Microglial phenotypes in the human epileptic temporal lobe

What value can TSPO PET bring for epilepsy treatment?

Epilepsy is one of the most common neurological disorders and affects both the young and adult populations. The question we asked for this review was how positron emission tomography (PET) imaging with translocator protein (TSPO) radioligands can help inform the epilepsy clinic and the development of future treatments targeting neuroinflammatory processes.Even though the first TSPO PET scans in epilepsy patients were performed over 20 years ago, this imaging modality has not seen wide adoption in the clinic. There is vast scientific evidence from preclinical studies in rodent models of temporal lobe epilepsy which have shown increased levels of TSPO corresponding to neuroinflammatory processes in the brain. These increases peaked sub-acutely (1-2 weeks) after the epileptogenic insult (e.g. status epilepticus) and remained chronically increased, albeit at lower levels. In addition, these studies have shown a correlation between TSPO levels and seizure outcome, pharmacoresistance and behavioural morbidities. Histological assessment points to a complex interplay between different cellular components such as microglial activation, astrogliosis and cell death changing dynamically over time.In epilepsy patients, a highly sensitive biomarker of neuroinflammation would provide value for the optimization of surgical assessment (particularly for extratemporal lobe epilepsy) and support the clinical development path of anti-inflammatory treatments. Clinical studies have shown a systematic increase in asymmetry indices of TSPO PET binding. However, region-based analysis typically does not yield statistical differences and changes are often not restricted to the epileptogenic zone, limiting the ability of this imaging modality to localise pathology for surgery. In this manuscript, we discuss the biological underpinnings of these findings and review for which applications in epilepsy TSPO PET could bring added value.

Sleep Disruption Worsens Seizures: Neuroinflammation as a Potential Mechanistic Link

Sleep disturbances, such as insomnia, obstructive sleep apnea, and daytime sleepiness, are common in people diagnosed with epilepsy. These disturbances can be attributed to nocturnal seizures, psychosocial factors, and/or the use of anti-epileptic drugs with sleep-modifying side effects. Epilepsy patients with poor sleep quality have intensified seizure frequency and disease progression compared to their well-rested counterparts. A better understanding of the complex relationship between sleep and epilepsy is needed, since approximately 20% of seizures and more than 90% of sudden unexpected deaths in epilepsy occur during sleep. Emerging studies suggest that neuroinflammation, (e.g., the CNS immune response characterized by the change in expression of inflammatory mediators and glial activation) may be a potential link between sleep deprivation and seizures. Here, we review the mechanisms by which sleep deprivation induces neuroinflammation and propose that neuroinflammation synergizes with seizure activity to worsen neurodegeneration in the epileptic brain. Additionally, we highlight the relevance of sleep interventions, often overlooked by physicians, to manage seizures, prevent epilepsy-related mortality, and improve quality of life.

Regulation of NRG-1-ErbB4 signaling and neuroprotection by exogenous neuregulin-1 in a mouse model of epilepsy

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy. Dysregulation of glutamate transporters has been a common finding across animal models of epilepsy and in patients with TLE. In this study, we investigate NRG-1/ErbB4 signaling in epileptogenesis and the neuroprotective effects of NRG-1 treatment in a mouse model of temporal lobe epilepsy. Using immunohistochemistry, we report the first evidence for NRG-1/ErbB4-dependent selective upregulation of glutamate transporter EAAC1 and bihemispheric neuroprotection by exogeneous NRG-1 in the intrahippocampal kainic acid (IHKA) model of TLE. Our findings provide evidence that dysregulation of glutamate transporter EAAC1 contributes to the development of epilepsy and can be therapeutically targeted to reduce neuronal death following IHKA-induced status epilepticus (SE).

Early-Released Interleukin-10 Significantly Inhibits Lipopolysaccharide-Elicited Neuroinflammation In Vitro

Anti-inflammatory cytokine interleukin (IL)-10 is pivotal for limiting excessive inflammation in the central nervous system. Reports show that lipopolysaccharide (LPS)-induced microglial IL-10 emerges in a delayed manner in vitro and in vivo, lagging behind proinflammatory cytokines to facilitate the resolution of neuroinflammation. We hypothesized that IL-10 releases quite quickly based on our pilot investigation. Here, we uncovered a bimodal expression of microglial IL-10 gene transcription induced by LPS in mouse primary mixed glial cultures. This pattern consisted of a short brief early-phase and a long-lived late-phase, enabling the production of IL-10 protein in a rapid manner. The removal and addition of IL-10 protein assays indicated that early-released IL-10 exerted potent modulatory effects on neuroinflammation at picomolar levels, and IL-10 released at the onset of neuroinflammation is tightly controlled. We further showed that the early-released, but not the late-released, IL-10 was crucial for mediating and potentiating the anti-inflammatory function of a β2-adrenergic receptor agonist salmeterol. This study in vitro highlights the essential role of early-released IL-10 in regulating the appropriate degree of neuroinflammation, overturning the previous notion that microglial IL-10 produces and functions in a delayed manner and providing new insights into anti-inflammatory mechanisms-mediated neuroimmune homeostasis.

Agmatine Alleviates Epileptic Seizures and Hippocampal Neuronal Damage by Inhibiting Gasdermin D-Mediated Pyroptosis

Background: Epilepsy is a common neurological disease, and neuroinflammation is one of the main contributors to epileptogenesis. Pyroptosis is a type of pro-inflammatory cell death that is related to epilepsy. Agmatine, has anti-inflammatory properties and exerts neuroprotective effects against seizures. Our study investigated the effect of agmatine on the core pyroptosis protein GSDMD in the context of epilepsy.

