute to rapid and sufficient glutamate uptake and removal. This finding reveals that astrocyte-driven glutamate reuptake protected the CNS from excitotoxicity by rapid reuptake of glutamate in 4-6 wk of CPZ feeding. At this stage, although NG2⁺ oligodendroglia progenitor cells (OPCs) were enhanced in the demyelination foci, the myelin sheath was still absent. In conclusion, we comprehensively observed the temporal and spatial evolution of various functional neurons. Our results will assist with understanding how demyelination affects neurons during CPZ-induced demyelination and provide novel information for neuroprotection in myelin regeneration and demyelinating diseases.NEW & NOTEWORTHY Our results further indicate temporal and spatial evolution of various functional neurons during the demyelination in a cuprizone (CPZ)-induced mouse model, which mainly occur 4-6 wk after CPZ feeding. At the same time, the axonal compartment is damaged and, consequently, neuronal death occurs, while glutamate neurons are lost obviously. The astrocyte-mediated glutamate reuptake could protect the neurons from the excitatory effects of glutamate.

A Multi-Ligand Imaging Study Exploring GABAergic Receptor Expression and Inflammation in Multiple Sclerosis

PURPOSE

The γ-aminobutyric acid (GABA) is the main inhibitory neurotransmitter and essential for normal brain function. The GABAergic system has been shown to have immunomodulatory effects and respond adaptively to excitatory toxicity. The association of the GABAergic system and inflammation in patients with multiple sclerosis (MS) remains unknown. In this pilot study, the in vivo relationship between GABA_A binding and the innate immune response is explored using positron emission tomography (PET) with [¹¹C] flumazenil (FMZ) and [¹¹C]-PK11195 PET (PK-PET), a measure of activated microglia/macrophages.

PROCEDURES

Sixteen MS patients had dynamic FMZ-PET and PK-PET imaging. Ten age-matched healthy controls (HC) had a single FMZ-PET. GABA_A receptor binding was calculated using Logan reference model with the pons as reference. Distribution of volume ratio (VTr) for PK-PET was calculated using image-derived input function. A hierarchical linear model was fitted to assess the linear association between PK-PET and FMZ-PET among six cortical regions of interest.

RESULTS

GABA_A receptor binding was higher throughout the cortex in MS patients (5.72 ± 0.91) as compared with HC (4.70 ± 0.41) (p = 0.002). A significant correlation was found between FMZ binding and PK-PET within the cortex (r = 0.61, p < 0.001) and among the occipital (r = 0.61, p = 0.012), parietal (r = 0.49, p = 0.041), and cingulate (r = 0.32, p = 0.006) regions.

CONCLUSIONS

A higher GABA_A receptor density in MS subjects compared with HC was observed and correlated with innate immune activity. Our observations demonstrate that immune-driven GABAergic abnormalities may be present in MS.

CHPG enhances BDNF and myelination in cuprizone-treated mice through astrocytic metabotropic glutamate receptor 5

It is well recognized that astrocytes can produce factors known to affect the myelination process. One such factor, brain-derived neurotrophic factor (BDNF), can enhance the differentiation of oligodendrocyte lineage cells following a demyelinating lesion. Our previous work indicated that enhancing astrocyte-derived BDNF via injection of a general agonist of Group I/II metabotropic glutamate receptors (mGluRs) into the lesion increased myelin proteins in the cuprizone model of demyelination after 4 hr. To determine if this observation has potential therapeutic significance, we now use a more specific mGluR agonist, 2-chloro-5-hydroxyphenylglycine (CHPG), which binds to mGluR5, to examine effects on myelination through the clinically relevant approach of a peripheral injection. In initial studies, intraperitoneal injection of CHPG resulted in an increase in myelin proteins within the lesioned corpus callosum. These effects were blocked when either BDNF or the CHPG receptor, mGluR5, was deleted from glial fibrillary acidic protein (GFAP)+ astrocytes or when the BDNF receptor, tropomyosin receptor kinase B (TrkB), was deleted from proteolipid protein (PLP)+ oligodendrocytes. Moreover, injection of CHPG over 2 weeks not only elevated BDNF and myelin proteins, but also enhanced myelination and reversed behavioral deficits. Interestingly, effects on myelin and myelin proteins were not seen in the control animals, indicating that a lesion is critical in eliciting effects. Taken together, the data suggest that the mGluR agonist CHPG may be a potential therapeutic strategy for treating demyelinating diseases and that it works by enhancing the release of BDNF from astrocytes.

Ubiquitination and the proteasome rather than caspase-3-mediated C-terminal cleavage are involved in the EAAT2 degradation by staurosporine-induced cellular stress

Diminished glutamate (Glu) uptake via the excitatory amino acid transporter EAAT2, which normally accounts for ~90% of total forebrain EAAT activity, may contribute to neurodegeneration via Glu-mediated excitotoxicity. C-terminal cleavage by caspase-3 (C3) was reported to mediate EAAT2 inactivation and down-regulation in the context of neurodegeneration. For a detailed analysis of C3-dependent EAAT2 degradation, we employed A172 glioblastoma as well as hippocampal HT22 cells and murine astrocytes over-expressing VSV-G-tagged EAAT2 constructs. C3 activation was induced by staurosporine (STR). In HT22 cells, STR-induced C3 activation-induced rapid EAAT2 protein degradation. The mutation of asparagine 504 to aspartate (D504N), which should inactivate the putative C3 cleavage site, increased EAAT2 activity in A172 cells. In contrast, the D504N mutation did not protect EAAT2 protein against STR-induced degradation in HT22 cells, whereas inhibition of caspases, ubiquitination and the proteasome did. Similar results were obtained in astrocytes. Phylogenetic analysis showed that C-terminal ubiquitin acceptor sites-but not the putative C3 cleavage site-exhibit a high degree of conservation. Moreover, C-terminal truncation mimicking C3 cleavage increased rather than decreased EAAT2 activity and stability as well as protected EAAT2 against STR-induced ubiquitination-dependent degradation. We conclude that cellular stress associated with endogenous C3 activation degrades EAAT2 via a pathway involving ubiquitination and the proteasome but not direct C3-mediated cleavage. In addition, C3 cleavage of EAAT2, described to occur in other models, is unlikely to inactivate EAAT2. However, mutation of the highly conserved D504 within the putative C3 cleavage site increases EAAT2 activity via an unknown mechanism.

