Noninflammatory Changes of Microglia Are Sufficient to Cause Epilepsy

Loss of insulin signaling in microglia impairs cellular uptake of Aβ and neuroinflammatory response exacerbating AD-like neuropathology

Insulin receptors are present on cells throughout the body, including the brain. Dysregulation of insulin signaling in neurons and astrocytes has been implicated in altered mood, cognition, and the pathogenesis of Alzheimer's disease (AD). To define the role of insulin signaling in microglia, the primary phagocytes in the brain critical for maintenance and damage repair, we created mice with an inducible microglia-specific insulin receptor knockout (MG-IRKO). RiboTag profiling of microglial mRNAs revealed that loss of insulin signaling results in alterations of gene expression in pathways related to innate immunity and cellular metabolism. In vitro, loss of insulin signaling in microglia results in metabolic reprogramming with an increase in glycolysis and impaired uptake of Aβ. In vivo, MG-IRKO mice exhibit alterations in mood and social behavior, and when crossed with the 5xFAD mouse model of AD, the resultant mice exhibit increased levels of Aβ plaque and elevated neuroinflammation. Thus, insulin signaling in microglia plays a key role in microglial cellular metabolism and the ability of the cells to take up Aβ, such that reduced insulin signaling in microglia alters mood and social behavior and accelerates AD pathogenesis. Together, these data indicate key roles of insulin action in microglia and the potential of targeting insulin signaling in microglia in treatment of AD.

Microglial MS4A4A Protects against Epileptic Seizures in Alzheimer's Disease

Alzheimer's disease (AD) is a predominant neurodegenerative disorder worldwide, with epileptic seizures being a common comorbidity that can exacerbate cognitive deterioration in affected individuals, thus highlighting the importance of early therapeutic intervention. It is determined that deletion of Ms4a4a, an AD-associated gene, exacerbates seizures in amyloid β (Aβ)-driven AD mouse model. MS4A4A is significantly upregulated in brain lesions in patients with epilepsy. Single-cell sequencing reveals that MS4A4A is highly expressed in microglia within these lesions, linked to enhanced phagocytic activity. Mechanistic investigation delineates that deletion of Ms4a4a impairs microglial phagocytosis, accompanied by diminished calcium influx and disruptions in mitochondrial metabolic fitness. The cytosolic fragment of Ms4a4a is anchored to the cytoskeletal components, supporting its critical role in mediating phagocytosis. Induction of Ms4a4a through central delivery of LNP-Il4 alleviates seizure conditions. Collectively, these findings identify Ms4a4a as a potential therapeutic target for managing seizures in AD treatment.

Inactivation of microglial LXRβ in early postnatal mice impairs microglia homeostasis and causes long-lasting cognitive dysfunction

Microglia, the largest population of brain immune cells, play an essential role in regulating neuroinflammation by removing foreign materials and debris and in cognition by pruning synapses. Since liver X receptor β (LXRβ) has been identified as a regulator of microglial homeostasis, this study examined whether its removal from microglia affects neuroinflammation and cognitive function. We used a cell-specific tamoxifen-inducible Cre-loxP-mediated recombination to remove LXRβ from microglia specifically. We now report that ablation of LXRβ in microglia in early postnatal life led to a reduction in microglial numbers, distinct morphological changes indicative of microglial activation, and enhanced synapse engulfment accompanied by cognitive deficits. Removal of LXRβ from microglia in adult mice caused no cognitive defects. RNAseq analysis of microglia revealed that loss of LXRβ led to reduced expression of SAll1, a master regulator of microglial homeostasis, while increasing expression of genes associated with microglial activation and CNS disease. This study demonstrates distinctly different functions of microglial LXRβ in developing and adult mice and points to long-term consequences of defective LXRβ signaling in microglia in early life.

Glial changes in the dentate gyrus of neuronal-specific PTEN knockout mice correlate with changes in cell proliferation

