Impaired striatal GABA transmission in experimental autoimmune encephalomyelitis

Lowered ratio of corticospinal excitation to inhibition predicts greater disability, poorer motor and cognitive function in multiple sclerosis

Objective

Investigate excitatory-inhibitory (E/I) (im)balance using transcranial magnetic stimulation (TMS) in individuals with Multiple Sclerosis (MS) and determine its validity as a neurophysiological biomarker of disability.

Methods

Participants with MS (n = 83) underwent TMS, cognitive, and motor function assessments. TMS-induced motor evoked potential amplitudes (excitability) and cortical silent periods (inhibition) were assessed bilaterally through recruitment curves. The E/I ratio was calculated as the ratio of excitation to inhibition.

Results

Participants with greater disability (Expanded Disability Status Scale, EDSS≥3) exhibited lower excitability and increased inhibition compared to those with lower disability (EDSS<3). This resulted in lower E/I ratios in the higher disability group. Individuals with higher disability presented with asymmetrical E/I ratios between brain hemispheres, a pattern not present in the group with lower disability. In regression analyses controlling for demographics, lowered TMS-probed E/I ratio predicted variance in disability (R2 = 0.37, p < 0.001), upper extremity function (R2 = 0.35, p < 0.001), walking speed (R2 = 0.22, p = 0.005), and cognitive performance (R2 = 0.25, p = 0.007). Receiver Operating Characteristic curve analysis confirmed 'excellent' discriminative ability of the E/I ratio in distinguishing high and low disability. Finally, excitation superiorly correlated with the E/I ratio than overall inhibition in both hemispheres (p ≤ 0.01).

Conclusion

The E/I ratio is a potential neurophysiological biomarker of disability level in MS, especially when assessed in the hemisphere corresponding to the weaker body side. Interventions aimed at increasing cortical excitation or reducing inhibition may restore E/I balance potentially stalling progression or improving function in MS.

Interleukin-10 contrasts inflammatory synaptopathy and central neurodegenerative damage in multiple sclerosis

Proinflammatory cytokines are implicated in promoting neurodegeneration in multiple sclerosis (MS) by affecting excitatory and inhibitory transmission at central synapses. Conversely, the synaptic effects of anti-inflammatory molecules remain underexplored, despite their potential neuroprotective properties and their presence in the cerebrospinal fluid (CSF) of patients. In a study involving 184 newly diagnosed relapsing-remitting (RR)-MS patients, we investigated whether CSF levels of the anti-inflammatory interleukin (IL)-10 were linked to disease severity and neurodegeneration measures. Additionally, we examined IL-10 impact on synaptic transmission in striatal medium spiny neurons and its role in counteracting inflammatory synaptopathy induced by IL-1β in female C57BL/6 mice with experimental autoimmune encephalomyelitis (EAE). Our findings revealed a significant positive correlation between IL-10 CSF levels and changes in EDSS (Expanded Disability Status Scale) scores one year after MS diagnosis. Moreover, IL-10 levels in the CSF were positively correlated with volumes of specific subcortical brain structures, such as the nucleus caudate. In both MS patients' CSF and EAE mice striatum, IL-10 and IL-1β expressions were upregulated, suggesting possible antagonistic effects of these cytokines. Notably, IL-10 exhibited the ability to decrease glutamate transmission, increase GABA transmission in the striatum, and reverse IL-1β-induced abnormal synaptic transmission in EAE. In conclusion, our data suggest that IL-10 exerts direct neuroprotective effects in MS patients by modulating both excitatory and inhibitory transmission and attenuating IL-1β-induced inflammatory synaptopathy. These findings underscore the potential therapeutic significance of IL-10 in mitigating neurodegeneration in MS.

Allopregnanolone and its antagonist modulate neuroinflammation and neurological impairment

Neuroinflammation accompanies several brain disorders, either as a secondary consequence or as a primary cause and may contribute importantly to disease pathogenesis. Neurosteroids which act as Positive Steroid Allosteric GABA-A receptor Modulators (Steroid-PAM) appear to modulate neuroinflammation and their levels in the brain may vary because of increased or decreased local production or import from the systemic circulation. The increased synthesis of steroid-PAMs is possibly due to increased expression of the mitochondrial cholesterol transporting protein (TSPO) in neuroinflammatory tissue, and reduced production may be due to changes in the enzymatic activity. Microglia and astrocytes play an important role in neuroinflammation, and their production of inflammatory mediators can be both activated and inhibited by steroid-PAMs and GABA. What is surprising is the finding that both allopregnanolone, a steroid-PAM, and golexanolone, a novel GABA-A receptor modulating steroid antagonist (GAMSA), can inhibit microglia and astrocyte activation and normalize their function. This review focuses on the role of steroid-PAMs in neuroinflammation and their importance in new therapeutic approaches to CNS and liver disease.

Interleukin-9 protects from microglia- and TNF-mediated synaptotoxicity in experimental multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a progressive neurodegenerative disease of the central nervous system characterized by inflammation-driven synaptic abnormalities. Interleukin-9 (IL-9) is emerging as a pleiotropic cytokine involved in MS pathophysiology.

METHODS

Through biochemical, immunohistochemical, and electrophysiological experiments, we investigated the effects of both peripheral and central administration of IL-9 on C57/BL6 female mice with experimental autoimmune encephalomyelitis (EAE), a model of MS.

RESULTS

We demonstrated that both systemic and local administration of IL-9 significantly improved clinical disability, reduced neuroinflammation, and mitigated synaptic damage in EAE. The results unveil an unrecognized central effect of IL-9 against microglia- and TNF-mediated neuronal excitotoxicity. Two main mechanisms emerged: first, IL-9 modulated microglial inflammatory activity by enhancing the expression of the triggering receptor expressed on myeloid cells-2 (TREM2) and reducing TNF release. Second, IL-9 suppressed neuronal TNF signaling, thereby blocking its synaptotoxic effects.

CONCLUSIONS

The data presented in this work highlight IL-9 as a critical neuroprotective molecule capable of interfering with inflammatory synaptopathy in EAE. These findings open new avenues for treatments targeting the neurodegenerative damage associated with MS, as well as other inflammatory and neurodegenerative disorders of the central nervous system.

Lipopolysaccharide augments microglial GABA uptake by increasing GABA transporter-1 trafficking and bestrophin-1 expression

Gamma-aminobutyric acid (GABA), the principal inhibitory neurotransmitter in the brain, affects numerous immune cell functions. Microglia, the brain's resident innate immune cells, regulate GABA signaling through GABA receptors and express the complete GABAergic machinery for GABA synthesis, uptake, and release. Here, the use of primary microglial cell cultures and ex vivo brain tissue sections allowed for demonstrating that treatment with lipopolysaccharide (LPS) increased microglial GABA uptake as well as GABA transporter (GAT)-1 trafficking. This effect was not entirely abolished by treatment with GAT inhibitors (GAT-Is). Notably, LPS also induced microglial upregulation of bestrophin-1 (BEST-1), a Ca2+ -activated Cl- channel permeable to GABA. Combined administration of GAT-Is and a BEST-1 inhibitor completely abolished LPS-induced microglial GABA uptake. Interestingly, increased microglial GAT-1 membrane turnover via syntaxin 1A was detected in LPS-treated cultures after BEST-1 blockade. Altogether, these findings provided evidence for a novel mechanism through which LPS may trigger the inflammatory response by directly altering microglial GABA clearance and identified the GAT-1/BEST-1 interplay as a potential novel mechanism involved in brain inflammation.

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.

Multiple sclerosis: Motor dysfunction

Multiple sclerosis is a chronic neurological disease characterized by inflammation and degeneration within the central nervous system. Over the course of the disease, most MS patients successively accumulate inflammatory lesions, axonal damage, and diffuse CNS pathology, along with an increasing degree of motor disability. While the pharmacological approach to MS targets inflammation to decrease relapse rates and relieve symptoms, disease-modifying therapy and immunosuppressive medications may not prevent the accumulation of pathology in most patients leading to long-term motor disability. This has been met with recent interest in promoting plasticity-guided concepts, enhanced by neurophysiological and neuroimaging approaches to address the preservation of motor function.