Alterations in white matter microstructure in individuals at persistent risk for psychosis

Abnormalities in brain white matter (WM) are reported in youth at-risk for psychosis. Yet, the neurodevelopmental time course of these abnormalities remains unclear. Thus, longitudinal diffusion-weighted imaging (DWI) was used to investigate WM abnormalities in youth at-risk for psychosis. A subset of individuals from the Philadelphia Neurodevelopmental Cohort (PNC) completed two DWI scans approximately 20 months apart. Youths were identified through structured interview as having subthreshold persistent psychosis risk symptoms (n = 46), and were compared to healthy typically developing participants (TD; n = 98). Analyses were conducted at voxelwise and regional levels. Nonlinear developmental patterns were examined using penalized splines within a generalized additive model. Compared to TD, youth with persistent psychosis risk symptoms had lower whole-brain WM fractional anisotropy (FA) and higher radial diffusivity (RD). Voxelwise analyses revealed clusters of significant WM abnormalities within the temporal and parietal lobes. Lower FA within the cingulum bundle of hippocampus and cerebrospinal tracts were the most robust deficits in individuals with persistent psychosis symptoms. These findings were consistent over two visits. Thus, it appears that WM abnormalities are present early in youth with persistent psychosis risk symptoms, however, there is little evidence to suggest that these features emerge in late adolescence or early adulthood. Future studies should seek to characterize WM abnormalities in younger individuals and follow individuals as subthreshold psychotic symptoms emerge.

Ionotropic Glutamate Receptors in Epilepsy: A Review Focusing on AMPA and NMDA Receptors

It is widely accepted that glutamate-mediated neuronal hyperexcitation plays a causative role in eliciting seizures. Among glutamate receptors, the roles of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors in physiological and pathological conditions represent major clinical research targets. It is well known that agonists of NMDA or AMPA receptors can elicit seizures in animal or human subjects, while antagonists have been shown to inhibit seizures in animal models, suggesting a potential role for NMDA and AMPA receptor antagonists in anti-seizure drug development. Several such drugs have been evaluated in clinical studies; however, the majority, mainly NMDA-receptor antagonists, failed to demonstrate adequate efficacy and safety for therapeutic use, and only an AMPA-receptor antagonist, perampanel, has been approved for the treatment of some forms of epilepsy. These results suggest that a misunderstanding of the role of each glutamate receptor in the ictogenic process may underlie the failure of these drugs to demonstrate clinical efficacy and safety. Accumulating knowledge of both NMDA and AMPA receptors, including pathological gene mutations, roles in autoimmune epilepsy, and evidence from drug-discovery research and pharmacological studies, may provide valuable information enabling the roles of both receptors in ictogenesis to be reconsidered. This review aimed to integrate information from several studies in order to further elucidate the specific roles of NMDA and AMPA receptors in epilepsy.

Sodium-Calcium Exchangers of the SLC8 Family in Oligodendrocytes: Functional Properties in Health and Disease

The solute carrier 8 (SLC8) family of sodium-calcium exchangers (NCXs) functions as an essential regulatory system that couples opposite fluxes of sodium and calcium ions across plasmalemmal membranes. NCXs, thereby, play key roles in maintaining an ion homeostasis that preserves cellular integrity. Hence, alterations in NCX expression and regulation have been found to lead to ionic imbalances that are often associated with intracellular calcium overload and cell death. On the other hand, intracellular calcium has been identified as a key driver for a multitude of downstream signaling events that are crucial for proper functioning of biological systems, thus highlighting the need for a tightly controlled balance. In the CNS, NCXs have been primarily characterized in the context of synaptic transmission and ischemic brain damage. However, a much broader picture is emerging. NCXs are expressed by virtually all cells of the CNS including oligodendrocytes (OLGs), the cells that generate the myelin sheath. With a growing appreciation of dynamic calcium signals in OLGs, NCXs are becoming increasingly recognized for their crucial roles in shaping OLG function under both physiological and pathophysiological conditions. In order to provide a current update, this review focuses on the importance of NCXs in cells of the OLG lineage. More specifically, it provides a brief introduction into plasmalemmal NCXs and their modes of activity, and it discusses the roles of OLG expressed NCXs in regulating CNS myelination and in contributing to CNS pathologies associated with detrimental effects on OLG lineage cells.

Reduction of AMPA receptor activity on mature oligodendrocytes attenuates loss of myelinated axons in autoimmune neuroinflammation

Glutamate dysregulation occurs in multiple sclerosis (MS), but whether excitotoxic mechanisms in mature oligodendrocytes contribute to demyelination and axonal injury is unexplored. Although current treatments modulate the immune system, long-term disability ensues, highlighting the need for neuroprotection. Glutamate is elevated before T2-visible white matter lesions appear in MS. We previously reported that myelin-reactive T cells provoke microglia to release glutamate from the system xc - transporter promoting myelin degradation in experimental autoimmune encephalomyelitis (EAE). Here, we explore the target for glutamate in mature oligodendrocytes. Most glutamate-stimulated calcium influx into oligodendrocyte somas is AMPA receptor (AMPAR)-mediated, and genetic deletion of AMPAR subunit GluA4 decreased intracellular calcium responses. Inducible deletion of GluA4 on mature oligodendrocytes attenuated EAE and loss of myelinated axons was selectively reduced compared to unmyelinated axons. These data link AMPAR signaling in mature oligodendrocytes to the pathophysiology of myelinated axons, demonstrating glutamate regulation as a potential neuroprotective strategy in MS.