Dysregulated hippocampal neurogenesis is a feature of temporal lobe epilepsy (TLE), marked by increased neuronal proliferation. The tumor suppressor gene phosphatase and tensin homolog (PTEN) regulates neuronal proliferation, and its deletion is implicated in TLE. We have previously shown that deletion of neuronal subset-specific (NS)-PTEN in mice increases the number of proliferating cells throughout the dentate gyrus, including subregions that are typically devoid of neurons but rich in glial cells, most notably the Hilus and Molecular Layer. In this study, we hypothesized that NS-PTEN knockout mice would exhibit increased numbers of microglia and astrocytes in these same dentate gyrus subregions. We performed immunohistochemistry for Iba1 (microglia) and GFAP (reactive astrocytes) on wild-type and NS-PTEN knockout mice at 4 and 10 weeks of age. Our data reveal that NS-PTEN knockout mice exhibit increased Iba1+ cell density at both ages, with some male-specific effects. Subregional analysis of the dentate gyrus showed that at 4 weeks, NS-PTEN knockout mice had greater Iba1+ cell density in the Granule Cell Layer (GCL) and Hilus, and at 10 weeks, increases were observed in the GCL, Hilus, and Molecular Layer. Additionally, we observed an increased number of microglia with an amoeboid morphology and fewer with thin, ramified processes. Contrast to Iba1+ microglia, GFAP+ reactive astrocytes were localized to the neurogenic GCL. Importantly, increases in both glial types strongly correlated with heightened cell proliferation (Ki67+ cells), as reported in our previous study, underscoring the role of glial cells in the spatial dysregulation of neurogenesis in NS-PTEN knockout mice.

GPNMB and ATP6V1A interact to mediate microglia phagocytosis of multiple types of pathological particles

Pronounced elevation of glycoprotein non-metastatic melanoma B (GPNMB) is a common phenomenon in a variety of brain diseases, but the expression patterns, functions, and molecular signaling of GPNMB have not been well studied. Here, we showed that pathological factors, including neuronal degeneration caused by seizures, caspase-3-induced neuronal apoptosis, neuronal debris, and β-amyloid, induced "on-demand" GPNMB expression in hippocampal microglia. Genetic ablation of GPNMB did not affect acute seizures but worsened chronic epileptogenesis. We found that GPNMB functioned in phagocytosis, deficiency of which resulted in defects in both phagocytic engulfment and degradation. GPNMB could be internalized into cells, where it wrapped engulfed pathogenic particles and presented them to lysosomes through interaction with lysosomal vacuolar-type proton ATPase catalytic subunit A (ATP6V1A). Activating ATP6V1A was able to rescue GPNMB-deficiency-caused phagocytosis impairment. Thus, microglial GPNMB-ATP6V1A might be a common treatment target of a batch of chronic neurological disorders, and clearing the degenerative neurons might be more valuable than reserving them to protect the brain.

Immunity and neuroinflammation in early stages of life and epilepsy

The immune system is crucial for the correct brain development, and recent findings also point toward central control of immune response. As the immune system is not fully developed at birth, the early years become an important window for infections and for the development of epilepsy. Both central and even peripheral inflammation may impact brain function, promoting opening of the blood-brain/blood and cerebrospinal barriers and allowing entry of immune cells and cytokines, which in turn may affect neuron function and connections. The resident brain immune cells, microglia, besides providing protection, also affect neurons, myelination, and astrocyte function. They may, via the complement system, remove synapses, both physiologically and pathologically. After seizures during development, activated microglia releases proinflammatory molecules, which are detrimental for neurons, and inhibition of microglial activation shows promising antiepileptogenic effects. In addition to cytokines, seizures and excessive excitability stimulate calpain 2 expression, which can promote neuron loss and contribute to amplification of inflammatory responses via stimulation of proinflammatory cytokines. In summary, the immature immune system during postnatal early life may be an important target for the development of long-desired antiepileptogenic drugs.

CX3CL1-CX3CR1 pathway mediates hyperthermia-induced microglial processes retraction

In infants, high fever is associated with robust microglial morphological changes, including process retraction and soma enlargement, which contribute to fever-induced seizures. The molecular mechanisms underlying dynamic process retraction during hyperthermia remain poorly understood. Using a hyperthermia-induced microglial activation model in postnatal day 8 mice, we identified the CX3CL1-CX3CR1 interaction as a key regulator of process retraction. The CX3CL1 is mainly cleaved by metalloproteases ADAM10 under hyperthermia stimulation. Pharmacological inhibition or genetic knockdown of ADAM10 prevented microglial process retraction. Hyperthermia is known to induce the release of glutamate and activation of the NMDA receptor. We found that NMDA mimicked the effects of hyperthermia on microglia, while the NMDA blocker MK801 attenuated hyperthermia-induced process retraction. Collectively, our findings suggest that the CX3CL1-CX3CR1 pathway plays a critical role in mediating dynamic microglial process retraction in response to hyperthermia.