Methods: A chronic epilepsy model and BV2 microglial cellular inflammation model were established by pentylenetetrazole (PTZ)-induced kindling or lipopolysaccharide (LPS) stimulation. H&E and Nissl staining were used to evaluate hippocampal neuronal damage. The expression of pyroptosis and inflammasome factors was examined by western blotting, quantitative real-time PCR, immunofluorescence and enzyme-linked immunosorbent assay (ELISA).

Results: Agmatine disrupted the kindling acquisition process, which decreased seizure scores and the incidence of full kindling and blocked hippocampal neuronal damage. In addition, agmatine increased BV2 microglial cell survival in vitro and alleviated seizures in vivo by suppressing the levels of PTZ-induced pyroptosis. Finally, the expression of TLR4, MYD88, phospho-IκBα, phospho-NF-κB and the NLRP3 inflammasome was significantly upregulated in LPS-induced BV2 microglial cells, while agmatine suppressed the expression of these proteins.

Conclusions: Our results indicate that agmatine affects epileptogenesis and exerts neuroprotective effects by inhibiting neuroinflammation, GSDMD activation, and pyroptosis. The inhibitory effect of agmatine on pyroptosis was mediated by the suppression of the TLR4/MYD88/NF-κB/NLRP3 inflammasome pathway. Therefore, agmatine may be a potential treatment option for epilepsy.

CXCL1-CXCR1/2 signaling is induced in human temporal lobe epilepsy and contributes to seizures in a murine model of acquired epilepsy

CXCL1, a functional murine orthologue of the human chemokine CXCL8 (IL-8), and its CXCR1 and CXCR2 receptors were investigated in a murine model of acquired epilepsy developing following status epilepticus (SE) induced by intra-amygdala kainate. CXCL8 and its receptors were also studied in human temporal lobe epilepsy (TLE). The functional involvement of the chemokine in seizure generation and neuronal cell loss was assessed in mice using reparixin (formerly referred to as repertaxin), a non-competitive allosteric inhibitor of CXCR1/2 receptors. We found a significant increase in hippocampal CXCL1 level within 24 h of SE onset that lasted for at least 1 week. No changes were measured in blood. In analogy with human TLE, immunohistochemistry in epileptic mice showed that CXCL1 and its two receptors were increased in hippocampal neuronal cells. Additional expression of these molecules was found in glia in human TLE. Mice were treated with reparixin or vehicle during SE and for additional 6 days thereafter, using subcutaneous osmotic minipumps. Drug-treated mice showed a faster SE decay, a reduced incidence of acute symptomatic seizures during 48 h post-SE, and a delayed time to spontaneous seizures onset compared to vehicle controls. Upon reparixin discontinuation, mice developed spontaneous seizures similar to vehicle mice, as shown by EEG monitoring at 14 days and 2.5 months post-SE. In the same epileptic mice, reparixin reduced neuronal cell loss in the hippocampus vs vehicle-injected mice, as assessed by Nissl staining at completion of EEG monitoring. Reparixin administration for 2 weeks in mice with established chronic seizures, reduced by 2-fold on average seizure number vs pre-treatment baseline, and this effect was reversible upon drug discontinuation. No significant changes in seizure number were measured in vehicle-injected epileptic mice that were EEG monitored in parallel. Data show that CXCL1-IL-8 signaling is activated in experimental and human epilepsy and contributes to acute and chronic seizures in mice, therefore representing a potential new target to attain anti-ictogenic effects.

Life and death in the hippocampus: What's bad?

The hippocampal formation is crucial for the generation and regulation of several brain functions, including memory and learning processes; however, it is vulnerable to neurological disorders, such as epilepsy. Temporal lobe epilepsy (TLE), the most common type of epilepsy, changes the hippocampal circuitry and excitability, under the contribution of both neuronal degeneration and abnormal neurogenesis. Classically, neurodegeneration affects sensitive areas of the hippocampus, such as dentate gyrus (DG) hilus, as well as specific fields of the Ammon's horn, CA3, and CA1. In addition, the proliferation, migration, and abnormal integration of newly generated hippocampal granular cells (GCs) into the brain characterize TLE neurogenesis. Robust studies over the years have intensely discussed the effects of death and life in the hippocampus, though there are still questions to be answered about their possible benefits and risks. Here, we review the impacts of death and life in the hippocampus, discussing its influence on TLE, providing new perspectives or insights for the implementation of new possible therapeutic targets. This article is part of the Special Issue "NEWroscience 2018".

Microglia proliferation plays distinct roles in acquired epilepsy depending on disease stages

OBJECTIVE

Microgliosis occurs in animal models of acquired epilepsy and in patients. It includes cell proliferation that is associated with seizure frequency and decreased neuronal cells in human epilepsy. The role of microglia proliferation in the development of acquired epilepsy is unknown; thus, we examined its contribution to spontaneous seizure, neurodegeneration, and cognitive deficits in different disease phases.

METHODS

We used a model of acquired epilepsy triggered by intra-amygdala kainic acid in C57BL6N adult male mice. Mice were electroencephalographically (EEG) monitored (24/7) during status epilepticus and in early and chronic disease. Microglia proliferation was blocked by GW2580, a selective CSF1 receptor inhibitor, supplemented in the diet for 21 days from status epilepticus onset. Then, mice were returned to placebo diet until experiment completion. Control mice were exposed to status epilepticus and fed with placebo diet. Experimental mice were tested in the novel object recognition test (NORT) and in Barnes maze, and compared to control and sham mice. At the end of the behavioral test, mice were killed for brain histopathological analysis. Additionally, seizure baseline was monitored in chronic epileptic mice, then mice were fed for 14 days with GW2580 or placebo diet under 24/7 EEG recording.