Reactive Astrocytes as Therapeutic Targets for Brain Degenerative Diseases: Roles Played by Metabotropic Glutamate Receptors

Astrocytes are well known to play critical roles in the development and maintenance of the central nervous system (CNS). Moreover, recent reports indicate that these cells are heterogeneous with respect to the molecules they express and the functions they exhibit in the quiescent or activated state. Because astrocytes also contribute to pathology, promising new results raise the possibility of manipulating specific astroglial populations for therapeutic roles. In this mini-review, we highlight the function of metabotropic glutamate receptors (mGluRs), in particular mGluR3 and mGluR5, in reactive astrocytes and relate these to three degenerative CNS diseases: multiple sclerosis, Alzheimer's disease and Amyotrophic Lateral Sclerosis. Previous studies demonstrate that effects of these receptors may be beneficial, but this varies depending on the subtype of receptor, the state of the astrocytes, and the specific disease to which they are exposed. Elucidating the role of mGluRs on astrocytes at specific times during development and disease will provide novel insights in understanding how to best use these to serve as therapeutic targets.

The Na+/Ca2+ exchangers in demyelinating diseases

Intracellular [Na⁺]_i and [Ca²⁺]_i imbalance significantly contribute to neuro-axonal dysfunctions and maladaptive myelin repair or remyelination failure in chronic inflammatory demyelinating diseases such as multiple sclerosis. Progress in recent years has led to significant advances in understanding how [Ca²⁺]_i signaling network drive degeneration or remyelination of demyelinated axons. The Na⁺/Ca²⁺ exchangers (NCXs), a transmembrane protein family including three members encoded by ncx1, ncx2, and ncx3 genes, are emerging important regulators of [Na⁺]_i and [Ca²⁺]_i both in neurons and glial cells. Here we review recent advance highlighting the role of NCX exchangers in axons and myelin-forming cells, i.e. oligodendrocytes, which represent the major targets of the aberrant inflammatory attack in multiple sclerosis. The contribution of NCX subtypes to axonal pathology and myelin synthesis will be discussed. Although a definitive understanding of mechanisms regulating axonal pathology and remyelination failure in chronic demyelinating diseases is still lacking and requires further investigation, current knowledge suggest that NCX activity plays a crucial role in these processes. Defining the relative contributions of each NCX transporter in axon pathology and myelinating glia will constitute not only a major advance in understanding in detail the intricate mechanism of neurodegeneration and remyelination failure in demyelinating diseases but also will help to identify neuroprotective or remyelinating strategies targeting selective NCX exchangers as a means of treating MS.

Outer Retinal Dysfunction on Multifocal Electroretinography May Help Differentiating Multiple Sclerosis From Neuromyelitis Optica Spectrum Disorder

Purpose: To evaluate the intermediate and outer retina of patients with multiple sclerosis (MS) and neuromyelitis optica spectrum disorder (NMOSD) using OCT and multifocal electroretinography (mf-ERG).

Methods: Patients with MS (n = 30), NMOSD (n = 30), and healthy controls (n = 29) underwent visual field (VF), OCT, and mf-ERG testing. The eyes were distributed into 5 groups: MS with or without history of ON (MS+ON, MS–ON), NMOSD with or without ON (NMOSD+ON, NMOSD–ON), and controls. The thickness of the macular retinal nerve fiber layer (mRNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer, outer plexiform layer, outer nuclear layer, and photoreceptor layer was measured. mf-ERG P1 and N1 responses were registered and grouped in 3 sets of rings. The groups were compared using GEE models, and effect size (ES) calculated.

Results: Compared to controls, GCL and IPL thickness was significantly smaller in MS+ON (both p < 0.01), MS–ON (p < 0.01 and p = 0.015, respectively), NMOSD+ON (both p < 0.01) and NMOSD–ON (p = 0.03 and p = 0.018, respectively). ES was >0.80. mRNFL was smaller in three of the above groups (p < 0.01, p < 0.001, and p = 0.028; ES > 0.80) but not in MS–ON eyes (p = 0.18). No significant difference was observed for the remaining layers. Compared to controls, P1 and N1 peak times were shorter in MS (p-values in the range 0.049–0.002, ES < 0.50; and 0.049–0.010; ES < 0.50, respectively) but not in NMOSD. These abnormalities were strongly correlated with intermediate and outer retinal layer thickness.

Conclusions: mf-ERG data suggest outer retinal abnormalities in MS, but not in NMOSD. Our results may help understand how the two conditions differ regarding retinal damage.

Perineuronal oligodendrocytes in health and disease: the journey so far

Perineuronal oligodendrocytes (pn-Ols) are located in the cerebral gray matter in close proximity to neuronal perikarya and less frequently near dendrites and neurites. Although their morphology is indistinguishable from that of other oligodendrocytes, it is not known if pn-Ols have a similar or different cell signature from that of typical myelinating oligodendroglial cells. In this review, we discussed the potential roles of these cells in myelination under normal and pathophysiologic conditions as functional and nutritional supporters of neurons, as restrainers of neuronal firing, and as possible players in glutamate-glutamine homeostasis. We also highlighted the occurrences in which perineuronal oligodendroglia are altered, such as in experimental demyelination, multiple sclerosis, cerebral ischemia, epilepsy, Alzheimer’s disease, schizophrenia, major depression, and bipolar disorder.