Upregulation of Cathepsin S Expression Contributes to Neuronal Damage Following Kainic Acid-Induced Status Epilepticus

Status epilepticus (SE) is a life-threatening neurological emergency characterized by persistent seizures, leading to brain damage that increases the risk of recurrent seizures due to abnormal electrical impulses produced by damaged neurons. However, the molecular mechanism by which convulsive SE leads to neuronal damage is not completely understood. Cathepsin S (Ctss), a lysosomal cysteine protease, has been implicated in secondary injury after traumatic brain injury. This study sought to explore whether Ctss is also involved in SE-induced neuronal damage in the hippocampus. Immunohistochemistry and Western blotting were utilized to detect the expression of Ctss in the hippocampal subregions of male C57BL/6J mice at various times following kainic acid (KA)-induced SE. The reactivity of microglia was assessed using immunohistochemistry, and Fluoro-Jade C (FJC) staining was employed to identify damaged neurons. We found that the mature form of Ctss is barely observed in naïve adult (12-week-old) mouse hippocampus, but its expression is significantly evident at 50 weeks of age. In adult mice, the expression of both pro-and mature forms of Ctss in the hippocampal CA3 region was increased as early as 16 h following KA-induced SE. The increased Ctss immunoreactivity was mainly found in microglia following KA-induced SE. The damaged neurons visualized by FJC staining were prominent in the CA3 region at 16 h following KA-induced SE. Ctss knockdown did not affect KA-induced behavioral seizures but significantly reduced SE-induced microglia activation and neuronal damage. An increase in chemokine CX3C motif ligand 1 (CX3CL1) immunoreactivity on microglia was observed following KA-induced SE, and CX3C motif chemokine receptor 1 (CX3CR1) antagonist AZD8797 treatment significantly attenuated SE-induced microglia activation and neuronal damage. Altogether, these results indicate a crucial role of Ctss in SE-induced neuronal damage, possibly through CXC3L1-mediated microglial activation, and provide a new perspective for preventing SE-induced neuronal damage.

Age differences in organophosphorus nerve agent-induced seizure, blood brain barrier integrity, and neurodegeneration in midazolam-treated rats

Exposure to organophosphorus nerve agents irreversibly inhibits acetylcholinesterase and may lead to cholinergic crisis and seizures. Although benzodiazepines are the standard of care after nerve agent-induced status epilepticus, when treatment is delayed for up to 30 min or more, refractory status epilepticus can develop. Adult male rodents are often utilized for evaluation of therapeutic efficacy against nerve agent exposure. However, there may be age and sex differences in toxicity and in therapeutic response. We previously reported that juvenile rats are less susceptible to the lethal effects of soman compared to adults, while pups are the most susceptible. Here, we report on age and sex differences in delayed midazolam treatment efficacy on survival, seizures and brain pathology. Male and female pups, juvenile and adult rats were exposed to an equitoxic dose of soman and treated with atropine sulfate and the oxime asoxime chloride (HI-6 dimethanesulphonate) 1 min after exposure and with midazolam 40 min after seizure onset, determined by EEG in juvenile and adult rats, and by behavior in pups. Survival, seizure data, and spontaneous recurrent seizures were evaluated. Brains were processed to assess neurodegeneration, neuroinflammation, and blood brain barrier (BBB) integrity. Juvenile and adult rats exposed to soman and treated with midazolam had BBB disruption, epileptogenesis, neurodegeneration, microglial activation, and astrogliosis; adult rats had poorer outcomes. Pups and juvenile rats exposed to soman had poor survival prior to midazolam treatment but most survived once treated; overall, neurodegeneration or disrupted BBB integrity was not detected in midazolam-treated pups. We found that age is a determinant factor in soman-induced toxicity and response to standard medical countermeasures. In addition, we observed sex differences in response to soman in juveniles and males with respect to body weight growth curves and in neuronal loss in juveniles and adults. Adjunct therapies to midazolam are warranted and it is important to evaluate both age and sex as factors in therapeutic response.

Microglial Regulation of Neural Networks in Neuropsychiatric Disorders

Microglia serve as vital innate immune cells in the central nervous system, playing crucial roles in the generation and development of brain neurons, as well as mediating a series of immune and inflammatory responses. The morphologic transitions of microglia are closely linked to their function. With the advent of single-cell sequencing technology, the diversity of microglial subtypes is increasingly recognized. The intricate interactions between microglia and neuronal networks have significant implications for psychiatric disorders and neurodegenerative diseases. A deeper investigation of microglia in neurologic diseases such as Alzheimer disease, depression, and epilepsy can provide valuable insights in understanding the pathogenesis of diseases and exploring novel therapeutic strategies, thereby addressing issues related to central nervous system disorders.