Chronic experimental autoimmune encephalomyelitis is an excellent model to study neuroaxonal degeneration in multiple sclerosis

Animal models of multiple sclerosis (MS), specifically experimental autoimmune encephalomyelitis (EAE), have been used extensively to develop anti-inflammatory treatments. However, the similarity between MS and one particular EAE model does not end at inflammation. MS and chronic EAE induced in C57BL/6 mice using myelin oligodendrocyte glycoprotein (MOG) peptide 35-55 share many neuropathologies. Beyond both having white matter lesions in spinal cord, both also have widespread neuropathology in the cerebral cortex, hippocampus, thalamus, striatum, cerebellum, and retina/optic nerve. In this review, we compare neuropathologies in each of these structures in MS with chronic EAE in C57BL/6 mice, and find evidence that this EAE model is well suited to study neuroaxonal degeneration in MS.

Oxidative Stress as a Potential Mechanism Underlying Membrane Hyperexcitability in Neurodegenerative Diseases

Neurodegenerative diseases are characterized by gradually progressive, selective loss of anatomically or physiologically related neuronal systems that produce brain damage from which there is no recovery. Despite the differences in clinical manifestations and neuronal vulnerability, the pathological processes appear to be similar, suggesting common neurodegenerative pathways. It is well known that oxidative stress and the production of reactive oxygen radicals plays a key role in neuronal cell damage. It has been proposed that this stress, among other mechanisms, could contribute to neuronal degeneration and might be one of the factors triggering the development of these pathologies. Another common feature in most neurodegenerative diseases is neuron hyperexcitability, an aberrant electrical activity. This review, focusing mainly on primary motor cortex pyramidal neurons, critically evaluates the idea that oxidative stress and inflammation may be involved in neurodegeneration via their capacity to increase membrane excitability.

Multiple sclerosis: Inflammation, autoimmunity and plasticity

In recent years, experimental studies have clarified that immune system influences the functioning of the central nervous system (CNS) in both physiologic and pathologic conditions. The neuro-immune crosstalk plays a crucial role in neuronal development and may be critically involved in mediating CNS response to neuronal damage. Multiple sclerosis (MS) represents a good model to investigate how the immune system regulates neuronal activity. Accordingly, a growing body of evidence has demonstrated that increased levels of pro-inflammatory mediators may significantly impact synaptic mechanisms, influencing overall neuronal excitability and synaptic plasticity expression. In this chapter, we provide an overview of preclinical data and clinical studies exploring synaptic functioning noninvasively with transcranial magnetic stimulation (TMS) in patients with MS. Moreover, we examine how inflammation-driven synaptic dysfunction could affect synaptic plasticity expression, negatively influencing the MS course. Contrasting CSF inflammation together with pharmacologic enhancement of synaptic plasticity and application of noninvasive brain stimulation, alone or in combination with rehabilitative treatments, could improve the clinical compensation and prevent the accumulating deterioration in MS.

Adult hippocampal neurogenesis in the context of lipopolysaccharide-induced neuroinflammation: A molecular, cellular and behavioral review

The continuous generation of new neurons occurs in at least two well-defined niches in the adult rodent brain. One of these areas is the subgranular zone of the dentate gyrus (DG) in the hippocampus. While the DG is associated with contextual and spatial learning and memory, hippocampal neurogenesis is necessary for pattern separation. Hippocampal neurogenesis begins with the activation of neural stem cells and culminates with the maturation and functional integration of a portion of the newly generated glutamatergic neurons into the hippocampal circuits. The neurogenic process is continuously modulated by intrinsic factors, one of which is neuroinflammation. The administration of lipopolysaccharide (LPS) has been widely used as a model of neuroinflammation and has yielded a body of evidence for unveiling the detrimental impact of inflammation upon the neurogenic process. This work aims to provide a comprehensive overview of the current knowledge on the effects of the systemic and central administration of LPS upon the different stages of neurogenesis and discuss their effects at the molecular, cellular, and behavioral levels.

Cerebrospinal fluid levels of L-glutamate signal central inflammatory neurodegeneration in multiple sclerosis

Excessive extracellular concentrations of L-glutamate (L-Glu) can be neurotoxic and contribute to neurodegenerative processes in multiple sclerosis (MS). The association between cerebrospinal fluid (CSF) L-Glu levels, clinical features, and inflammatory biomarkers in patients with MS remains unclear. In 179 MS patients (relapsing remitting, RR, N = 157; secondary progressive/primary progressive, SP/PP, N = 22), CSF levels of L-Glu at diagnosis were determined and compared with those obtained in a group of 40 patients with non-inflammatory/non-degenerative disorders. Disability at the time of diagnosis, and after 1 year follow-up, was assessed using the Expanded Disability Status Scale (EDSS). CSF concentrations of lactate and of a large set of pro-inflammatory and anti-inflammatory molecules were explored. CSF levels of L-Glu were slightly reduced in MS patients compared to controls. In RR-MS patients, L-Glu levels correlated with EDSS after 1 year follow-up. Moreover, in MS patients, significant correlations were found between L-Glu and both CSF levels of lactate and the inflammatory molecules interleukin (IL)-2, IL-6, and IL-1 receptor antagonist. Altered expression of L-Glu is associated with disability progression, oxidative stress, and inflammation. These findings identify CSF L-Glu as a candidate neurochemical marker of inflammatory neurodegeneration in MS.

Synaptic Dysfunction in Multiple Sclerosis: A Red Thread from Inflammation to Network Disconnection

Multiple sclerosis (MS) has been clinically considered a chronic inflammatory disease of the white matter; however, in the last decade growing evidence supported an important role of gray matter pathology as a major contributor of MS-related disability and the involvement of synaptic structures assumed a key role in the pathophysiology of the disease. Synaptic contacts are considered central units in the information flow, involved in synaptic transmission and plasticity, critical processes for the shaping and functioning of brain networks. During the course of MS, the immune system and its diffusible mediators interact with synaptic structures leading to changes in their structure and function, influencing brain network dynamics. The purpose of this review is to provide an overview of the existing literature on synaptic involvement during experimental and human MS, in order to understand the mechanisms by which synaptic failure eventually leads to brain networks alterations and contributes to disabling MS symptoms and disease progression.

Connecting Neuroinflammation and Neurodegeneration in Multiple Sclerosis: Are Oligodendrocyte Precursor Cells a Nexus of Disease?

The pathology in neurodegenerative diseases is often accompanied by inflammation. It is well-known that many cells within the central nervous system (CNS) also contribute to ongoing neuroinflammation, which can promote neurodegeneration. Multiple sclerosis (MS) is both an inflammatory and neurodegenerative disease in which there is a complex interplay between resident CNS cells to mediate myelin and axonal damage, and this communication network can vary depending on the subtype and chronicity of disease. Oligodendrocytes, the myelinating cell of the CNS, and their precursors, oligodendrocyte precursor cells (OPCs), are often thought of as the targets of autoimmune pathology during MS and in several animal models of MS; however, there is emerging evidence that OPCs actively contribute to inflammation that directly and indirectly contributes to neurodegeneration. Here we discuss several contributors to MS disease progression starting with lesion pathology and murine models amenable to studying particular aspects of disease. We then review how OPCs themselves can play an active role in promoting neuroinflammation and neurodegeneration, and how other resident CNS cells including microglia, astrocytes, and neurons can impact OPC function. Further, we outline the very complex and pleiotropic role(s) of several inflammatory cytokines and other secreted factors classically described as solely deleterious during MS and its animal models, but in fact, have many neuroprotective functions and promote a return to homeostasis, in part via modulation of OPC function. Finally, since MS affects patients from the onset of disease throughout their lifespan, we discuss the impact of aging on OPC function and CNS recovery. It is becoming clear that OPCs are not simply a bystander during MS progression and uncovering the active roles they play during different stages of disease will help uncover potential new avenues for therapeutic intervention.