Glial Cell AMPA Receptors in Nervous System Health, Injury and Disease

Glia form a central component of the nervous system whose varied activities sustain an environment that is optimised for healthy development and neuronal function. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA)-type glutamate receptors (AMPAR) are a central mediator of glutamatergic excitatory synaptic transmission, yet they are also expressed in a wide range of glial cells where they influence a variety of important cellular functions. AMPAR enable glial cells to sense the activity of neighbouring axons and synapses, and as such many aspects of glial cell development and function are influenced by the activity of neural circuits. However, these AMPAR also render glia sensitive to elevations of the extracellular concentration of glutamate, which are associated with a broad range of pathological conditions. Excessive activation of AMPAR under these conditions may induce excitotoxic injury in glial cells, and trigger pathophysiological responses threatening other neural cells and amplifying ongoing disease processes. The aim of this review is to gather information on AMPAR function from across the broad diversity of glial cells, identify their contribution to pathophysiological processes, and highlight new areas of research whose progress may increase our understanding of nervous system dysfunction and disease.

Role of Glutamatergic Excitotoxicity in Neuromyelitis Optica Spectrum Disorders

Neuromyelitis optica spectrum disorder (NMOSD) is an inflammatory disorder mediated by immune-humoral responses directed against central nervous system (CNS) antigens. Most patients are positive for specific immunoglobulin G (IgG) auto-antibodies for aquaporin-4 (AQP4), a water channel present in astrocytes. Antigen-antibody binding promotes complement system cascade activation, immune system cell infiltration, IgG deposition, loss of AQP4 and excitatory amino acid transporter 2 (EAAT2) expression on the astrocytic plasma membrane, triggering necrotic destruction of spinal cord tissue and optic nerves. Astrocytes are very important cells in the CNS and, in addition to supporting other nerve cells, they also regulate cerebral homeostasis and control glutamatergic synapses by modulating neurotransmission in the cleft through the high-affinity glutamate transporters present in their cell membrane. Specific IgG binding to AQP4 in astrocytes blocks protein functions and reduces EAAT2 activity. Once compromised, EAAT2 cannot take up free glutamate from the extracellular space, triggering excitotoxicity in the cells, which is characterized by overactivation of glutamate receptors in postsynaptic neurons. Therefore, the longitudinally extensive myelitis and optic neuritis lesions observed in patients with NMOSD may be the result of primary astrocytic damage triggered by IgG binding to AQP4, which can activate the immune-system cascade and, in addition, downregulate EAAT2. All these processes may explain the destructive lesions in NMOSD secondary to neuroinflammation and glutamatergic excitotoxicity. New or repurposed existing drugs capable of controlling glutamatergic excitotoxicity may provide new therapeutic options to reduce tissue damage and permanent disability after NMOSD attacks.

Glutamate versus GABA in neuron-oligodendroglia communication

In the central nervous system (CNS), myelin sheaths around axons are formed by glial cells named oligodendrocytes (OLs). In turn, OLs are generated by oligodendrocyte precursor cells (OPCs) during postnatal development and in adults, according to a process that depends on the proliferation and differentiation of these progenitors. The maturation of OL lineage cells as well as myelination by OLs are complex and highly regulated processes in the CNS. OPCs and OLs express an array of receptors for neurotransmitters, in particular for the two main CNS neurotransmitters glutamate and GABA, and are therefore endowed with the capacity to respond to neuronal activity. Initial studies in cell cultures demonstrated that both glutamate and GABA signaling mechanisms play important roles in OL lineage cell development and function. However, much remains to be learned about the communication of glutamatergic and GABAergic neurons with oligodendroglia in vivo. This review focuses on recent major advances in our understanding of the neuron-oligodendroglia communication mediated by glutamate and GABA in the CNS, and highlights the present controversies in the field. We discuss the expression, activation modes and potential roles of synaptic and extrasynaptic receptors along OL lineage progression. We review the properties of OPC synaptic connectivity with presynaptic glutamatergic and GABAergic neurons in the brain and consider the implication of glutamate and GABA signaling in activity-driven adaptive myelination.

Developing RNA aptamers for potential treatment of neurological diseases

AMPA receptor antagonists are drug candidates for potential treatment of a number of CNS diseases that involve excessive receptor activation. To date, small-molecule compounds are the dominating drug candidates in the field. However, lower potency, cross activity and poor water solubility are generally associated with these compounds. Here we show the potential of RNA-based antagonists or RNA aptamers as drug candidates and some strategies to discover these aptamers from a random sequence library (∼10¹⁴ sequences). As an alternative to small molecule compounds, our aptamers exhibit higher potency and selectivity toward AMPA receptors. Because aptamers are RNA molecules, they are naturally water soluble. We also discuss the major challenges of translating RNA aptamers as lead molecules into drugs/treatment options.

Fingolimod reduces neuropathic pain behaviors in a mouse model of multiple sclerosis by a sphingosine-1 phosphate receptor 1-dependent inhibition of central sensitization in the dorsal horn

Multiple sclerosis (MS) is an autoimmune-inflammatory neurodegenerative disease that is often accompanied by a debilitating neuropathic pain. Disease-modifying agents slow down the progression of multiple sclerosis and prevent relapses, yet it remains unclear if they yield analgesia. We explored the analgesic potential of fingolimod (FTY720), an agonist and/or functional antagonist at the sphingosine-1-phosphate receptor 1 (S1PR1), because it reduces hyperalgesia in models of peripheral inflammatory and neuropathic pain. We used a myelin oligodendrocyte glycoprotein 35 to 55 (MOG35-55) mouse model of experimental autoimmune encephalomyelitis, modified to avoid frank paralysis, and thus, allow for assessment of withdrawal behaviors to somatosensory stimuli. Daily intraperitoneal fingolimod reduced behavioral signs of central neuropathic pain (mechanical and cold hypersensitivity) in a dose-dependent and reversible manner. Both autoimmune encephalomyelitis and fingolimod changed hyperalgesia before modifying motor function, suggesting that pain-related effects and clinical neurological deficits were modulated independently. Fingolimod also reduced cellular markers of central sensitization of neurons in the dorsal horn of the spinal cord: glutamate-evoked Ca signaling and stimulus-evoked phospho-extracellular signal-related kinase ERK (pERK) expression, as well as upregulation of astrocytes (GFAP) and macrophage/microglia (Iba1) immunoreactivity. The antihyperalgesic effects of fingolimod were prevented or reversed by the S1PR1 antagonist W146 (1 mg/kg daily, i.p.) and could be mimicked by either repeated or single injection of the S1PR1-selective agonist SEW2871. Fingolimod did not change spinal membrane S1PR1 content, arguing against a functional antagonist mechanism. We conclude that fingolimod behaves as an S1PR1 agonist to reduce pain in multiple sclerosis by reversing central sensitization of spinal nociceptive neurons.