Molecular biomarkers of glial activation and injury in epilepsy

Increasing evidence from fluid biopsies suggests activation and injury of glial cells in epilepsy. The prevalence of clinical and subclinical seizures in neurodegenerative conditions such as Alzheimer's disease, frontotemporal dementia, and others merits review and comparison of the effects of seizures on glial markers in epilepsy and neurodegenerative diseases with concomitant seizures. Herein, we revisit preclinical and clinical reports of alterations in glial proteins in cerebrospinal fluid and blood associated with various types of epilepsy. We consider shared and distinct characteristics of changes in different age groups and sexes, in humans and animal models of epilepsy, and compare them with those reported in biofluids in neurodegenerative diseases. Our analysis indicates a significant overlap of glial response in these prevalent neurological conditions.

Microglial modulation of neuronal network function and plasticity

Microglia are the resident immune cells of the central nervous system (CNS), which have been classically viewed as involved in CNS responses to damage and tissue repair. However, microglia are constantly sensing neuronal network activity and changes in the CNS milieu, establishing complex state-dependent microglia-neuron interactions that impact their functions. By doing so, microglia perform a wide range of physiological roles, including brain homeostasis maintenance, control of neural connectivity, network function modulation, as well as functional and morphological plasticity regulation in health and disease. Here, the author reviews recent evidence of the modulations induced by microglia, a highly heterogeneous cell type, on synaptic and intrinsic neuronal properties, and on neuronal network patterns during perinatal development and adulthood. The reviewed evidence clearly indicates that microglia are important, if not essential, for brain function and plasticity in both health and disease.

Large Animal Epilepsy Model Platform: Kainic Acid Porcine Model of Mesial Temporal Lobe Epilepsy

Objective

Translational animal models are essential for advancing neuromodulation therapies for drug-resistant epilepsy. Large animal epilepsy models that accommodate implantable neuromodulation devices designed for humans are needed to advance electrical brain stimulation and sensing applicaitons. We establish a kainic acid (KA) porcine model of mesial temporal lobe epilepsy (mTLE) for pre-clinical research and development of novel stimulating therapies.

Methods

We developed a platform integrating MRI-guided stereotactic electrode and chemotoxin delivery with intraoperative and long-term electrophysiology monitoring. Six domestic pigs underwent MRI-guided stereotactic implantation of bilateral hippocampal (HPC) and anterior nucleus of the thalamus (ANT) electrodes, followed by KA infusion into the right HPC. Local field potential (LFP) monitoring was conducted intraoperatively with a bedside workstation and chronically with an implantable neural sensing and recording (INSR) device designed for human use. Cresyl violet staining, commonly used to visualize neurons, was used to assess the neuropathophysiological effect of the KA infusion.

Results

KA infusion led to the emergence of interictal epileptiform discharges (IEDs), acute status epilepticus (SE), and spontaneous seizure activity. Acute IEDs and electrographic SE was observed in all cases, with two pigs euthanized due to uncontrolled seizures. Of the surviving four pigs, spontaneous IEDs were detected in all, and spontaneous seizures within 2 weeks were observed in three. Seizures in freely behaving animals varied from subclinical events to focal and generalized tonic-clonic seizures. Single pulse evoked response potentials (SPEP) measured in two pigs demonstrated functional connectivity between ANT and HPC circuits. Neuronal disorganization and loss, both major components of mTLE neuropathology were observed.

Significance

The KA-induced porcine mTLE model supports the feasibility of testing human neuromodulation devices and provides a translational platform for studying therapeutic electrical sensing and stimulation. In particular, the model should be useful for advancing automated seizure diaries, closed-loop and adaptive therapies targeting Papez circuit nodes.

Microglial TREM2 promotes phagocytic clearance of damaged neurons after status epilepticus

In the central nervous system, triggering receptor expressed on myeloid cells 2 (TREM2) is exclusively expressed by microglia and is critical for microglial proliferation, migration, and phagocytosis. Microglial TREM2 plays an important role in neurodegenerative diseases, such as Alzheimer's disease and amyotrophic lateral sclerosis. However, little is known about how TREM2 affects microglial function within epileptogenesis. To investigate this, we utilized male TREM2 knockout (KO) mice within the intra-amygdala kainic acid seizure model. Electroencephalographic analysis, immunocytochemistry, and RNA sequencing revealed that TREM2 deficiency significantly promoted seizure-induced pathology. We found that TREM2 KO increased both the severity of acute status epilepticus and the number of spontaneous recurrent seizures characteristic of chronic focal epilepsy. Phagocytic clearance of damaged neurons by microglia was also impaired by TREM2 KO and reduced phagocytic activity correlated with increased spontaneous seizures. Analysis of human tissue from patients who underwent surgical resection for drug resistant temporal lobe epilepsy also showed a negative correlation between expression of the microglial phagocytic marker CD68 and focal to bilateral tonic-clonic generalized seizure history. These results indicate that microglial TREM2 and phagocytic activity are important to epileptogenic pathology.