Acute and chronic synaptic pathology in multiple sclerosis gray matter

OBJECTIVES

To investigate the extent of synaptic loss, and the contribution of gray matter (GM) inflammation and demyelination to synaptic loss, in multiple sclerosis (MS) brain tissue.

METHODS

This study was performed on two different post-mortem series of MS and control brains, including deep GM and cortical GM. MS brain samples had been specifically selected for the presence of active demyelinating GM lesions. Over 1,000,000 individual synapses were identified and counted using confocal microscopy, and further characterized as glutamatergic/GABAergic. Synaptic counts were also correlated with neuronal/axonal loss.

RESULTS

Important synaptic loss was observed in active demyelinating GM lesions (-58.9%), while in chronic inactive GM lesions, synaptic density was only mildly reduced compared to adjacent non-lesional gray matter (NLGM) (-12.6%). Synaptic loss equally affected glutamatergic and GABAergic synapses. Diffuse synaptic loss was observed in MS NLGM compared to control GM (-21.2% overall).

CONCLUSION

This study provides evidence, in MS brain tissue, of acute synaptic damage/loss during active GM inflammatory demyelination and of synaptic reorganization in chronically demyelinated GM, affecting equally glutamatergic and GABAergic synapses. Furthermore, this study provides a strong indication of widespread synaptic loss in MS NLGM also independently from focal GM demyelination.

Positivity of oligoclonal bands in the cerebrospinal fluid predisposed to metabolic changes and rearrangement of inhibitory/excitatory neurotransmitters in subcortical brain structures in multiple sclerosis

BACKGROUND

The latest diagnostic criteria for multiple sclerosis (MS) have revitalized the role of oligoclonal bands synthesis in the cerebrospinal fluid (CSF-OCB). This study identifies predictors of CSF-OCB-positivity among in vivo metabolic markers in the subcortical gray/white matter in MS patients after their first episode (CIS) and in patients with relapsing-remitting course (RRMS).

METHODS

The study enrolled 13 CIS and 23 RRMS patients. Metabolism was evaluated using Mescher-Garwood-edited proton-magnetic resonance spectroscopy on a 3T MR scanner. In addition to N-acetyl-aspartate (tNAA), myoinositol (mIns), and choline- and creatine compounds (tCho, tCr) were also evaluated γ-aminobutyric acid (GABA) and glutamate-glutamine (Glx) ratios.

RESULTS

CSF-OCB-positivity was found in 76.9% of CIS and 78.2% of RRMS patients. GABA and Glx ratios in putamen and corpus callosum strongly determined CSF-OCB-positive CIS patients. Other essential predictors of CSF-OCB-positive CIS were mIns and Glx ratios in the putamen, and tCho/tNAA in the corpus callosum. In RRMS, GABA ratios in the right thalamus and Glx ratios in the left hippocampus strongly predicted CSF-OCB-positive patients. tCho/tNAA and tNAA/tCr in the left hippocampus were also identified as essential predictors of CSF-OCB-positive RRMS patients.

CONCLUSION

This is the first in vivo evidence of GABA-Glx rearrangement in CSF-OCB-positive patients since its early stages of MS.

Alternative splicing of neurexins 1-3 is modulated by neuroinflammation in the prefrontal cortex of a murine model of multiple sclerosis

Mounting evidence points to immune-mediated synaptopathy and impaired plasticity as early pathogenic events underlying cognitive decline (CD) in Multiple sclerosis (MS) and in the experimental autoimmune encephalomyelitis (EAE) mouse model of the disease. However, knowledge of the neurobiology of synaptic dysfunction is still incomplete. Splicing regulation represents a flexible and powerful mechanism involved in dynamic remodeling of the synapse, which allows the expression of synaptic protein variants that dynamically control the specificity of contacts between neurons. The pre-synaptic adhesion molecules neurexins (NRXNs) 1-3 play a relevant role in cognition and are alternatively spliced to yield variants that differentially cluster specific ligands in the postsynaptic compartment and modulate functional properties of the synaptic contact. Notably, mutations in these genes or disruption of their splicing program are associated with neuropsychiatric disorders. Herein, we have investigated how inflammatory changes imposed by EAE impact on alternative splicing of the Nrxn 1-3 mouse genes in the acute phase of disease. Due to its relevance in cognition, we focused on the prefrontal cortex (PFC) of SJL/J mice, in which EAE-induced inflammatory lesions extend to the rostral forebrain. We found that inclusion of the Nrxn 1-3 AS4 exon is significantly increased in the PFC of EAE mice and that splicing changes are correlated with local Il1β-expression levels. This correlation is sustained by the concomitant downregulation of SLM2, the main splicing factor involved in skipping of the AS4 exon, in EAE mice displaying high levels of Il1β- expression. We also observed that Il1β-expression levels correlate with changes in parvalbumin (PV)-positive interneuron connectivity. Moreover, exposure to environmental enrichment (EE), a condition known to stimulate neuronal connectivity and to improve cognitive functions in mice and humans, modified PFC phenotypes of EAE mice with respect to Il1β-, Slm2-expression, Nrxn AS4 splicing and PV-expression, by limiting changes associated with high levels of inflammation. Our results reveal that local inflammation results in early splicing modulation of key synaptic proteins and in remodeling of GABAergic circuitry in the PFC of SJL/J mice. We also suggest EE as a tool to counteract these inflammation-associated events, thus highlighting potential therapeutic targets for limiting the progressive CD occurring in MS.

The novel atypical dopamine transport inhibitor CT-005404 has pro-motivational effects in neurochemical and inflammatory models of effort-based dysfunctions related to psychopathology

Depressed individuals suffer from effort-related motivational symptoms such as anergia and fatigue, which are resistant to treatment with many common antidepressants. While drugs that block dopamine transport (DAT) reportedly have positive motivational effects, DAT inhibitors such as cocaine and amphetamines produce undesirable side effects. Thus, there is a need to develop and characterize novel atypical DAT inhibitors with unique and selective binding profiles. Rodent effort-based choice tasks provide useful models of motivational dysfunctions. With these tasks, animals choose between a high-effort instrumental action leading to highly valued reinforcement vs. a low effort/low reward option. The present studies focused on the initial characterization of a novel atypical DAT inhibitor, CT-005404, which binds to DAT with high selectivity relative to serotonin and norepinephrine transport, and produces long-term elevations of extracellular DA. CT-005404 was assessed for its ability to attenuate the effort-related motivational effects of the DA depleting agent tetrabenazine and the pro-inflammatory cytokine interleukin-1β (IL-1β) using a fixed ratio 5/chow feeding choice test. Tetrabenazine (1.0 mg/kg i.p.) shifted choice behavior, decreasing lever pressing and increasing chow intake. IL-1β (4.0 μg/kg i.p.) also decreased lever pressing. CT-005404 was co-administered (7.5-30.0 mg/kg p.o.) with either tetrabenazine or IL-1β, and the 15.0 and 30.0 mg/kg doses significantly reversed the effects of tetrabenazine and IL-1β. CT-005404 administered alone produced a dose-related increase in lever pressing in rats tested on a progressive ratio/chow feeding choice task. Atypical DAT inhibitors such as CT-005404 offer potential as a new avenue for drug treatment of motivational dysfunctions in humans.