RESULTS

GW2580 prevented microglia proliferation in mice undergoing epilepsy, whereas it did not affect microglia or basal excitatory neurotransmission in the hippocampus of naive mice. Mice with occluded microglia proliferation during early disease development underwent status epilepticus and subsequent epilepsy similar to placebo diet mice, and were similarly impaired in NORT, with improvement in Barnes maze. GW2580-treated mice displayed neuroprotection in the hippocampus. In contrast, blockade of microglia proliferation in chronic epileptic mice resulted in spontaneous seizure reduction versus placebo mice.

SIGNIFICANCE

Microglia proliferation during early disease contributes to neurodegeneration, whereas in late chronic disease it contributes to seizures. Timely pharmacological interference with microglia proliferation may offer a potential target for improving disease outcomes.

Sublayer- and cell-type-specific neurodegenerative transcriptional trajectories in hippocampal sclerosis

Hippocampal sclerosis, the major neuropathological hallmark of temporal lobe epilepsy, is characterized by different patterns of neuronal loss. The mechanisms of cell-type-specific vulnerability and their progression and histopathological classification remain controversial. Using single-cell electrophysiology in vivo and immediate-early gene expression, we reveal that superficial CA1 pyramidal neurons are overactive in epileptic rodents. Bulk tissue and single-nucleus expression profiling disclose sublayer-specific transcriptomic signatures and robust microglial pro-inflammatory responses. Transcripts regulating neuronal processes such as voltage channels, synaptic signaling, and cell adhesion are deregulated differently by epilepsy across sublayers, whereas neurodegenerative signatures primarily involve superficial cells. Pseudotime analysis of gene expression in single nuclei and in situ validation reveal separated trajectories from health to epilepsy across cell types and identify a subset of superficial cells undergoing a later stage in neurodegeneration. Our findings indicate that sublayer- and cell-type-specific changes associated with selective CA1 neuronal damage contribute to progression of hippocampal sclerosis.

The effect of dipeptidyl peptidase IV on disease-associated microglia phenotypic transformation in epilepsy

BACKGROUND

Accumulating evidence suggests that disease-associated microglia (DAM), a recently discovered subset of microglia, plays a protective role in neurological diseases. Targeting DAM phenotypic transformation may provide new therapeutic options. However, the relationship between DAM and epilepsy remains unknown.

METHODS

Analysis of public RNA-sequencing data revealed predisposing factors (such as dipeptidyl peptidase IV; DPP4) for epilepsy related to DAM conversion. Anti-epileptic effect was assessed by electroencephalogram recordings and immunohistochemistry in a kainic acid (KA)-induced mouse model of epilepsy. The phenotype, morphology and function of microglia were assessed by qPCR, western blotting and microscopic imaging.

RESULTS

Our results demonstrated that DPP4 participated in DAM conversion and epilepsy. The treatment of sitagliptin (a DPP4 inhibitor) attenuated KA-induced epilepsy and promoted the expression of DAM markers (Itgax and Axl) in both mouse epilepsy model in vivo and microglial inflammatory model in vitro. With sitagliptin treatment, microglial cells did not display an inflammatory activation state (enlarged cell bodies). Furthermore, these microglia exhibited complicated intersections, longer processes and wider coverage of parenchyma. In addition, sitagliptin reduced the activation of NF-κB signaling pathway and inhibited the expression of iNOS, IL-1β, IL-6 and the proinflammatory DAM subset gene CD44.

CONCLUSION

The present results highlight that the DPP4 inhibitor sitagliptin can attenuate epilepsy and promote DAM phenotypic transformation. These DAM exhibit unique morphological features, greater migration ability and better surveillance capability. The possible underlying mechanism is that sitagliptin can reduce the activation of NF-κB signaling pathway and suppress the inflammatory response mediated by microglia. Thus, we propose DPP4 may act as an attractive direction for DAM research and a potential therapeutic target for epilepsy.

Serotonin and dopamine receptors profile on peripheral immune cells from patients with temporal lobe epilepsy

The role of inflammation and immune cells has been demonstrated in neurological diseases, including epilepsy. Leukocytes, as well as inflammatory mediators, contribute to abnormal processes that lead to a reduction in seizure threshold and synaptic reorganization. In this sense, identifying different phenotypes of circulating immune cells is essential to understanding the role of these cells in epilepsy. Immune cells can express a variety of surface markers, including neurotransmitter receptors, such as serotonin and dopamine. Alteration in these receptors expression patterns may affect the level of inflammatory mediators and the pathophysiology of epilepsy. Therefore, in the current study, we evaluated the expression of dopamine and serotonin receptors on white blood cells from patients with temporal lobe epilepsy with hippocampal sclerosis (TLE-HS). Blood samples from 17 patients with TLE-HS and 21 controls were collected. PBMC were isolated and stained ex vivo for flow cytometry. We evaluated the expression of serotonin (5-HT_1A, 5-HT_1B, 5-HT₂, 5-HT_2B, 5-HT_2C, 5-HT₃, 5-HT₄), and dopamine receptors (D₁, D₂, D₃, D₄, and D₅) on the cell surface of lymphocytes and innate immune cells (monocytes and granulocytes). Our results demonstrated that innate cells and lymphocytes from patients with TLE-HS showed high mean fluorescent intensity (MFI) for 5-HT_1A, 5-HT_1B, 5-HT_2A, and 5-HT₄ compared to controls. No difference was observed for 5-HT_2B. For dopamine receptors, the expression of D₁, D₂, D_4, and D₅ receptors was higher on innate cells from patients with TLE-HS when compared to controls for the MFI. Regarding lymphocytes population, D₂ expression was increased in patients with TLE-HS. In conclusion, there are alterations in the expression of serotonin and dopamine receptors on immune blood cells of patients with TLE-HS. Although the biological significance of these findings still needs to be further investigated, these changes may contribute to the understanding of TLE-HS pathophysiology.