Thiamine and benfotiamine counteract ultrasound-induced aggression, normalize AMPA receptor expression and plasticity markers, and reduce oxidative stress in mice

The negative societal impacts associated with the increasing prevalence of violence and aggression is increasing, and, with this rise, is the need to understand the molecular and cellular changes that underpin ultrasound-induced aggressive behavior. In mice, stress-induced aggression is known to alter AMPA receptor subunit expression, plasticity markers, and oxidative stress within the brain. Here, we induced aggression in BALB/c mice using chronic ultrasound exposure and examined the impact of the psychoactive anti-oxidant compounds thiamine (vitamin B1), and its derivative benfotiamine, on AMPA receptor subunit expression, established plasticity markers, and oxidative stress. The administration of thiamine or benfotiamine (200 mg/kg/day) in drinking water decreased aggressive behavior following 3-weeks of ultrasound exposure and benfotiamine, reduced floating behavior in the swim test. The vehicle-treated ultrasound-exposed mice exhibited increases in protein carbonyl and total glutathione, altered AMPA receptor subunits expression, and decreased expression of plasticity markers. These ultrasound-induced effects were ameliorated by thiamine and benfotiamine treatment; in particular both antioxidants were able to reverse ultrasound-induced changes in GluA1 and GluA2 subunit expression, and, within the prefrontal cortex, significantly reversed the changes in protein carbonyl and polysialylated form of neural cell adhesion molecule (PSA-NCAM) expression levels. Benfotiamine was usually more efficacious than thiamine. Thus, the thiamine compounds were able to counteract ultrasound-induced aggression, which was accompanied by the normalization of markers that have been showed to be associated with ultrasound-induced aggression. These commonly used, orally-active compounds may have considerable potential for use in the control of aggression within the community. This article is part of the Special Issue entitled 'Current status of the neurobiology of aggression and impulsivity'.

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

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

Topical moringin-cream relieves neuropathic pain by suppression of inflammatory pathway and voltage-gated ion channels in murine model of multiple sclerosis

Background

Neuropathic pain represents the major public health burden with a strong impact on quality life in multiple sclerosis patients. Although some advances have been obtained in the last years, the conventional therapies remain poorly effective. Thus, the discovery of innovative approaches to improve the outcomes for multiple sclerosis patients is a goal of primary importance. With this aim, we investigated the efficacy of the 4-(α−L-rhamnopyranosyloxy)benzyl isothiocyanate (moringin), purified from Moringa oleifera seeds and ready-to-use as topical treatment in experimental autoimmune encephalomyelitis, murine model of multiple sclerosis. Female C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein (MOG_35–55) were topically treated with 2% moringin cream twice daily from the onset of the symptoms until the sacrifice occurred about 21 days after experimental autoimmune encephalomyelitis induction.

Results

Our observations showed the efficacy of 2% moringin cream treatment in reducing clinical and histological disease score, as well as in alleviating neuropathic pain with consequent recovering of the hind limbs and response to mechanical stimuli. In particular, Western blot analysis and immunohistochemical evaluations revealed that 2% moringin cream was able to counteract the inflammatory cascade by reducing the production of pro-inflammatory cytokines (interleukin-17 and interferon-γ) and in parallel by increasing the expression of anti-inflammatory cytokine (interleukin-10). Interestingly, 2% moringin cream treatment was found to modulate the expression of voltage-gated ion channels (results focused on P2X7, Nav 1.7, Nav 1.8 KV4.2, and α2δ-1) as well as metabotropic glutamate receptors (mGluR5 and xCT) involved in neuropathic pain initiation and maintenance.

Conclusions

Finally, our evidences suggest 2% moringin cream as a new pharmacological trend in the management of multiple sclerosis-induced neuropathic pain.

Regulation and dysregulation of axon infrastructure by myelinating glia

Pan and Chan discuss the role of myelinating glia in axonal development and the impact of demyelination on axon degeneration.

Axon loss and neurodegeneration constitute clinically debilitating sequelae in demyelinating diseases such as multiple sclerosis, but the underlying mechanisms of secondary degeneration are not well understood. Myelinating glia play a fundamental role in promoting the maturation of the axon cytoskeleton, regulating axon trafficking parameters, and imposing architectural rearrangements such as the nodes of Ranvier and their associated molecular domains. In the setting of demyelination, these changes may be reversed or persist as maladaptive features, leading to axon degeneration. In this review, we consider recent insights into axon–glial interactions during development and disease to propose that disruption of the cytoskeleton, nodal architecture, and other components of axon infrastructure is a potential mediator of pathophysiological damage after demyelination.