Inflammation and Corticospinal Functioning in Multiple Sclerosis: A TMS Perspective

Transcranial magnetic stimulation (TMS) has been employed in multiple sclerosis (MS) to assess the integrity of the corticospinal tract and the corpus callosum and to explore some physiological properties of the motor cortex. Specific alterations of TMS measures have been strongly associated to different pathophysiological mechanisms, particularly to demyelination and neuronal loss. Moreover, TMS has contributed to investigate the neurophysiological basis of MS symptoms, particularly those not completely explained by conventional structural damage, such as fatigue. However, variability existing between studies suggests that alternative mechanisms should be involved. Knowledge of MS pathophysiology has been enriched by experimental studies in animal models (i.e., experimental autoimmune encephalomyelitis) demonstrating that inflammation alters synaptic transmission, promoting hyperexcitability and neuronal damage. Accordingly, TMS studies have demonstrated an imbalance between cortical excitation and inhibition in MS. In particular, cerebrospinal fluid concentrations of different proinflammatory and anti-inflammatory molecules have been associated to corticospinal hyperexcitability, highlighting that inflammatory synaptopathy may represent a key pathophysiological mechanism in MS. In this perspective article, we discuss whether corticospinal excitability alterations assessed with TMS in MS patients could be useful to explain the pathophysiological correlates and their relationships with specific MS clinical characteristics and symptoms. Furthermore, we discuss evidence indicating that, in MS patients, inflammatory synaptopathy could be present since the early phases, could specifically characterize relapses, and could progressively increase during the disease course.

Central Modulation of Selective Sphingosine-1-Phosphate Receptor 1 Ameliorates Experimental Multiple Sclerosis

Future treatments of multiple sclerosis (MS), a chronic autoimmune neurodegenerative disease of the central nervous system (CNS), aim for simultaneous early targeting of peripheral immune function and neuroinflammation. Sphingosine-1-phosphate (S1P) receptor modulators are among the most promising drugs with both “immunological” and “non-immunological” actions. Selective S1P receptor modulators have been recently approved for MS and shown clinical efficacy in its mouse model, the experimental autoimmune encephalomyelitis (EAE). Here, we investigated the anti-inflammatory/neuroprotective effects of ozanimod (RPC1063), a S1P_1/5 modulator recently approved in the United States for the treatment of MS, by performing ex vivo studies in EAE brain. Electrophysiological experiments, supported by molecular and immunofluorescence analysis, revealed that ozanimod was able to dampen the EAE glutamatergic synaptic alterations, through attenuation of local inflammatory response driven by activated microglia and infiltrating T cells, the main CNS-cellular players of EAE synaptopathy. Electrophysiological studies with selective S1P₁ (AUY954) and S1P₅ (A971432) agonists suggested that S1P₁ modulation is the main driver of the anti-excitotoxic activity mediated by ozanimod. Accordingly, in vivo intra-cerebroventricular treatment of EAE mice with AUY954 ameliorated clinical disability. Altogether these results strengthened the relevance of S1P₁ agonists as immunomodulatory and neuroprotective drugs for MS therapy.

Potential of activated microglia as a source of dysregulated extracellular microRNAs contributing to neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron degeneration in adults, and several mechanisms underlying the disease pathology have been proposed. It has been shown that glia communicate with other cells by releasing extracellular vesicles containing proteins and nucleic acids, including microRNAs (miRNAs), which play a role in the post-transcriptional regulation of gene expression. Dysregulation of miRNAs is commonly observed in ALS patients, together with inflammation and an altered microglial phenotype. However, the role of miRNA-containing vesicles in microglia-to-neuron communication in the context of ALS has not been explored in depth. This review summarises the evidence for the presence of inflammation, pro-inflammatory microglia and dysregulated miRNAs in ALS, then explores how microglia may potentially be responsible for this miRNA dysregulation. The possibility of pro-inflammatory ALS microglia releasing miRNAs which may then enter neuronal cells to contribute to degeneration is also explored. Based on the literature reviewed here, microglia are a likely source of dysregulated miRNAs and potential mediators of neurodegenerative processes. Therefore, dysregulated miRNAs may be promising candidates for the development of therapeutic strategies.

Synaptic Elimination in Neurological Disorders

Synapses are well known as the main structures responsible for transmitting information through the release and recognition of neurotransmitters by pre- and post-synaptic neurons. These structures are widely formed and eliminated throughout the whole lifespan via processes termed synaptogenesis and synaptic pruning, respectively. Whilst the first pro-cess is needed for ensuring proper connectivity between brain regions and also with the periphery, the second phenomenon is important for their refinement by eliminating weaker and unnecessary synapses and, at the same time, maintaining and fa-voring the stronger ones, thus ensuring proper synaptic transmission. It is well-known that synaptic elimination is modulated by neuronal activity. However, only recently the role of the classical complement cascade in promoting this phenomenon has been demonstrated. Specifically, microglial cells recognize activated complement component 3 (C3) bound to synapses tar-geted for elimination, triggering their engulfment. As this is a highly relevant process for adequate neuronal functioning, dis-ruptions or exacerbations in synaptic pruning could lead to severe circuitry alterations that could underlie neuropathological alterations typical of neurological and neuropsychiatric disorders. In this review, we focus on discussing the possible in-volvement of excessive synaptic elimination in Alzheimer’s disease, as it has already been reported dendritic spine loss in post-synaptic neurons, increased association of complement proteins with its synapses and, hence, augmented microglia-mediated pruning in animal models of this disorder. In addition, we briefly discuss how this phenomenon could be related to other neurological disorders, including multiple sclerosis and schizophrenia.

'Prototypical' proinflammatory cytokine (IL-1) in multiple sclerosis: role in pathogenesis and therapeutic targeting

Introduction: It has been recognized for about 20 years that interleukin (IL)-1 signaling is implicated in Multiple Sclerosis (MS), a disabling, chronic, inflammatory and neurodegenerative disease of the central nervous system (CNS). Only recently, multifaceted roles of IL-1 emerged in MS pathophysiology as a result of both clinical and preclinical studies. Notably, drugs that directly target the IL-1 system have not been tested so far in MS.Areas covered: Recent studies in animal models, together with the development of ex vivo chimeric MS models, have disclosed a critical role for IL-1 not only at the peripheral level but also within the CNS. In the present review, we highlight the IL-1-dependent neuropathological aspects of MS, by providing an overview of the cells of the immune and CNS systems that respond to IL-1 signaling, and by emphasizing the subsequent effects on the CNS, from demyelinating processes, to synaptopathy, and excitotoxicity.Expert opinion: Drugs that act on the IL-1 system show a therapeutic potential in several autoinflammatory diseases and preclinical studies have highlighted the effects of these compounds in MS. We will discuss why anti-IL-1 therapies in MS have been neglected to date.

Interleukin-6 Disrupts Synaptic Plasticity and Impairs Tissue Damage Compensation in Multiple Sclerosis

Background: Synaptic plasticity helps in reducing the clinical expression of brain damage and represents a useful mechanism to compensate the negative impact of new brain lesions in multiple sclerosis (MS). Inflammation, altering synaptic plasticity, could negatively influence the disease course in relapsing-remitting MS (RR-MS). Objective: In the present study, we explored whether interleukin (IL)-6, a major proinflammatory cytokine involved in MS pathogenesis, alters synaptic plasticity and affects the ability to compensate for ongoing brain damage. Methods: The effect of IL-6 incubation on long-term potentiation (LTP) induction was explored in vitro, in mice hippocampal slices. We also explored the correlation between the cerebrospinal fluid (CSF) levels of this cytokine and the LTP-like effect induced by the paired associative stimulation (PAS) in a group of RR-MS patients. Finally, we examined the correlation between the CSF levels of IL-6 at the time of diagnosis and the prospective disease activity in a cohort of 150 RR-MS patients. Results: In vitro LTP induction was abolished by IL-6. Consistently, in patients with MS, a negative correlation emerged between IL-6 CSF concentrations and the effect of PAS. In MS patients, longer disease duration before diagnosis was associated with higher IL-6 CSF concentrations. In addition, elevated CSF levels of IL-6 were associated with greater clinical expression of new inflammatory brain lesions, unlike in patients with low or absent IL-6 concentrations, who had a better disease course. Conclusions: IL-6 interfering with synaptic plasticity mechanisms may impair the ability to compensate the clinical manifestation of new brain lesions in RR-MS patients.