Dissecting the role of subiculum in epilepsy: Research update and translational potential

The subiculum serves as the strategic core output of the hippocampus, through which neural activity exits the hippocampal proper and targets the entorhinal cortex and other more distant subcortical and cortical areas. The past decade has witnessed a growing interest in the subiculum, owing to discoveries revealing its critical role in regulating many physiological and pathophysiological processes. Notably, accumulating evidence from both clinical and experimental studies suggests that the subiculum plays a vital role in seizure initiation and propagation, in epilepsy. In this review, we briefly describe the structure and connectivity of the subiculum and then summarize the molecular and cellular mechanisms in the subiculum underlying the epileptic brain, in both epilepsy patients and animal models. Next, we review some translational approaches targeting the malfunctioned subiculum to treat epilepsy. Finally, we pose open questions for future research in the subiculum and their clinical translation challenges.

d-Serine Intervention In The Medial Entorhinal Area Alters TLE-Related Pathology In CA1 Hippocampus Via The Temporoammonic Pathway

Entrainment of the hippocampus by the medial entorhinal area (MEA) in Temporal Lobe Epilepsy (TLE), the most common type of drug-resistant epilepsy in adults, is believed to be mediated primarily through the perforant pathway (PP), which connects stellate cells in layer (L) II of the MEA with granule cells of the dentate gyrus (DG) to drive the hippocampal tri-synaptic circuit. Using immunohistochemistry, high-resolution confocal microscopy and the rat pilocarpine model of TLE, we show here that the lesser known temporoammonic pathway (TAP) plays a significant role in transferring MEA pathology to the CA1 region of the hippocampus independently of the PP. The pathology observed was region-specific and restricted primarily to the CA1c subfield of the hippocampus. As shown previously, daily intracranial infusion of d-serine (100 μm), an antagonist of GluN3-containing triheteromeric N-Methyl d-aspartate receptors (t-NMDARs), into the MEA prevented loss of LIII neurons and epileptogenesis. This intervention in the MEA led to the rescue of hippocampal CA1 neurons that would have otherwise perished in the epileptic animals, and down regulation of the expression of astrocytes and microglia thereby mitigating the effects of neuroinflammation. Interestingly, these changes were not observed to a similar extent in other regions of vulnerability like the hilus, DG or CA3, suggesting that the pathology manifest in CA1 is driven predominantly through the TAP. This work highlights TAP's role in the entrainment of the hippocampus and identifies specific areas for therapeutic intervention in dealing with TLE.

Past, present and future of zebrafish in epilepsy research

Animal models contribute greatly to our understanding of brain development and function as well as its dysfunction in neurological diseases. Epilepsy research is a very good example of how animal models can provide us with a mechanistic understanding of the genes, molecules, and pathophysiological processes involved in disease. Over the course of the last two decades, zebrafish came in as a new player in epilepsy research, with an expanding number of laboratories using this animal to understand epilepsy and to discover new strategies for preventing seizures. Yet, zebrafish as a model offers a lot more for epilepsy research. In this viewpoint, we aim to highlight some key contributions of zebrafish to epilepsy research, and we want to emphasize the great untapped potential of this animal model for expanding these contributions. We hope that our suggestions will trigger further discussions between clinicians and researchers with a common goal to understand and cure epilepsy.

Pro- and anti-epileptic roles of microglia

Microglia are brain-resident immune cells that contribute to the maintenance of brain homeostasis. In the epileptic brain, microglia show various activation phenotypes depending on the stage of epileptogenesis. Therefore, it remains unclear whether microglial activation acts in a pro-epileptic or anti-epileptic manner. In mesial temporal lobe epilepsy, one of the most common form of epilepsies, microglia exhibit at least two distinct morphologies, amoeboid shape and ramified shape. Amoeboid microglia are often found in sclerotic area, whereas ramified microglia are mainly found in non-sclerotic area; however, it remains unclear whether these structurally distinct microglia share separate roles in the epileptic brain. Here, we review the roles of the two distinct microglial phenotypes, focusing on their pro- and anti-epileptic roles in terms of inflammatory response, regulation of neurogenesis and microglia-neuron interaction.

Increased frequency of proinflammatory CD4 T cells and pathological levels of serum neurofilament light chain in adult drug-resistant epilepsy

OBJECTIVE

Adult drug-resistant epilepsy (DRE) is associated with significant morbidity. Infiltration of immune cells is observed in DRE epileptic foci; however, the relation between DRE and the peripheral immune cell compartment remains only partially understood. We aimed to investigate differences in immune cell populations, cytokines, and neurodegenerative biomarkers in the peripheral blood of subjects with epilepsy versus healthy controls, and in DRE compared to well-controlled epilepsy (WCE).

METHODS

Peripheral blood mononuclear cells and serum from >120 age- and sex-matched adults suffering from focal onset epilepsy and controls were analyzed by multipanel flow cytometry, multiplex immunoassays, and ultrasensitive single molecule array.