WONOEP appraisal: The role of glial cells in focal malformations associated with early onset epilepsies

Epilepsy represents a common neurological disorder in patients with developmental brain lesions, particularly in association with malformations of cortical development and low-grade glioneuronal tumors. In these diseases, genetic and molecular alterations in neurons are increasingly discovered that can trigger abnormalities in the neuronal network, leading to higher neuronal excitability levels. However, the mechanisms underlying epilepsy cannot rely solely on assessing the neuronal component. Growing evidence has revealed the high degree of complexity underlying epileptogenic processes, in which glial cells emerge as potential modulators of neuronal activity. Understanding the role of glial cells in developmental brain lesions such as malformations of cortical development and low-grade glioneuronal tumors is crucial due to the high degree of pharmacoresistance characteristic of these lesions. This has prompted research to investigate the role of glial and immune cells in epileptiform activity to find new therapeutic targets that could be used as combinatorial drug therapy. In a special session of the XVI Workshop of the Neurobiology of Epilepsy (WONOEP, Talloires, France, July 2022) organized by the Neurobiology Commission of the International League Against Epilepsy, we discussed the evidence exploring the genetic and molecular mechanisms of glial cells and immune response and their implications in the pathogenesis of neurodevelopmental pathologies associated with early life epilepsies.

Microglia-Impaired Phagocytosis Contributes to the Epileptogenesis in a Mouse Model of Dravet Syndrome

Dravet syndrome (DS) is a genetic disorder caused by a deficit in the Nav1.1 channel, leading to drug-resistant epilepsy. The Nav1.1 channel plays a crucial role in microglial cell activation, and microglia are recognized as key mediators of seizures. In this study, we explored the role of microglia in DS-related epileptogenesis using a knock-in mouse model (Scn1aE1099X/+) that mimics a subset of DS patients. In these DS mice, we observed a significant downregulation of the Nav1.1 channel in microglia. This channel deficit led microglia to adopt a pro-inflammatory state in their quiescent phase. In the LPS-activated state, microglia predominantly exhibited an intermediate morphology rather than the expected fully activated form. The reduced expression of pro-inflammatory cytokines was detected in microglia following treatment with LPS. Notably, we found a significant decrease in the phagocytic ability of microglia in DS mice. Electrophysiological studies revealed an increased immature synaptic activity in the dentate gyrus in DS mice. The impaired microglial phagocytosis of damaged cells, combined with reduced cytokine secretion, may result in an excess of immature synaptic connections, neuronal hyperexcitation, and the formation of abnormal neural circuits in the hippocampus of Scn1aE1099X/+ mice. These changes could potentially contribute to mechanisms relevant to epileptogenesis in DS.

(+)-Borneol inhibits neuroinflammation and M1 phenotype polarization of microglia in epileptogenesis through the TLR4-NFκB signaling pathway

Objective

To investigate the effect of (+)-borneol on neuroinflammation and microglia phenotype polarization in epileptogenesis and its possible mechanism.

Methods

Based on mouse models of status epilepticus (SE) induced by pilocarpine, and treated with 15 mg/kg (+)-borneol, western-blot was used to detect the expressions of NeuN, Iba-1, TLR4, p65 and p-p65 in the hippocampus. Immunofluorescence was used to detect the expression of apoptosis-related proteins Bax and Bcl-2. To explore the effect of (+)-borneol on microglia in vitro, we used the kainic acid-induced microglia model and the concentration of (+)-borneol was 25 μM according to CCK-8 results. The levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-10 (IL-10) in the supernatant of each group was detected by ELISA. The nitric oxide (NO) content in the supernatant was detected by Griess method. The expressions of Iba-1 and TLR4-NFκB signaling pathway-related proteins (TLR4, p65, p-p65) were detected by Western-Blot. Immunofluorescence was used to detect microglia's M1 and M2 phenotype polarization and the expression of Iba-1 and TLR4.