Cytokine inflammatory threat, but not LPS one, shortens GABAergic synaptic currents in the mouse spinal cord organotypic cultures

Background

Synaptic dysfunction, named synaptopathy, due to inflammatory status of the central nervous system (CNS) is a recognized factor potentially underlying both motor and cognitive dysfunctions in neurodegenerative diseases. To gain knowledge on the mechanistic interplay between local inflammation and synapse changes, we compared two diverse inflammatory paradigms, a cytokine cocktail (CKs; IL-1β, TNF-α, and GM-CSF) and LPS, and their ability to tune GABAergic current duration in spinal cord cultured circuits.

Methods

We exploit spinal organotypic cultures, single-cell electrophysiology, immunocytochemistry, and confocal microscopy to explore synaptic currents and resident neuroglia reactivity upon CK or LPS incubation.

Results

Local inflammation in slice cultures induced by CK or LPS stimulations boosts network activity; however, only CKs specifically reduced GABAergic current duration. We pharmacologically investigated the contribution of GABA_AR α-subunits and suggested that a switch of GABA_AR α1-subunit might have induced faster GABA_AR decay time, weakening the inhibitory transmission.

Conclusions

Lower GABAergic current duration could contribute to providing an aberrant excitatory transmission critical for pre-motor circuit tasks and represent a specific feature of a CK cocktail able to mimic an inflammatory reaction that spreads in the CNS. Our results describe a selective mechanism that could be triggered during specific inflammatory stress.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1519-z) contains supplementary material, which is available to authorized users.

Mechanisms of neurobehavioral abnormalities in multiple sclerosis: Contributions from neural and immune components

Multiple sclerosis-related neurobehavioral abnormalities are one of the main components of disability in this disease. The same pathological processes that explain demyelination periods and neurodegeneration also allow the comprehension of neurobehavioral abnormalities. Inflammation in the central nervous system caused by cells of the immune system, especially lymphocytes, and by resident cells, such as astrocytes and microglia, directly modulate neurotransmission and synaptic physiology, resulting in behavioral changes (such as sickness behavior) and amplifying the degenerative mechanisms that occur in multiple sclerosis. In addition, neuronal death caused by glutamate-mediated excitotoxicity, alterations in GABAergic, serotonergic, and dopaminergic neurotransmission, and the mechanisms of axon damage are of foremost importance to explain the reduction in brain volume and the associated cognitive decline. Neuroinflammation and neurodegeneration are not isolated phenomena and various instances of interaction between them have been described. This presents attractive targets for the development of therapeutic strategies for this neglected component of multiple sclerosis related disability.

Mechanism of Siponimod: Anti-Inflammatory and Neuroprotective Mode of Action

Multiple sclerosis (MS) is a neuroinflammatory disorder of the central nervous system (CNS), and represents one of the main causes of disability in young adults. On the histopathological level, the disease is characterized by inflammatory demyelination and diffuse neurodegeneration. Although on the surface the development of new inflammatory CNS lesions in MS may appear consistent with a primary recruitment of peripheral immune cells, questions have been raised as to whether lymphocyte and/or monocyte invasion into the brain are really at the root of inflammatory lesion development. In this review article, we discuss a less appreciated inflammation-neurodegeneration interplay, that is: Neurodegeneration can trigger the formation of new, focal inflammatory lesions. We summarize old and recent findings suggesting that new inflammatory lesions develop at sites of focal or diffuse degenerative processes within the CNS. Such a concept is discussed in the context of the EXPAND trial, showing that siponimod exerts anti-inflammatory and neuroprotective activities in secondary progressive MS patients. The verification or rejection of such a concept is vital for the development of new therapeutic strategies for progressive MS.

Enhanced GABAergic Tonic Inhibition Reduces Intrinsic Excitability of Hippocampal CA1 Pyramidal Cells in Experimental Autoimmune Encephalomyelitis

Cognitive impairment (CI), a debilitating and pervasive feature of multiple sclerosis (MS), is correlated with hippocampal atrophy. Findings from postmortem MS hippocampi indicate that expression of genes involved in both excitatory and inhibitory neurotransmission are altered in MS, and although deficits in excitatory neurotransmission have been reported in the MS model experimental autoimmune encephalomyelitis (EAE), the functional consequence of altered inhibitory neurotransmission remains poorly understood. In this study, we used electrophysiological and biochemical techniques to examine inhibitory neurotransmission in the CA1 region of the hippocampus in EAE. We find that tonic, GABAergic inhibition is enhanced in CA1 pyramidal cells from EAE mice. Although plasma membrane expression of the GABA transporter GAT-3 was decreased in the EAE hippocampus, an increased surface expression of α5 subunit-containing GABA_A receptors appears to be primarily responsible for the increase in tonic inhibition during EAE. Enhanced tonic inhibition during EAE was associated with decreased CA1 pyramidal cell excitability and inhibition of α5 subunit-containing GABA_A receptors with the negative allosteric modulator L-655,708 enhanced pyramidal cell excitability in EAE mice. Together, our results suggest that altered GABAergic neurotransmission may underlie deficits in hippocampus-dependent cognitive function in EAE and MS.

Influence of lipopolysaccharide on diazepam-modified loss of righting reflex duration by pentobarbital treatment in mice

Benzodiazepine receptor agonists are widely prescribed therapeutic agents, alter gamma-aminobutyric acid (GABA)_A receptor function, and have hypnotic, anxiolytic, anticonvulsant, and antispastic effects. GABA_A receptor activity increases under systemic inflammatory conditions. We investigated the effect of benzodiazepine receptor agonists on pentobarbital-induced loss of righting reflex (LORR) duration using a mouse model of lipopolysaccharide (LPS)-induced inflammation. We assessed pentobarbital-induced LORR duration 24 h after LPS treatment in mice. Additionally, we examined the microglial response by immunohistochemistry and serum IL-6 and TNF-α concentrations in mice. LPS treatment significantly increased the duration of pentobarbital-induced LORR in mice treated with benzodiazepine receptor agonists (diazepam and brotizolam) and a GABA_A receptor agonist (muscimol) compared to that of mice treated with vehicle. These effects were blocked by bicuculline, a GABA_A receptor antagonist. LPS significantly increased the number of ionized calcium binding adapter molecule-1-positive hippocampal cells 2 and 24 h after treatment. The enhancing effect of diazepam in LPS-treated mice was significantly reduced by minocycline. These findings suggest that LPS enhances pentobarbital-induced LORR duration in mice treated with benzodiazepine via GABA_A receptor activity.

Visualization of the Breakdown of the Axonal Transport Machinery: a Comparative Ultrastructural and Immunohistochemical Approach

Axonal damage is a major factor contributing to disease progression in multiple sclerosis (MS) patients. On the histological level, acute axonal injury is most frequently analyzed by anti-amyloid precursor protein immunohistochemistry. To what extent this method truly detects axonal injury, and whether other proteins and organelles are as well subjected to axonal transport deficits in demyelinated tissues is not known. The aim of this study was to correlate ultrastructural morphology with the immunohistochemical appearance of acute axonal injury in a model of toxin-induced oligodendrocyte degeneration. C57BL/6J mice were intoxicated with 0.25% cuprizone to induce demyelination. The corpus callosum was investigated by serial block-face scanning electron microscopy (i.e., 3D EM), immunohistochemistry, and immunofluorescence microscopy. Brain tissues of progressive MS patients were included to test the relevance of our findings in mice for MS. Volumes of axonal swellings, determined by 3D EM, were comparable to volumes of axonal spheroids, determined by anti-APP immunofluorescence stains. Axonal swellings were present at myelinated and non-myelinated axonal internodes. Densities of amyloid precursor protein (APP)⁺ spheroids were highest during active demyelination. Besides APP, vesicular glutamate transporter 1 and mitochondrial proteins accumulated at sites of axonal spheroids. Such accumulations were found as well in lesions of progressive MS patients. In this correlative ultrastructural-immunohistochemical study, we provide strong evidence that breakdown of the axonal transport machinery results in focal accumulations of mitochondria and different synaptic proteins. We provide new marker proteins to visualize acute axonal injury, which helps to further understand the complex nature of axonal damage in progressive MS.