RESULTS

Using a data-driven analytical approach, we identified that CD4 T cells in the peripheral blood are present in a higher proportion in DRE patients. Moreover, we observed that the frequency of CD4 T cells expressing proinflammatory cytokines interleukin (IL)-17A, IL-22, tumor necrosis factor, interferon-γ, and granulocyte-macrophage colony-stimulating factor, but not anti-inflammatory cytokines IL-10 and IL-4, is elevated in the peripheral blood of DRE subjects compared to WCE. In parallel, we found that Th17-related circulating proinflammatory cytokines are elevated, but Th2-related cytokine IL-4 is reduced, in the serum of epilepsy and DRE subjects. As Th17 cells can exert neurotoxicity, we measured levels of serum neurofilament light chain (sNfL), a marker of neuronal injury. We found significantly elevated levels of sNfL in DRE compared to controls, especially among older individuals.

SIGNIFICANCE

Our data support that DRE is associated with an expansion of the CD4 Tcell subset in the peripheral blood and with a shift toward a proinflammatory Th17/Th1 CD4 Tcell immune profile. Our results further show that pathological levels of sNfL are more frequent in DRE, supporting a potential neurodegenerative component in adult DRE. With this work, we provide evidence for novel potential inflammatory and degenerative biomarkers in DRE.

NLRP3 and NLRP1 inflammasomes are up-regulated in patients with mesial temporal lobe epilepsy and may contribute to overexpression of caspase-1 and IL-β in sclerotic hippocampi

Inflammation plays a role in the pathophysiology of mesial temporal lobe epilepsy (MTLE). Inflammasome pathways, including the NLRP1 and NLRP3-induced ones, promote neuroinflammation and pyroptosis through interleukin (IL)-1β and caspase-1 action. Evaluation of NLRP1 in sclerotic hippocampi is scarce and there are no data on NLRP3 in human TLE. The aim of this study was to evaluate the expression of these proteins alongside caspase-1 and IL-1β in the hippocampi of patients with TLE compared to control samples. We also sought to investigate peripheral levels of caspase-1 and IL-1β in an independent cohort. Sclerotic and control hippocampi were collected for both histological and immunohistochemical analyses of NLRP1, NLRP3, caspase-1 and IL-1β; plasma was sampled for the measurement of caspase-1 and IL-1β levels through enzyme-linked immunoassay (ELISA) and cytometric bead array (CBA). Sclerotic hippocampi displayed higher expression of the measured proteins than control. Both glia and neurons showed activation of these pathways. Additionally, increased expression of NLRP1 and NLRP3 was associated with elevated plasma levels of IL-1β and in TLE, and increased levels of peripheral caspase-1 were associated with bilateral hippocampal sclerosis (HS). In conclusion, NLRP1 and NLRP3 are up-regulated in sclerotic hippocampi, what may be responsible, at least in part, for the increased hippocampal expression of caspase-1 and IL-1β. Our data suggest a role for inflammasome activation in central and peripheral inflammation in TLE.

Microglial innate memory and epigenetic reprogramming in neurological disorders

Microglia are myeloid-derived cells recognized as brain-resident macrophages. They act as the first and main line of immune defense in the central nervous system (CNS). Microglia have high phenotypic plasticity and are essential for regulating healthy brain homeostasis, and their dysregulation underlies the onset and progression of several CNS pathologies through impaired inflammatory responses. Aberrant microglial activation, following an inflammatory insult, is associated with epigenetic dysregulation in various CNS pathologies. Emerging data suggest that certain stimuli to myeloid cells determine enhanced or attenuated responses to subsequent stimuli. These phenomena, generally termed innate immune memory (IIM), are highly dependent on epigenetic reprogramming. Microglial priming has been reported in several neurological diseases and corresponds to a state of increased permissiveness or exacerbated response, promoted by continuous exposure to a chronic pro-inflammatory environment. In this article, we provide extensive evidence of these epigenetic-mediated phenomena under neurological conditions and discuss their contribution to pathogenesis and their clinical implications, including those concerning potential novel therapeutic approaches.

Microglial Cells in Epilepsy: Not That Bad After All?

Microglial Depletion Aggravates the Severity of Acute and Chronic Seizures in Mice

Wu W, Li Y, Wei Y, et al. Brain Behav Immun. 2020;89:245-255. doi:10.1016/j.bbi.2020.06.028. PMID: 32621847.

Microglia are the resident immune cells of the center nervous system and participate in various neurological diseases. Here, we determined the function of microglia in epileptogenesis using microglial ablation approaches. Three different microglia-specific genetic tools were used, CX3CR1^CreER/+: R26^iDTA/+, CX3CR1^CreER/+: R26^iDTR/+, and CX3CR1^CreER/+: Csf1r^Flox/Flox mice. We found that microglial depletion led to worse kainic acid (KA)–induced status epilepticus, higher mortality rate, and increased neuronal degeneration in the hippocampus. In KA-induced chronic spontaneous recurrent seizures, microglial depletion increased seizure frequency, interictal spiking, and seizure duration. Therefore, microglial depletion aggravates the severity of KA-induced acute and chronic seizures. Interestingly, microglial repopulation reversed the effects of depletion upon KA-induced status epilepticus. Our results demonstrate a beneficial role of microglia in suppressing both acute and chronic seizures, suggesting that microglia are a potential therapeutic target for the management of epilepsy.

Bioinformatic analysis identifies key transcriptome signatures in temporal lobe epilepsy

Aims

To identify transcriptome signatures underlying epileptogenesis in temporal lobe epilepsy (TLE).