Results

(+)-borneol reduced hippocampal neuronal injury, apoptosis, and microglia activation by inhibiting the TLR-NFκB signaling pathway in SE mice. TLR4 agonist LPS partially reversed the neuroprotective effect of (+)-borneol. In the KA-induced microglia model, (+)-borneol inhibited microglia activation, M1 phenotype polarization, and secretion of pro-inflammatory cytokines through the TLR4-NFκB signaling pathway. LPS treatment inhibited the therapeutic effects of (+)-borneol.

Conclusion

(+)-borneol inhibits microglial neuroinflammation and M1 phenotype polarization through TLR4-NFκB signaling pathway and reduces neuronal damage and apoptosis in SE mice. Therefore, (+)-borneol may be a potential drug for epilepsy modification therapy.

Constitutive expression of CX3CR1-BAC-Cre introduces minimal off-target effects in microglia

CX3CR1-Cre mouse lines have produced important advancements in our understanding of microglial biology. Recent studies have demonstrated the adverse effects of tamoxifen-induced CX3CR1-Cre expression during development, which include changes in microglial density, phenotype, and DNA damage, as well as anxiety-like behavior. However, the unintended effects of constitutive CX3CR1-BAC-Cre expression remain unexplored. Here, we characterized the effects of CX3CR1-BAC-Cre expression on microglia in CX3CR1-BAC-Cre+/- and CX3CR1-BAC-Cre-/- male and female littermates during early postnatal development and adulthood in multiple brain regions. Additionally, we performed anxiety-like behavior tests to assess changes caused by Cre expression. We found that CX3CR1-BAC-Cre expression causes subtle region- and sex-specific changes in microglial density, volume, and morphology during development, but these changes normalized by adulthood in all brain regions except the hippocampus. No behavioral effects were found. Our findings suggest that the constitutive-Cre model might be less detrimental than the inducible model, and highlight the need for proper controls.

NS-Pten knockout mice exhibit sex and hippocampal subregion-specific increases in microglia/macrophage density

Seizures induce hippocampal subregion dependent enhancements in microglia/macrophage phagocytosis and cytokine release that may contribute to the development of epilepsy. As a model of hyperactive mTOR induced epilepsy, neuronal subset specific phosphatase and tensin homolog (NS-Pten) knockout (KO) mice exhibit hyperactive mTOR signaling in the hippocampus, seizures that progress with age, and enhanced hippocampal microglia/macrophage activation. However, it is unknown where microglia/macrophages are most active within the hippocampus of NS-Pten KO mice. We quantified the density of IBA1 positive microglia/macrophages in the CA1, CA2/3, and dentate gyrus of NS-Pten KO and wildtype (WT) male and female mice at 4, 10, and 15 weeks of age. NS-Pten KO mice exhibited an overall increase in the number of IBA1 positive microglia/macrophages in each subregion and in the entire hippocampus. After accounting for differences in size, the whole hippocampus of NS-Pten KO mice still exhibited an increased density of IBA1 positive microglia/macrophages. Subregion analyses showed that this increase was restricted to the dentate gyrus of both male and female NS-Pten KO mice and to the CA1 of male NS-Pten KO mice. These data suggest enhanced microglia/macrophage activity may occur in the NS-Pten KO mice in a hippocampal subregion and sex-dependent manner. Future work should seek to determine whether these region-specific increases in microgliosis play a role in the progression of epilepsy in this model.

Rethinking the role of microglia in obesity

Microglia are the macrophages of the central nervous system (CNS), implying their role in maintaining brain homeostasis. To achieve this, these cells are sensitive to a plethora of endogenous and exogenous signals, such as neuronal activity, cellular debris, hormones, and pathological patterns, among many others. More recent research suggests that microglia are highly responsive to nutrients and dietary variations. In this context, numerous studies have demonstrated their significant role in the development of obesity under calorie surfeit. Because many reviews already exist on this topic, we have chosen to present the state of our reflections on various concepts put forth in the literature, bringing a new perspective whenever possible. Our literature review focuses on studies conducted in the arcuate nucleus of the hypothalamus, a key structure in the control of food intake. Specifically, we present the recent data available on the modifications of microglial energy metabolism following the consumption of an obesogenic diet and their consequences on hypothalamic neuron activity. We also highlight the studies unraveling the mechanisms underlying obesity-related sexual dimorphism. The review concludes with a list of questions that remain to be addressed in the field to achieve a comprehensive understanding of the role of microglia in the regulation of body energy metabolism. This article is part of the Special Issue on "Microglia".