Effects of Peripheral Immune Challenge on In Vivo Firing of Basolateral Amygdala Neurons in Adult Male Rats

Peripheral inflammation often causes changes in mood and emergence of depressive behavior, and is characterized by a group of physical manifestations including lethargy, malaise, listlessness, decreased appetite, anhedonia, and fever. These behavioral changes are induced at the molecular level by pro-inflammatory cytokines like interleukin (IL)-1β, IL-6 and TNF-α. The basolateral amygdala (BLA) is a key brain region involved in mood and may mediate some of the behavioral effects of inflammation. However, it is unknown whether peripheral inflammatory state affects the activity of BLA neurons. To test this, adult male Sprague-Dawley rats were treated with IL-1β (1 μg, intraperitoneal (i.p.)), and behavioral and electrophysiological measures were obtained. IL-1β reduced locomotion in the open-field test and also reduced home-cage mobility, consistent with features of sickness-like behavior. Using in vivo single-unit extracellular electrophysiological recordings from anesthetized rats, we found that spontaneous BLA neuronal firing was acutely (<30 min) increased after IL-1β, followed by a return to baseline level, particularly in the basal nucleus of the BLA complex. To verify and expand on effects of peripheral inflammation, we tested whether another, long-lasting inflammagen also changes BLA neuronal firing. Lipopolysaccharide (250 μg/kg, i.p.) increased BLA firing rate acutely (<30 min) and persistently. The findings demonstrate a rapid effect of peripheral inflammation on BLA activity and suggest a link between BLA neuronal firing and triggering of behavioral consequences of peripheral inflammation. These findings are a first step toward understanding the neuronal basis of depressive behavior caused by acute peripheral inflammation.

Interplay Between Age and Neuroinflammation in Multiple Sclerosis: Effects on Motor and Cognitive Functions

Aging is one of the main risk factors for the development of many neurodegenerative diseases. Emerging evidence has acknowledged neuroinflammation as potential trigger of the functional changes occurring during normal and pathological aging. Two main determinants have been recognized to cogently contribute to neuroinflammation in the aging brain, i.e., the systemic chronic low-grade inflammation and the decline in the regulation of adaptive and innate immune systems (immunosenescence, ISC). The persistence of the inflammatory status in the brain in turn may cause synaptopathy and synaptic plasticity impairments that underlie both motor and cognitive dysfunctions. Interestingly, such inflammation-dependent synaptic dysfunctions have been recently involved in the pathophysiology of multiple sclerosis (MS). MS is an autoimmune neurodegenerative disease, typically affecting young adults that cause an early and progressive deterioration of both cognitive and motor functions. Of note, recent controlled studies have clearly shown that age at onset modifies prognosis and exerts a significant effect on presenting phenotype, suggesting that aging is a significant factor associated to the clinical course of MS. Moreover, some lines of evidence point to the different impact of age on motor disability and cognitive deficits, being the former most affected than the latter. The precise contribution of aging-related factors to MS neurological disability and the underlying molecular and cellular mechanisms are still unclear. In the present review article, we first emphasize the importance of the neuroinflammatory dependent mechanisms, such as synaptopathy and synaptic plasticity impairments, suggesting their potential exacerbation or acceleration with advancing age in the MS disease. Lastly, we provide an overview of clinical and experimental studies highlighting the different impact of age on motor disability and cognitive decline in MS, raising challenging questions on the putative age-related mechanisms involved.

Tumor Necrosis Factor and Interleukin-1β Modulate Synaptic Plasticity during Neuroinflammation

Cytokines are constitutively released in the healthy brain by resident myeloid cells to keep proper synaptic plasticity, either in the form of Hebbian synaptic plasticity or of homeostatic plasticity. However, when cytokines dramatically increase, establishing a status of neuroinflammation, the synaptic action of such molecules remarkably interferes with brain circuits of learning and cognition and contributes to excitotoxicity and neurodegeneration. Among others, interleukin-1β (IL-1β) and tumor necrosis factor (TNF) are the best studied proinflammatory cytokines in both physiological and pathological conditions and have been invariably associated with long-term potentiation (LTP) (Hebbian synaptic plasticity) and synaptic scaling (homeostatic plasticity), respectively. Multiple sclerosis (MS) is the prototypical neuroinflammatory disease, in which inflammation triggers excitotoxic mechanisms contributing to neurodegeneration. IL-β and TNF are increased in the brain of MS patients and contribute to induce the changes in synaptic plasticity occurring in MS patients and its animal model, the experimental autoimmune encephalomyelitis (EAE). This review will introduce and discuss current evidence of the role of IL-1β and TNF in the regulation of synaptic strength at both physiological and pathological levels, in particular speculating on their involvement in the synaptic plasticity changes observed in the EAE brain.

Activation of central GABAB receptors offsets the cyclosporine counteraction of endotoxic cardiovascular outcomes in conscious rats

We have previously shown that cyclosporine (CSA) counteracts cardiovascular manifestations induced by endotoxemia (lipopolysaccharide, LPS) such as hypotension and cardiac autonomic dysfunction in conscious rats. In this study, we investigated whether the facilitation of central γ-amino butyric acid (GABA) neurotransmission blunts these favorable influences of CSA. The LPS-CSA interaction was determined in the absence and presence of drugs that activate GABA_A or GABA_B receptors or elevate synaptic GABA levels in the central nervous system. The consequent i.v. administration of CSA (10 mg/kg) blunted the LPS-evoked hypotension, tachycardia, and reductions in time- and frequency-domain indices of heart rate variability (measures of cardiac autonomic control) evoked by LPS (10 mg/kg i.v.). The ability of CSA to reverse the LPS effects disappeared in rats treated intracisternally (i.c.) with baclofen (selective GABA_B agonist, 2 μg/rat) but not muscimol (selective GABA_A agonist, 1 μg/rat), indicating a preferential compromising action for central GABA_B receptors on the advantageous effects of CSA. Moreover, the improvement by CSA of LPS-evoked cardiovascular derangements was also eliminated after concurrent i.c. administration of vigabatrin (GABA transaminase inhibitor, 200 μg/rat) or tiagabine (GABA reuptake inhibitor, 100 μg/rat). These results demonstrate that the activation of central GABA_B receptors either directly via baclofen or indirectly following interventions that boost GABA levels in central synapses counterbalances the rectifying action of CSA on endotoxemia.

Glia-to-neuron transfer of miRNAs via extracellular vesicles: a new mechanism underlying inflammation-induced synaptic alterations

Recent evidence indicates synaptic dysfunction as an early mechanism affected in neuroinflammatory diseases, such as multiple sclerosis, which are characterized by chronic microglia activation. However, the mode(s) of action of reactive microglia in causing synaptic defects are not fully understood. In this study, we show that inflammatory microglia produce extracellular vesicles (EVs) which are enriched in a set of miRNAs that regulate the expression of key synaptic proteins. Among them, miR-146a-5p, a microglia-specific miRNA not present in hippocampal neurons, controls the expression of presynaptic synaptotagmin1 (Syt1) and postsynaptic neuroligin1 (Nlg1), an adhesion protein which play a crucial role in dendritic spine formation and synaptic stability. Using a Renilla-based sensor, we provide formal proof that inflammatory EVs transfer their miR-146a-5p cargo to neuron. By western blot and immunofluorescence analysis we show that vesicular miR-146a-5p suppresses Syt1 and Nlg1 expression in receiving neurons. Microglia-to-neuron miR-146a-5p transfer and Syt1 and Nlg1 downregulation do not occur when EV-neuron contact is inhibited by cloaking vesicular phosphatidylserine residues and when neurons are exposed to EVs either depleted of miR-146a-5p, produced by pro-regenerative microglia, or storing inactive miR-146a-5p, produced by cells transfected with an anti-miR-146a-5p. Morphological analysis reveals that prolonged exposure to inflammatory EVs leads to significant decrease in dendritic spine density in hippocampal neurons in vivo and in primary culture, which is rescued in vitro by transfection of a miR-insensitive Nlg1 form. Dendritic spine loss is accompanied by a decrease in the density and strength of excitatory synapses, as indicated by reduced mEPSC frequency and amplitude. These findings link inflammatory microglia and enhanced EV production to loss of excitatory synapses, uncovering a previously unrecognized role for microglia-enriched miRNAs, released in association to EVs, in silencing of key synaptic genes.