Methods

Robust rank aggregation analysis was used to integrate multiple microarrays in rodent models of TLE and determine differentially expressed genes (DEGs) in acute, latent, and chronic stages. Functional annotation and protein‐protein interaction analysis were performed to explore the potential functions of the DEGs and identify hub genes with the highest intramodular connectivity. The association between hub genes and hippocampal sclerosis/seizure frequency was analyzed using publicly available RNA‐sequencing datasets from TLE patients. We subsequently established a pilocarpine‐induced status epilepticus (SE) model in rats and validated mRNA expression of hub genes by quantitative reverse transcription PCR (qRT‐PCR).

Results

The DEGs in the acute, latent, and chronic phases of TLE in animal models were prominently enriched in inflammatory response. Hub genes identified in the acute phase mainly participated in biological processes including inflammation, blood‐brain barrier damage, and cell adhesion. The hub genes in the latent phase were related to microglia/macrophage activation (Emr1 and Aif1) and phagocytosis (Cd68, Tyrobp, and Lyz). In the chronic phase, the hub genes were associated with activation of complements and microglia/macrophages. We further found that some hub genes identified in human TLE, such as Tlr2, Lgals3, and Stat3, were positively correlated with seizure frequency. Other hub genes, including Lgals3 and Serpine1, were associated with hippocampus sclerosis. qRT‐PCR analysis confirmed that the mRNA levels of hub genes in rat hippocampus were significantly up‐regulated after SE induction.

Conclusions

Our integrated analysis identified hub genes in different stages of epilepsy. The functional annotations suggest that the activation and phagocytic activities of microglia/macrophages may play critical roles in epileptogenesis of TLE.

In vivo deep-brain imaging of microglia enabled by three-photon fluorescence microscopy

Microglia act as the first and main form of active immune defense in brain. However, in animal models, research on these cells is limited to the superficial layer of the brain, due to the lack of a deep-imaging technique. Here we break this depth limit using three-photon fluorescence (3PF) microscopy excited at the 1700-nm window. Three-photon action cross-section (ησ₃) measurement lays the basis for dye selection and the resultant maximization of 3PF generation. 3PF imaging suppresses the surface background, leading to a much improved signal-to-background ratio compared to the commonly used two-photon microscopy (2PM). We can image microglia 1124 µm below the brain surface in vivo, 3.7 times deeper than previous results using 2PM for microglia imaging. This technique enables us to visualize microglia in the white matter layer in vivo for the first time.

Interleukin 4 Affects Epilepsy by Regulating Glial Cells: Potential and Possible Mechanism

Epilepsy is a chronic brain dysfunction induced by an abnormal neuronal discharge that is caused by complicated psychopathologies. Recently, accumulating studies have revealed a close relationship between inflammation and epilepsy. Specifically, microglia and astrocytes are important inflammatory cells in the central nervous system (CNS) that have been proven to be related to the pathogenesis and development of epilepsy. Additionally, interleukin 4 (IL-4) is an anti-inflammatory factor that can regulate microglia and astrocytes in many aspects. This review article focuses on the regulatory role of IL-4 in the pathological changes of glial cells related to epilepsy. We additionally propose that IL-4 may play a protective role in epileptogenesis and suggest that IL-4 may be a novel therapeutic target for the treatment of epilepsy.

Genetic deletion of microRNA-22 blunts the inflammatory transcriptional response to status epilepticus and exacerbates epilepsy in mice

MicroRNAs perform important roles in the post-transcriptional regulation of gene expression. Sequencing as well as functional studies using antisense oligonucleotides indicate important roles for microRNAs during the development of epilepsy through targeting transcripts involved in neuronal structure, gliosis and inflammation. MicroRNA-22 (miR-22) has been reported to protect against the development of epileptogenic brain networks through suppression of neuroinflammatory signalling. Here, we used mice with a genetic deletion of miR-22 to extend these insights. Mice lacking miR-22 displayed normal behaviour and brain structure and developed similar status epilepticus after intraamygdala kainic acid compared to wildtype animals. Continuous EEG monitoring after status epilepticus revealed, however, an accelerated and exacerbated epilepsy phenotype whereby spontaneous seizures began sooner, occurred more frequently and were of longer duration in miR-22-deficient mice. RNA sequencing analysis of the hippocampus during the period of epileptogenesis revealed a specific suppression of inflammatory signalling in the hippocampus of miR-22-deficient mice. Taken together, these findings indicate a role for miR-22 in establishing early inflammatory responses to status epilepticus. Inflammatory signalling may serve anti-epileptogenic functions and cautions the timing of anti-inflammatory interventions for the treatment of status epilepticus.

Reactive Glia Inflammatory Signaling Pathways and Epilepsy

Neuroinflammation and epilepsy are interconnected. Brain inflammation promotes neuronal hyper-excitability and seizures, and dysregulation in the glia immune-inflammatory function is a common factor that predisposes or contributes to the generation of seizures. At the same time, acute seizures upregulate the production of pro-inflammatory cytokines in microglia and astrocytes, triggering a downstream cascade of inflammatory mediators. Therefore, epileptic seizures and inflammatory mediators form a vicious positive feedback loop, reinforcing each other. In this work, we have reviewed the main glial signaling pathways involved in neuroinflammation, how they are affected in epileptic conditions, and the therapeutic opportunities they offer to prevent these disorders.