Role of ginsenoside Rb1 in attenuating depression-like symptoms through astrocytic and microglial complement C3 pathway

Ginsenoside Rb1, known as gypenoside III, exerts antidepressant-like effects in previous studies. It has also been indicated that ginsenoside Rb1 regulated neuroinflammation via inhibiting NF-κB signaling. According to the evidence that astrocytes can regulate microglia and neuroinflammation by secreting complement C3, the present study aimed to demonstrate the molecular mechanisms underlying ginsenoside Rb1-induced antidepressant-like effects from the astrocytic and microglial complement C3 pathway. The complement C3 mediated mechanism of ginsenoside Rb1 was investigated in mice exposed to chronic restraint stress (CRS). The results showed that ginsenoside Rb1 reversed the depressive-like behaviors in CRS. Treatment with ginsenoside Rb1 reduced both the number of astrocytes and microglia. In addition, ginsenoside Rb1 suppressed TLR4/NF-κB/C3 signaling in the astrocytes of the hippocampus. Furthermore, ginsenoside Rb1 attenuated the contents of synaptic protein including synaptophysin and PSD95 in microglia, suggesting the inhibition of microglia-mediated synaptic elimination caused by CRS. Importantly, ginsenoside Rb1 also maintained the dendritic spines in mice. In conclusion, our results demonstrate that ginsenoside Rb1 produces the antidepressant-like effects by inhibiting astrocyte TLR4/NF-κB/C3 signaling to covert microglia from a pro-inflammatory phenotype (amoeboid) towards an anti-inflammatory phenotype (ramified), which inhibit the synaptic pruning in the hippocampus.

Acute rapamycin treatment reveals novel mechanisms of behavioral, physiological, and functional dysfunction in a maternal inflammation mouse model of autism and sensory over-responsivity

Maternal inflammatory response (MIR) during early gestation in mice induces a cascade of physiological and behavioral changes that have been associated with autism spectrum disorder (ASD). In a prior study and the current one, we find that mild MIR results in chronic systemic and neuro-inflammation, mTOR pathway activation, mild brain overgrowth followed by regionally specific volumetric changes, sensory processing dysregulation, and social and repetitive behavior abnormalities. Prior studies of rapamycin treatment in autism models have focused on chronic treatments that might be expected to alter or prevent physical brain changes. Here, we have focused on the acute effects of rapamycin to uncover novel mechanisms of dysfunction and related to mTOR pathway signaling. We find that within 2 hours, rapamycin treatment could rapidly rescue neuronal hyper-excitability, seizure susceptibility, functional network connectivity and brain community structure, and repetitive behaviors and sensory over-responsivity in adult offspring with persistent brain overgrowth. These CNS-mediated effects are also associated with alteration of the expression of several ASD-,ion channel-, and epilepsy-associated genes, in the same time frame. Our findings suggest that mTOR dysregulation in MIR offspring is a key contributor to various levels of brain dysfunction, including neuronal excitability, altered gene expression in multiple cell types, sensory functional network connectivity, and modulation of information flow. However, we demonstrate that the adult MIR brain is also amenable to rapid normalization of these functional changes which results in the rescue of both core and comorbid ASD behaviors in adult animals without requiring long-term physical alterations to the brain. Thus, restoring excitatory/inhibitory imbalance and sensory functional network modularity may be important targets for therapeutically addressing both primary sensory and social behavior phenotypes, and compensatory repetitive behavior phenotypes.

The cGAS/STING signaling pathway is involved in sevoflurane induced neuronal necroptosis via regulating microglia M1 polarization

OBJECTIVE

The specific mechanisms of sevoflurane-induced neurotoxicity are still undetermined. The aim of the current study was to investigate the role of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway in sevoflurane-induced neuronal necroptosis.

METHODS

BV2 microglial cells were divided into a control group and a 4% sevoflurane exposure group. Western blotting was used to detect expression of the M1 polarization marker inducible nitric oxide synthase (iNOS). RNA was collected for RNA sequencing analysis. After STING knockdown in microglia, western blotting was performed to examine expression of the pro-inflammatory markers CD16 and CD32. The tumor necrosis factor-α (TNF-α) level in media was detected using an enzyme-linked immunosorbent assay. BV2 microglia conditioned media was collected to incubate HT22 neuronal cells, and their cell activity was measured using a CCK8 assay. Calcium was observed by fluorescence. Western blotting was performed to evaluate receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like (MLKL) expression. Neuronal necroptosis rate were detected using flow cytometry.