Synapsin I deletion reduces neuronal damage and ameliorates clinical progression of experimental autoimmune encephalomyelitis

The classical view of multiple sclerosis (MS) pathogenesis states that inflammation-mediated demyelination is responsible for neuronal damage and loss. However, recent findings show that impairment of neuronal functions and demyelination can be independent events, suggesting the coexistence of other pathogenic mechanisms. Due to the inflammatory milieu, subtle alterations in synaptic function occur, which are probably at the basis of the early cognitive decline that often precedes the neurodegenerative phases in MS patients. In particular, it has been reported that inflammation enhances excitatory synaptic transmission while it decreases GABAergic transmission in vitro and ex vivo. This evidence points to the idea that an excitation/inhibition imbalance occurs in the inflamed MS brain, even though the exact molecular mechanisms leading to this synaptic dysfunction are as yet not completely clear. Along this line, we observed that acute treatment of primary hippocampal neurons in culture with pro-inflammatory cytokines leads to an increased phosphorylation of synapsin I (SynI) by ERK1/2 kinase and to an increase in the frequency of spontaneous synaptic vesicle release events, which is prevented by SynI deletion. In vivo, the ablation of SynI expression is protective in terms of disease progression and neuronal damage in the experimental autoimmune encephalomyelitis mouse model of MS. Our results point to a possible key role in MS pathogenesis of the neuronal protein SynI, a regulator of excitation/inhibition balance in neuronal networks.

Bridging pro-inflammatory signals, synaptic transmission and protection in spinal explants in vitro

Multiple sclerosis is characterized by tissue atrophy involving the brain and the spinal cord, where reactive inflammation contributes to the neurodegenerative processes. Recently, the presence of synapse alterations induced by the inflammatory responses was suggested by experimental and clinical observations, in experimental autoimmune encephalomyelitis mouse model and in patients, respectively. Further knowledge on the interplay between pro-inflammatory agents, neuroglia and synaptic dysfunction is crucial to the design of unconventional protective molecules. Here we report the effects, on spinal cord circuits, of a cytokine cocktail that partly mimics the signature of T lymphocytes sub population Th1. In embryonic mouse spinal organ-cultures, containing neuronal cells and neuroglia, cytokines induced inflammatory responses accompanied by a significant increase in spontaneous synaptic activity. We suggest that cytokines specifically altered signal integration in spinal networks by speeding the decay of GABA_A responses. This hypothesis is supported by the finding that synapse protection by a non-peptidic NGF mimetic molecule prevented both the changes in the time course of GABA events and in network activity that were left unchanged by the cytokine production from astrocytes and microglia present in the cultured tissue. In conclusion, we developed an important tool for the study of synaptic alterations induced by inflammation, that takes into account the role of neuronal and not neuronal resident cells.

Remodeling Functional Connectivity in Multiple Sclerosis: A Challenging Therapeutic Approach

Neurons in the central nervous system are organized in functional units interconnected to form complex networks. Acute and chronic brain damage disrupts brain connectivity producing neurological signs and/or symptoms. In several neurological diseases, particularly in Multiple Sclerosis (MS), structural imaging studies cannot always demonstrate a clear association between lesion site and clinical disability, originating the "clinico-radiological paradox." The discrepancy between structural damage and disability can be explained by a complex network perspective. Both brain networks architecture and synaptic plasticity may play important roles in modulating brain networks efficiency after brain damage. In particular, long-term potentiation (LTP) may occur in surviving neurons to compensate network disconnection. In MS, inflammatory cytokines dramatically interfere with synaptic transmission and plasticity. Importantly, in addition to acute and chronic structural damage, inflammation could contribute to reduce brain networks efficiency in MS leading to worse clinical recovery after a relapse and worse disease progression. These evidence suggest that removing inflammation should represent the main therapeutic target in MS; moreover, as synaptic plasticity is particularly altered by inflammation, specific strategies aimed at promoting LTP mechanisms could be effective for enhancing clinical recovery. Modulation of plasticity with different non-invasive brain stimulation (NIBS) techniques has been used to promote recovery of MS symptoms. Better knowledge of features inducing brain disconnection in MS is crucial to design specific strategies to promote recovery and use NIBS with an increasingly tailored approach.

Reduced GABA levels correlate with cognitive impairment in patients with relapsing-remitting multiple sclerosis

OBJECTIVES

To investigate if brain gamma-aminobutyric acid (GABA) levels in patients with relapsing-remitting multiple sclerosis (RRMS) are abnormal compared with healthy controls, and their relationship to cognitive function in RRMS.

METHODS

Twenty-eight RRMS patients and twenty-six healthy controls underwent magnetic resonance spectroscopy (MRS) at 3-T to detect GABA signals from posterior cingulate cortex (PCC), medial prefrontal cortex (mPFC) and left hippocampus using the 'MEGAPoint Resolved Spectroscopy Sequence' (MEGA-PRESS) technique. All subjects also underwent a cognitive assessment.

RESULTS

In RRMS patients, GABA+ were lower in the PCC (p = 0.036) and left hippocampus (p = 0.039) compared with controls, decreased GABA+ in the PCC and left hippocampus were associated with specific cognitive functions (r = -0.452, p = 0.016 and r = 0.451, p = 0.016 respectively); GABA+ in the mPFC were not significantly decreased or related to any cognitive scores (p > 0.05).

CONCLUSIONS

This study demonstrates that abnormalities of the GABAergic system may be present in the pathogenesis of RRMS and suggests a potential link between regional GABA levels and cognitive impairment in patients with RRMS.

KEY POINTS

• GABA levels may decrease in patients with RRMS. • Lower GABA levels correlated with worse cognitive performance in patients with RRMS. • Dysfunctional GABAergic neurotransmission may have a role in cognitive impairment in RRMS.

TGFβ signaling is associated with changes in inflammatory gene expression and perineuronal net degradation around inhibitory neurons following various neurological insults

Brain damage due to stroke or traumatic brain injury (TBI), both leading causes of serious long-term disability, often leads to the development of epilepsy. Patients who develop post-injury epilepsy tend to have poor functional outcomes. Emerging evidence highlights a potential role for blood-brain barrier (BBB) dysfunction in the development of post-injury epilepsy. However, common mechanisms underlying the pathological hyperexcitability are largely unknown. Here, we show that comparative transcriptome analyses predict remodeling of extracellular matrix (ECM) as a common response to different types of injuries. ECM-related transcriptional changes were induced by the serum protein albumin via TGFβ signaling in primary astrocytes. In accordance with transcriptional responses, we found persistent degradation of protective ECM structures called perineuronal nets (PNNs) around fast-spiking inhibitory interneurons, in a rat model of TBI as well as in brains of human epileptic patients. Exposure of a naïve brain to albumin was sufficient to induce the transcriptional and translational upregulation of molecules related to ECM remodeling and the persistent breakdown of PNNs around fast-spiking inhibitory interneurons, which was contingent on TGFβ signaling activation. Our findings provide insights on how albumin extravasation that occurs upon BBB dysfunction in various brain injuries can predispose neural circuitry to the development of chronic inhibition deficits.

miR-142-3p Is a Key Regulator of IL-1β-Dependent Synaptopathy in Neuroinflammation