Distinct P2Y Receptors Mediate Extension and Retraction of Microglial Processes in Epileptic and Peritumoral Human Tissue

Microglia exhibit multiple, phenotype-dependent motility patterns often triggered by purinergic stimuli. However, little data exist on motility of human microglia in pathological situations. Here we examine motility of microglia stained with a fluorescent lectin in tissue slices from female and male epileptic patients diagnosed with mesial temporal lobe epilepsy or cortical glioma (peritumoral cortex). Microglial shape varied from ramified to amoeboid cells predominantly in regions of high neuronal loss or closer to a tumor. Live imaging revealed unstimulated or purine-induced microglial motilities, including surveillance movements, membrane ruffling, and process extension or retraction. At different concentrations, ADP triggered opposing motilities. Low doses triggered process extension. It was suppressed by P2Y12 receptor antagonists, which also reduced process length and surveillance movements. Higher purine doses caused process retraction and membrane ruffling, which were blocked by joint application of P2Y1 and P2Y13 receptor antagonists. Purinergic effects on motility were similar for all microglia tested. Both amoeboid and ramified cells from mesial temporal lobe epilepsy or peritumoral cortex tissue expressed P2Y12 receptors. A minority of microglia expressed the adenosine A2A receptor, which has been linked with process withdrawal of rodent cells. Laser-mediated tissue damage let us test the functional significance of these effects. Moderate damage induced microglial process extension, which was blocked by P2Y12 receptor antagonists. Overall, the purine-induced motility of human microglia in epileptic tissue is similar to that of rodent microglia in that the P2Y12 receptor initiates process extension. It differs in that retraction is triggered by joint activation of P2Y1/P2Y13 receptors.SIGNIFICANCE STATEMENT Microglial cells are brain-resident immune cells with multiple functions in healthy or diseased brains. These diverse functions are associated with distinct phenotypes, including different microglial shapes. In the rodent, purinergic signaling is associated with changes in cell shape, such as process extension toward tissue damage. However, there are little data on living human microglia, especially in diseased states. We developed a reliable technique to stain microglia from epileptic and glioma patients to examine responses to purines. Low-intensity purinergic stimuli induced process extension, as in rodents. In contrast, high-intensity stimuli triggered a process withdrawal mediated by both P2Y1 and P2Y13 receptors. P2Y1/P2Y13 receptor activation has not previously been linked to microglial morphological changes.

Emerging Roles for Microglial Phagocytic Signaling in Epilepsy

Microglia are the resident immune cells and professional phagocytes of the central nervous system. However, little is known about the contribution of their phagocytic signaling to the neuropathology and pathophysiology of epilepsy. Here, we summarize and discuss the implications of recent evidence supporting that aberrant microglia phagocytic activity and alterations in phagocytosis signaling molecules occur in association with microglia-neuronal contacts, neuronal/synaptic loss, and spontaneous recurrent seizures in human and preclinical models of epilepsy. This body of evidence provides strong support that the microglial contribution to epileptogenic networks goes beyond inflammation, and suggests that phagocytic signaling molecules may be novel therapeutic targets for epilepsy.

Fiery Cell Death: Pyroptosis in the Central Nervous System

Pyroptosis ('fiery death') is an inflammatory type of regulated cell death (RCD), which occurs downstream of inflammasome activation. Pyroptosis is mediated directly by the recently identified family of pore-forming proteins known as gasdermins, the best characterized of which is gasdermin D (GSDMD). Recent investigations implicate pyroptosis in the pathogenesis of multiple neurological diseases. In this review, we discuss molecular mechanisms that drive pyroptosis, evidence for pyroptosis within the CNS, and emerging therapeutic strategies for its inhibition in the context of neurological disease.

Synaptic Pruning by Microglia in Epilepsy

Structural and functional collapse of the balance between excitatory (E) and inhibitory (I) synapses, i.e., synaptic E/I balance, underlies the pathogeneses of various central nervous system (CNS) disorders. In epilepsy, the synaptic E/I balance tips toward excitation; thus, most of the existing epileptic remedies have focused on how to directly suppress the activity of neurons. However, because as many as 30% of patients with epilepsy are drug resistant, the discovery of new therapeutic targets is strongly desired. Recently, the roles of glial cells in epilepsy have gained attention because glial cells manipulate synaptic structures and functions in addition to supporting neuronal survival and growth. Among glial cells, microglia, which are brain-resident immune cells, have been shown to mediate inflammation, neuronal death and aberrant neurogenesis after epileptic seizures. However, few studies have investigated the involvement of synaptic pruning—one of the most important roles of microglia—in the epileptic brain. In this review, we propose and discuss the hypothesis that synaptic pruning by microglia is enhanced in the epileptic brain, drawing upon the findings of previous studies. We further discuss the possibility that aberrant synaptic pruning by microglia induces synaptic E/I imbalance, promoting the development and aggravation of epilepsy.

Microglia in neuropathology caused by protozoan parasites

Involvement of the central nervous system (CNS) is the most severe consequence of some parasitic infections. Protozoal infections comprise a group of diseases that together affect billions of people worldwide and, according to the World Health Organization, are responsible for more than 500000 deaths annually. They include African and American trypanosomiasis, leishmaniasis, malaria, toxoplasmosis, and amoebiasis. Mechanisms underlying invasion of the brain parenchyma by protozoa are not well understood and may depend on parasite nature: a vascular invasion route is most common. Immunosuppression favors parasite invasion into the CNS and therefore the host immune response plays a pivotal role in the development of a neuropathology in these infectious diseases. In the brain, microglia are the resident immune cells active in defense against pathogens that target the CNS. Beside their direct role in innate immunity, they also play a principal role in coordinating the trafficking and recruitment of other immune cells from the periphery to the CNS. Despite their evident involvement in the neuropathology of protozoan infections, little attention has given to microglia-parasite interactions. This review describes the most prominent features of microglial cells and protozoan parasites and summarizes the most recent information regarding the reaction of microglial cells to parasitic infections. We highlight the involvement of the periphery-brain axis and emphasize possible scenarios for microglia-parasite interactions.