Serum Glutamate Is a Predictor for the Diagnosis of Multiple Sclerosis

One neurotransmitter, glutamate, has been implicated in the autoimmune demyelination seen in multiple sclerosis (MS). Glutamate is present in many tissues in the body, so consideration should be given to whether the serum level of glutamate is likely well correlated with the activity of the disease. This research aimed to compare the serum glutamate levels from patients diagnosed with MS with those from an age-matched control population. A review of this data could shed light upon whether the serum testing of glutamate using Enzyme-Linked Immunosorbent Assay (ELISA) is a reliable indicator of MS activity. Serum samples were obtained from 55 patients with different patterns of MS and from 25 healthy adults as a control group. The ELISA technique was used to determine the glutamate levels in the serum samples. The mean serum glutamate level for patients with MS was 1.318 ± 0.543 nmol/ml and that of the controls was 0.873 ± 0.341 nmol/ml. The serum glutamate levels showed an area under the curve via the receiver operating characteristics (ROC) of 0.738, which was significant (p value = 0.001). The present study is the first to establish a strong connection between the serum glutamate levels and MS patients, where there was statistically significant elevation of serum glutamate in MS patients; hence this elevation might be used as a monitor to help in the diagnosis of MS patients.

Fingolimod effects in neuroinflammation: Regulation of astroglial glutamate transporters?

Fingolimod is an oral sphingosine-1-phosphate-receptor modulator which reduces the recirculation of immune cells and may also directly target glial cells. Here we investigate effects of fingolimod on expression of astroglial glutamate transporters under pro-inflammatory conditions. In astrocyte cell culture, the addition of pro-inflammatory cytokines led to a significant downregulation of glutamate transporters glutamate transporter-1 (slc1a2/SLC1A2) and glutamate aspartate transporter (slc1a3/SLC1A3) expression on the mRNA or protein level. In this setting, the direct application of fingolimod-1 phosphate (F1P) on astrocytes did not change expression levels of slc1a2 and slc1a3 mRNA. The analysis of both transporters on the protein level by Western Blot and immunocytochemistry did also not reveal any effect of F1P. On a functional level, the addition of conditioned supernatants from F1P treated astrocytes to neuronal cell culture did not result in increased neurite growth. In experimental autoimmune encephalomyelitis as a model of multiple sclerosis, fingolimod treatment reduced T cell and macrophages/microglia mediated inflammation and also diminished astrocyte activation. At the same time, fingolimod restored the reduced expression of slc1a2 and slc1a3 in the inflamed spinal cord on the mRNA level and of SLC1A2 and SLC1A3 on the protein level, presumably via indirect, anti-inflammatory mechanisms. These findings provide further evidence for a predominantly peripheral effect of the compound in neuroinflammation.

Glutamate, T cells and multiple sclerosis

Glutamate is the major excitatory neurotransmitter in the nervous system, where it induces multiple beneficial and essential effects. Yet, excess glutamate, evident in a kaleidoscope of acute and chronic pathologies, is absolutely catastrophic, since it induces excitotoxicity and massive loss of brain function. Both the beneficial and the detrimental effects of glutamate are mediated by a large family of glutamate receptors (GluRs): the ionotropic glutamate receptors (iGluRs) and the metabotropic glutamate receptors (mGluRs), expressed by most/all cells of the nervous system, and also by many non-neural cells in various peripheral organs and tissues. T cells express on their cell surface several types of functional GluRs, and so do few other immune cells. Furthermore, glutamate by itself activates resting normal human T cells, and induces/elevates key T cell functions, among them: T cell adhesion, chemotactic migration, cytokine secretion, gene expression and more. Glutamate has also potent effects on antigen/mitogen/cytokine-activated T cells. Furthermore, T cells can even produce and release glutamate, and affect other cells and themselves via their own glutamate. Multiple sclerosis (MS) and its animal model Experimental Autoimmune Encephalomyelitis (EAE) are mediated by autoimmune T cells. In MS and EAE, there are excess glutamate levels, and multiple abnormalities in glutamate degrading enzymes, glutamate transporters, glutamate receptors and glutamate signaling. Some GluR antagonists block EAE. Enhancer of mGluR4 protects from EAE via regulatory T cells (Tregs), while mGluR4 deficiency exacerbates EAE. The protective effect of mGluR4 on EAE calls for testing GluR4 enhancers in MS patients. Oral MS therapeutics, namely Fingolimod, dimethyl fumarate and their respective metabolites Fingolimod-phosphate and monomethyl fumarate, can protect neurons against acute glutamatergic excitotoxic damage. Furthermore, Fingolimod reduce glutamate-mediated intracortical excitability in relapsing-remitting MS. Glatiramer acetate -COPAXONE®, an immunomodulator drug for MS, reverses TNF-α-induced alterations of striatal glutamate-mediated excitatory postsynaptic currents in EAE-afflicted mice. With regard to T cells of MS patients: (1) The cell surface expression of a specific GluR: the AMPA GluR3 is elevated in T cells of MS patients during relapse and with active disease, (2) Glutamate and AMPA (a selective agonist for glutamate/AMPA iGluRs) augment chemotactic migration of T cells of MS patients, (3) Glutamate augments proliferation of T cells of MS patients in response to myelin-derived proteins: MBP and MOG, (4) T cells of MS patients respond abnormally to glutamate, (5) Significantly higher proliferation values in response to glutamate were found in MS patients assessed during relapse, and in those with gadolinium (Gd)+ enhancing lesions on MRI. Furthermore, glutamate released from autoreactive T cells induces excitotoxic cell death of neurons. Taken together, the evidences accumulated thus far indicate that abnormal glutamate levels and signaling in the nervous system, direct activation of T cells by glutamate, and glutamate release by T cells, can all contribute to MS. This may be true also to other neurological diseases. It is postulated herein that the detrimental activation of autoimmune T cells by glutamate in MS could lead to: (1) Cytotoxicity in the CNS: T cell-mediated killing of neurons and glia cells, which would subsequently increase the extracellular glutamate levels, and by doing so increase the excitotoxicity mediated by excess glutamate, (2) Release of proinflammatory cytokines, e.g., TNFα and IFNγ that increase neuroinflammation. Finally, if excess glutamate, abnormal neuronal signaling, glutamate-induced activation of T cells, and glutamate release by T cells are indeed all playing a key detrimental role in MS, then optional therapeutic tolls include GluR antagonists, although these may have various side effects. In addition, an especially attractive therapeutic strategy is the novel and entirely different therapeutic approach to minimize excess glutamate and excitotoxicity, titled: 'brain to blood glutamate scavenging', designed to lower excess glutamate levels in the CNS by 'pumping it out' from the brain to the blood. The glutamate scavanging is achieved by lowering glutamate levels in the blood by intravenous injection of the blood enzyme glutamate oxaloacetate transaminase (GOT). The glutamate-scavenging technology, which is still experimental, validated so far for other brain pathologies, but not tested on MS or EAE yet, may be beneficial for MS too, since it could decrease both the deleterious effects of excess glutamate on neural cells, and the activation of autoimmune T cells by glutamate in the brain. The topic of glutamate scavenging, and also its potential benefit for MS, are discussed towards the end of the review, and call for research in this direction.