MicroRNAs (miRNA) play an important role in post-transcriptional gene regulation of several physiological and pathological processes. In multiple sclerosis (MS), a chronic inflammatory and degenerative disease of the CNS, and in its mouse model, the experimental autoimmune encephalomyelitis (EAE), miRNA dysregulation has been mainly related to immune system dysfunction and white matter (WM) pathology. However, little is known about their role in gray matter pathology. Here, we explored miRNA involvement in the inflammation-driven alterations of synaptic structure and function, collectively known as synaptopathy, a neuropathological process contributing to excitotoxic neurodegeneration in MS/EAE. Particularly, we observed that miR-142-3p is increased in the CSF of patients with active MS and in EAE brains. We propose miR-142-3p as a molecular mediator of the IL-1β-dependent downregulation of the glial glutamate-aspartate transporter (GLAST), which causes an enhancement of the glutamatergic transmission in the EAE cerebellum. The synaptic abnormalities mediated by IL-1β and the clinical and neuropathological manifestations of EAE disappeared in miR-142 knock-out mice. Furthermore, we observed that in vivo miR-142-3p inhibition, either by a preventive and local treatment or by a therapeutic and systemic strategy, abolished IL-1β- and GLAST-dependent synaptopathy in EAE wild-type mice. Consistently, miR-142-3p was responsible for the glutamatergic synaptic alterations caused by CSF of patients with MS, and CSF levels of miR-142-3p correlated with prospective MS disease progression. Our findings highlight miR-142-3p as key molecular player in IL-1β-mediated synaptic dysfunction, possibly leading to excitotoxic damage in both EAE and MS diseases. Inhibition of miR-142-3p could be neuroprotective in MS.

SIGNIFICANCE STATEMENT

Current studies suggest the role of glutamate excitotoxicity in the development and progression of multiple sclerosis (MS) and of its mouse model experimental autoimmune encephalomyelitis (EAE). The molecular mechanisms linking inflammation and synaptic alterations in MS/EAE are still unknown. Here, we identified miR-142-3p as a determinant molecular actor in inflammation-dependent synaptopathy typical of both MS and EAE. miR-142-3p was upregulated in the CSF of MS patients and in EAE cerebellum. Inhibition of miR-142-3p, locally in EAE brain and in a MS chimeric ex vivo model, recovered glutamatergic synaptic enhancement typical of EAE/MS. We proved that miR-142-3p promoted the IL-1β-dependent glutamate dysfunction by targeting glutamate-aspartate transporter (GLAST), a crucial glial transporter involved in glutamate homeostasis. Finally, we suggest miR-142-3p as a negative prognostic factor in patients with relapsing-remitting multiple sclerosis.

Interferon-γ causes mood abnormalities by altering cannabinoid CB1 receptor function in the mouse striatum

Interferon-γ (IFN-γ) has been implicated in the pathogenesis of multiple sclerosis (MS) and in its animal model, experimental autoimmune encephalomyelitis (EAE). The type-1 cannabinoid receptors (CB1Rs) are heavily involved in MS pathophysiology, and a growing body of evidence suggests that mood disturbances reflect specific effects of proinflammatory cytokines on neuronal activity. Here, we investigated whether IFN-γ could exert a role in the anxiety- and depressive-like behavior observed in mice with EAE, and in the modulation of CB1Rs. Anxiety and depression in fact are often diagnosed in MS, and have already been shown to depend on cannabinoid system. We performed biochemical, behavioral and electrophysiological experiments to assess the role of IFN-γ on mood control and on synaptic transmission in mice. Intracerebroventricular delivery of IFN-γ caused a depressive- and anxiety-like behavior in mice, associated with the selective dysfunction of CB1Rs controlling GABA transmission in the striatum. EAE induction was associated with increased striatal expression of IFN-γ, and with CB1R transmission deficits, which were rescued by pharmacological blockade of IFN-γ. IFN-γ was unable to replicate the effects of EAE on excitatory and inhibitory transmission in the striatum, but mimicked the effects of EAE on CB1R function in this brain area. Overall these results indicate that IFN-γ exerts a relevant control on mood, through the modulation of CB1R function. A better understanding of the biological pathways underling the psychological disorders during neuroinflammatory conditions is crucial for developing effective therapeutic strategies.

Prophylactic versus Therapeutic Fingolimod: Restoration of Presynaptic Defects in Mice Suffering from Experimental Autoimmune Encephalomyelitis

Fingolimod, the first oral, disease-modifying therapy for MS, has been recently proposed to modulate glutamate transmission in the central nervous system (CNS) of mice suffering from Experimental Autoimmune Encephalomyelitis (EAE) and in MS patients. Our study aims at investigating whether oral fingolimod recovers presynaptic defects that occur at different stages of disease in the CNS of EAE mice. In vivo prophylactic (0.3 mg/kg for 14 days, from the 7th day post immunization, d.p.i, the drug dissolved in the drinking water) fingolimod significantly reduced the clinical symptoms and the anxiety-related behaviour in EAE mice. Spinal cord inflammation, demyelination and glial cell activation are markers of EAE progression. These signs were ameliorated following oral fingolimod administration. Glutamate exocytosis was shown to be impaired in cortical and spinal cord terminals isolated from EAE mice at 21 ± 1 d.p.i., while GABA alteration emerged only at the spinal cord level. Prophylactic fingolimod recovered these presynaptic defects, restoring altered glutamate and GABA release efficiency. The beneficial effect occurred in a dose-dependent, region-specific manner, since lower (0.1-0.03 mg/kg) doses restored, although to a different extent, synaptic defects in cortical but not spinal cord terminals. A delayed reduction of glutamate, but not of GABA, exocytosis was observed in hippocampal terminals of EAE mice at 35 d.p.i. Therapeutic (0.3 mg/kg, from 21 d.p.i. for 14 days) fingolimod restored glutamate exocytosis in the cortex and in the hippocampus of EAE mice at 35 ± 1 d.p.i. but not in the spinal cord, where also GABAergic defects remained unmodified. These results improve our knowledge of the molecular events accounting for the beneficial effects elicited by fingolimod in demyelinating disorders.

Siponimod (BAF312) prevents synaptic neurodegeneration in experimental multiple sclerosis

Background

Data from multiple sclerosis (MS) and the MS rodent model, experimental autoimmune encephalomyelitis (EAE), highlighted an inflammation-dependent synaptopathy at the basis of the neurodegenerative damage causing irreversible disability in these disorders. This synaptopathy is characterized by an imbalance between glutamatergic and GABAergic transmission and has been proposed to be a potential therapeutic target.

Siponimod (BAF312), a selective sphingosine 1-phosphate1,5 receptor modulator, is currently under investigation in a clinical trial in secondary progressive MS patients. We investigated whether siponimod, in addition to its peripheral immune modulation, may exert direct neuroprotective effects in the central nervous system (CNS) of mice with chronic progressive EAE.

Methods

Minipumps allowing continuous intracerebroventricular (icv) infusion of siponimod for 4 weeks were implanted into C57BL/6 mice subjected to MOG_35-55-induced EAE. Electrophysiology, immunohistochemistry, western blot, qPCR experiments, and peripheral lymphocyte counts were performed. In addition, the effect of siponimod on activated microglia was assessed in vitro to confirm the direct effect of the drug on CNS-resident immune cells.

Results

Siponimod administration (0.45 μg/day) induced a significant beneficial effect on EAE clinical scores with minimal effect on peripheral lymphocyte counts. Siponimod rescued defective GABAergic transmission in the striatum of EAE, without correcting the EAE-induced alterations of glutamatergic transmission. We observed a significant attenuation of astrogliosis and microgliosis together with reduced lymphocyte infiltration in the striatum of EAE mice treated with siponimod. Interestingly, siponimod reduced the release of IL-6 and RANTES from activated microglial cells in vitro, which might explain the reduced lymphocyte infiltration. Furthermore, the loss of parvalbumin-positive (PV+) GABAergic interneurons typical of EAE brains was rescued by siponimod treatment, providing a plausible explanation of the selective effects of this drug on inhibitory synaptic transmission.

Conclusions

Altogether, our results show that siponimod has neuroprotective effects in the CNS of EAE mice, which are likely independent of its peripheral immune effect, suggesting that this drug could be effective in limiting neurodegenerative pathological processes in MS.