Electrophysiological Investigation of Microglia

Although microglial cells are not electrically excitable, they express a large repertoire of ion channels that are activated by voltage, stretch, extracellular ligands, or intracellular pathways (e.g. Ca²⁺, G-proteins). The patch-clamp technique is the electrophysiological method of choice to study these channels whose expression varies largely in pathological conditions but also during normal development and aging. This chapter focuses on protocols allowing the recording and the analysis of these channels in acute brain slices, with a particular emphasis on the study of channels activated by extracellular ligands.

Amentoflavone Affects Epileptogenesis and Exerts Neuroprotective Effects by Inhibiting NLRP3 Inflammasome

Brain inflammation is one of the main causes of epileptogenesis, a chronic process triggered by various insults, including genetic or acquired factors that enhance susceptibility to seizures. Amentoflavone, a naturally occurring biflavonoid compound that has anti-inflammatory effects, exerts neuroprotective effects against nervous system diseases. In the present study, we aimed to investigate the effects of amentoflavone on epilepsy in vivo and in vitro and elucidate the underlying mechanism. The chronic epilepsy model and BV2 microglial cellular inflammation model were established by pentylenetetrazole (PTZ) kindling or lipopolysaccharide (LPS) stimulation. Cognitive dysfunction was tested by Morris water maze while hippocampal neuronal apoptosis was evaluated by immunofluorescence staining. The levels of nucleotide oligomerization domain-like receptor protein 3 (NLRP3) inflammasome complexes and inflammatory cytokines were determined using quantitative real-time polymerase chain reaction, Western blotting, immunofluorescence staining, and enzyme-linked immunosorbent assay. Amentoflavone reduced seizure susceptibility, minimized PTZ-induced cognitive dysfunction, and blocked the apoptosis of hippocampal neurons in PTZ-induced kindling mice. Amentoflavone also inhibited the activation of the NLRP3 inflammasome and decreased the levels of inflammatory cytokines in the hippocampus of PTZ-induced kindling mice. Additionally, amentoflavone could alleviate the LPS-induced inflammatory response by inhibiting the NLRP3 inflammasome in LPS-induced BV2 microglial cells. Our results indicated that amentoflavone affects epileptogenesis and exerts neuroprotective effects by inhibiting the NLRP3 inflammasome and, thus, mediating the inflammatory process in PTZ-induced kindling mice and LPS-induced BV2 microglial cells. Therefore, amentoflavone may be a potential treatment option for epilepsy.

Microglia-neuron crosstalk: Signaling mechanism and control of synaptic transmission

The continuous crosstalk between microglia and neurons is required for microglia housekeeping functions and contributes to brain homeostasis. Through these exchanges, microglia take part in crucial brain functions, including development and plasticity. The alteration of neuron-microglia communication contributes to brain disease states with consequences, ranging from synaptic function to neuronal survival. This review focuses on the signaling pathways responsible for neuron-microglia crosstalk, highlighting their physiological roles and their alteration or specific involvement in disease. In particular, we discuss studies, establishing how these signaling allow microglial cells to control relevant physiological functions during brain development, including synaptic formation and circuit refinement. In addition, we highlight how microglia and neurons interact functionally to regulate highly dynamical synaptic functions. Microglia are able to release several signaling molecules involved in the regulation of synaptic activity and plasticity. On the other side, molecules of neuronal origin control microglial processes motility in an activity-dependent manner. Indeed, the continuous crosstalk between microglia and neurons is required for the sensing and housekeeping functions of microglia and contributes to the maintenance of brain homeostasis and, particularly, to the sculpting of neuronal connections during development. These interactions lay on the delicate edge between physiological processes and homeostasis alteration in pathology and are themselves altered during neuroinflammation. The full description of these processes could be fundamental for understanding brain functioning in health and disease.

Human Microglia Seize the Chance to be Different

Human microglia regional heterogeneity and phenotypes determined by multiplexed single-cell mass cytometry

Böttcher C, Schlickeiser S, Sneeboer MAM, et al. Nat Neurosci. 2019;22(1):78-90. doi:10.1038/s41593-018-0290-2

Microglia, the specialized innate immune cells of the central nervous system, play crucial roles in neural development and function. Different phenotypes and functions have been ascribed to rodent microglia, but little is known about human microglia (huMG) heterogeneity. Difficulties in procuring huMG and their susceptibility to cryopreservation damage have limited large-scale studies. Here we applied multiplexed mass cytometry for a comprehensive characterization of postmortem huMG (103-104 cells). We determined expression levels of 57 markers on huMG isolated from up to 5 different brain regions of 9 donors. We identified the phenotypic signature of huMG, which was distinct from peripheral myeloid cells but was comparable to fresh huMG. We detected microglia regional heterogeneity using a hybrid workflow combining Cytobank and R/Bioconductor for multidimensional data analysis. Together, these methodologies allowed us to perform high-dimensional, large-scale Immunophenotyping of huMG at the single-cell level, which facilitates their unambiguous profiling in health and disease.

Transcriptomics and Live Imaging to Define Functional Phenotypes of Microglia in Pathological Human Tissue

Human microglia, as those of rodents, may possess multiple functional phenotypes. Here we present protocols to determine elements of these microglial phenotypes obtained after therapeutic excision of brain tissue from patients with epilepsies of the temporal lobe and cortical gliomas. This technique permits to identify microglia, to determine their shape and expression of state-specific markers, to measure resting and induced motilities, to define a human microglial transcriptome, and to determine how it changes after a seizure.