Excitotoxins, Mitochondrial and Redox Disturbances in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS). There is increasing evidence that MS is not only characterized by immune mediated inflammatory reactions, but also by neurodegenerative processes. There is cumulating evidence that neurodegenerative processes, for example mitochondrial dysfunction, oxidative stress, and glutamate (Glu) excitotoxicity, seem to play an important role in the pathogenesis of MS. The alteration of mitochondrial homeostasis leads to the formation of excitotoxins and redox disturbances. Mitochondrial dysfunction (energy disposal failure, apoptosis, etc.), redox disturbances (oxidative stress and enhanced reactive oxygen and nitrogen species production), and excitotoxicity (Glu mediated toxicity) may play an important role in the progression of the disease, causing axonal and neuronal damage. This review focuses on the mechanisms of mitochondrial dysfunction (including mitochondrial DNA (mtDNA) defects and mitochondrial structural/functional changes), oxidative stress (including reactive oxygen and nitric species), and excitotoxicity that are involved in MS and also discusses the potential targets and tools for therapeutic approaches in the future.

Cortical Gray Matter MR Imaging in Multiple Sclerosis

Several neuropathologic and imaging studies have consistently confirmed that multiple sclerosis affects both white (WM) and gray matter (GM) and that GM damage plays a key role in disability progression. However, differently from WM damage, the less inflammatory cell infiltration, the absence of significant blood-brain barrier damage, the low myelin density in upper cortical layers, as well as technical constraints, make the GM damage almost undetectable by means of conventional MR imaging.

Absence of system xc- on immune cells invading the central nervous system alleviates experimental autoimmune encephalitis

BACKGROUND

Multiple sclerosis (MS) is an autoimmune demyelinating disease that affects the central nervous system (CNS), leading to neurodegeneration and chronic disability. Accumulating evidence points to a key role for neuroinflammation, oxidative stress, and excitotoxicity in this degenerative process. System x_c^- or the cystine/glutamate antiporter could tie these pathological mechanisms together: its activity is enhanced by reactive oxygen species and inflammatory stimuli, and its enhancement might lead to the release of toxic amounts of glutamate, thereby triggering excitotoxicity and neurodegeneration.

METHODS

Semi-quantitative Western blotting served to study protein expression of xCT, the specific subunit of system x_c^-, as well as of regulators of xCT transcription, in the normal appearing white matter (NAWM) of MS patients and in the CNS and spleen of mice exposed to experimental autoimmune encephalomyelitis (EAE), an accepted mouse model of MS. We next compared the clinical course of the EAE disease, the extent of demyelination, the infiltration of immune cells and microglial activation in xCT-knockout (xCT^-/-) mice and irradiated mice reconstituted in xCT^-/- bone marrow (BM), to their proper wild type (xCT^+/+) controls.

RESULTS

xCT protein expression levels were upregulated in the NAWM of MS patients and in the brain, spinal cord, and spleen of EAE mice. The pathways involved in this upregulation in NAWM of MS patients remain unresolved. Compared to xCT^+/+ mice, xCT^-/- mice were equally susceptible to EAE, whereas mice transplanted with xCT^-/- BM, and as such only exhibiting loss of xCT in their immune cells, were less susceptible to EAE. In none of the above-described conditions, demyelination, microglial activation, or infiltration of immune cells were affected.

CONCLUSIONS

Our findings demonstrate enhancement of xCT protein expression in MS pathology and suggest that system x_c^- on immune cells invading the CNS participates to EAE. Since a total loss of system x_c^- had no net beneficial effects, these results have important implications for targeting system x_c^- for treatment of MS.

Neurochemical Markers in the Mammalian Brain: Structure, Roles in Synaptic Communication, and Pharmacological Relevance

BACKGROUND

Knowledge of molecular marker (typically protein or mRNA) expression in neural systems can provide insight to the chemical blueprint of signal processing and transmission, assist in tracking developmental or pathological progressions, and yield key information regarding potential medicinal targets. These markers are particularly relevant in the mammalian brain in the light of its unsurpassed cellular diversity. Accordingly, molecular expression profiling is rapidly becoming a major approach to classify neuron types. Despite a profusion of research, however, the biological functions of molecular markers commonly used to distinguish neuron types remain incompletely understood. Furthermore, most molecular markers of mammalian neuron types are also present in other organs, therefore complicating considerations of their potential pharmacological interactions.

OBJECTIVE

Here, we survey 15 prominent neurochemical markers from five categories, namely membrane transporters, calcium-binding proteins, neuropeptides, receptors, and extracellular matrix proteins, explaining their relation and relevance to synaptic communication.

METHOD

For each marker, we summarize fundamental structural features, cellular functionality, distributions within and outside the brain, as well as known drug effectors and mechanisms of action.

CONCLUSION

This essential primer thus links together the cellular complexity of the brain, the chemical properties of key molecular players in neurotransmission, and possible biomedical opportunities.

Ionotropic glutamate receptors: Which ones, when, and where in the mammalian neocortex

A multitude of 18 iGluR receptor subunits, many of which are diversified by splicing and RNA editing, localize to >20 excitatory and inhibitory neocortical neuron types defined by physiology, morphology, and transcriptome in addition to various types of glial, endothelial, and blood cells. Here we have compiled the published expression of iGluR subunits in the areas and cell types of developing and adult cortex of rat, mouse, carnivore, bovine, monkey, and human as determined with antibody- and mRNA-based techniques. iGluRs are differentially expressed in the cortical areas and in the species, and all have a unique developmental pattern. Differences are quantitative rather than a mere absence/presence of expression. iGluR are too ubiquitously expressed and of limited use as markers for areas or layers. A focus has been the iGluR profile of cortical interneuron types. For instance, GluK1 and GluN3A are enriched in, but not specific for, interneurons; moreover, the interneurons expressing these subunits belong to different types. Adressing the types is still a major hurdle because type-specific markers are lacking, and the frequently used neuropeptide/CaBP signatures are subject to regulation by age and activity and vary as well between species and areas. RNA-seq reveals almost all subunits in the two morphofunctionally characterized interneuron types of adult cortical layer I, suggesting a fairly broad expression at the RNA level. It remains to be determined whether all proteins are synthesized, to which pre- or postsynaptic subdomains in a given neuron type they localize, and whether all are involved in synaptic transmission. J. Comp. Neurol. 525:976-1033, 2017. © 2016 Wiley Periodicals, Inc.

Mechanisms of glutamate toxicity in multiple sclerosis: biomarker and therapeutic opportunities

Research advances support the idea that excessive activation of the glutamatergic pathway plays an important part in the pathophysiology of multiple sclerosis. Beyond the well established direct toxic effects on neurons, additional sites of glutamate-induced cell damage have been described, including effects in oligodendrocytes, astrocytes, endothelial cells, and immune cells. Such toxic effects could provide a link between various pathological aspects of multiple sclerosis, such as axonal damage, oligodendrocyte cell death, demyelination, autoimmunity, and blood-brain barrier dysfunction. Understanding of the mechanisms underlying glutamate toxicity in multiple sclerosis could help in the development of new approaches for diagnosis, treatment, and follow-up in patients with this debilitating disease. While several clinical trials of glutamatergic modulators have had disappointing results, our growing understanding suggests that there is reason to remain optimistic about the therapeutic potential of these drugs.

Synaptic pathology: A shared mechanism in neurological disease

Synaptic proteomes have evolved a rich and complex diversity to allow the exquisite control of neuronal communication and information transfer. It is therefore not surprising that many neurological disorders are associated with alterations in synaptic function. As technology has advanced, our ability to study the anatomical and physiological function of synapses in greater detail has revealed a critical role for both central and peripheral synapses in neurodegenerative disease. Synapse loss has a devastating effect on cellular communication, leading to wide ranging effects such as network disruption within central neural systems and muscle wastage in the periphery. These devastating effects link synaptic pathology to a diverse range of neurological disorders, spanning Alzheimer's disease to multiple sclerosis. This review will highlight some of the current literature on synaptic integrity in animal models of disease and human post-mortem studies. Synaptic changes in normal brain ageing will also be discussed and finally the current and prospective treatments for neurodegenerative disorders will be summarised.

Glutamate signalling: A multifaceted modulator of oligodendrocyte lineage cells in health and disease

Myelin is essential for the mammalian brain to function efficiently. Whilst many factors have been associated with regulating the differentiation of oligodendroglia and myelination, glutamate signalling might be particularly important for learning-dependent myelination. The majority of myelinated projection neurons are glutamatergic. Oligodendrocyte precursor cells receive glutamatergic synaptic inputs from unmyelinated axons and oligodendrocyte lineage cells express glutamate receptors which enable them to monitor and respond to changes in neuronal activity. Yet, what role glutamate plays for oligodendroglia is not fully understood. Here, we review glutamate signalling and its effects on oligodendrocyte lineage cells, and myelination in health and disease. Furthermore, we discuss whether glutamate signalling between neurons and oligodendroglia might lay the foundation to activity-dependent white matter plasticity. This article is part of the Special Issue entitled 'Oligodendrocytes in Health and Disease'.

Imaging Axonal Degeneration and Repair in Preclinical Animal Models of Multiple Sclerosis

Multiple sclerosis (MS) is a central nervous system (CNS) disease characterized by chronic neuroinflammation, demyelination, and axonal damage. Infiltration of activated lymphocytes and myeloid cells are thought to be primarily responsible for white matter damage and axonopathy. Over time, this neurologic damage manifests clinically as debilitating motor and cognitive symptoms. Existing MS therapies focus on symptom relief and delay of disease progression through reduction of neuroinflammation. However, long-term strategies to remyelinate, protect, or regenerate axons have remained elusive, posing a challenge to treating progressive forms of MS. Preclinical mouse models and techniques, such as immunohistochemistry, flow cytometry, and genomic and proteomic analysis have provided advances in our understanding of discrete time-points of pathology following disease induction. More recently, in vivo and in situ two-photon (2P) microscopy has made it possible to visualize continuous real-time cellular behavior and structural changes occurring within the CNS during neuropathology. Research utilizing 2P imaging to study axonopathy in neuroinflammatory demyelinating disease has focused on five areas: (1) axonal morphologic changes, (2) organelle transport and health, (3) relationship to inflammation, (4) neuronal excitotoxicity, and (5) regenerative therapies. 2P imaging may also be used to identify novel therapeutic targets via identification and clarification of dynamic cellular and molecular mechanisms of axonal regeneration and remyelination. Here, we review tools that have made 2P accessible for imaging neuropathologies and advances in our understanding of axonal degeneration and repair in preclinical models of demyelinating diseases.