RESULTS

Sevoflurane exposure promoted microglial M1 polarization. The cGAS/STING pathway was screened and identified by RNA sequencing analysis of sevoflurane-exposed microglia and the control group. Compared with the control group, STING knockdown in microglia rescued the amoeboid morphology, inhibited TNF-α release, and significantly decreased iNOS, CD16, and CD32 expression. Moreover, calcium ions and necroptosis within neurons were decreased, and RIPK1, RIPK3, and p-MLKL expression was markedly decreased in microglia media culture with STING knockdown.

CONCLUSION

These results suggest that sevoflurane can regulate microglial M1 polarization by activating the cGAS/STING signaling pathway and increasing immune factor release, thus accelerating the neuronal necroptosis induced by calcium overload.

Aberrant activation of hippocampal astrocytes causes neuroinflammation and cognitive decline in mice

Reactive astrocytes are associated with neuroinflammation and cognitive decline in diverse neuropathologies; however, the underlying mechanisms are unclear. We used optogenetic and chemogenetic tools to identify the crucial roles of the hippocampal CA1 astrocytes in cognitive decline. Our results showed that repeated optogenetic stimulation of the hippocampal CA1 astrocytes induced cognitive impairment in mice and decreased synaptic long-term potentiation (LTP), which was accompanied by the appearance of inflammatory astrocytes. Mechanistic studies conducted using knockout animal models and hippocampal neuronal cultures showed that lipocalin-2 (LCN2), derived from reactive astrocytes, mediated neuroinflammation and induced cognitive impairment by decreasing the LTP through the reduction of neuronal NMDA receptors. Sustained chemogenetic stimulation of hippocampal astrocytes provided similar results. Conversely, these phenomena were attenuated by a metabolic inhibitor of astrocytes. Fiber photometry using GCaMP revealed a high level of hippocampal astrocyte activation in the neuroinflammation model. Our findings suggest that reactive astrocytes in the hippocampus are sufficient and required to induce cognitive decline through LCN2 release and synaptic modulation. This abnormal glial-neuron interaction may contribute to the pathogenesis of cognitive disturbances in neuroinflammation-associated brain conditions.

Microglia and infiltrating macrophages in ictogenesis and epileptogenesis

Phagocytes maintain homeostasis in a healthy brain. Upon injury, they are essential for repairing damaged tissue, recruiting other immune cells, and releasing cytokines as the first line of defense. However, there seems to be a delicate balance between the beneficial and detrimental effects of their activation in a seizing brain. Blocking the infiltration of peripheral phagocytes (macrophages) or their depletion can partially alleviate epileptic seizures and prevent the death of neurons in experimental models of epilepsy. However, the depletion of resident phagocytes in the brain (microglia) can aggravate disease outcomes. This review describes the role of resident microglia and peripheral infiltrating monocytes in animal models of acutely triggered seizures and epilepsy. Understanding the roles of phagocytes in ictogenesis and the time course of their activation and involvement in epileptogenesis and disease progression can offer us new biomarkers to identify patients at risk of developing epilepsy after a brain insult, as well as provide novel therapeutic targets for treating epilepsy.

mTOR and neuroinflammation in epilepsy: implications for disease progression and treatment

Epilepsy remains a major health concern as anti-seizure medications frequently fail, and there is currently no treatment to stop or prevent epileptogenesis, the process underlying the onset and progression of epilepsy. The identification of the pathological processes underlying epileptogenesis is instrumental to the development of drugs that may prevent the generation of seizures or control pharmaco-resistant seizures, which affect about 30% of patients. mTOR signalling and neuroinflammation have been recognized as critical pathways that are activated in brain cells in epilepsy. They represent a potential node of biological convergence in structural epilepsies with either a genetic or an acquired aetiology. Interventional studies in animal models and clinical studies give strong support to the involvement of each pathway in epilepsy. In this Review, we focus on available knowledge about the pathophysiological features of mTOR signalling and the neuroinflammatory brain response, and their interactions, in epilepsy. We discuss mitigation strategies for each pathway that display therapeutic effects in experimental and clinical epilepsy. A deeper understanding of these interconnected molecular cascades could enhance our strategies for managing epilepsy. This could pave the way for new treatments to fill the gaps in the development of preventative or disease-modifying drugs, thus overcoming the limitations of current symptomatic medications.

Bacteria Invade the Brain Following Sterile Intracortical Microelectrode Implantation

Brain-machine interface performance is largely affected by the neuroinflammatory responses resulting in large part from blood-brain barrier (BBB) damage following intracortical microelectrode implantation. Recent findings strongly suggest that certain gut bacterial constituents penetrate the BBB and are resident in various brain regions of rodents and humans, both in health and disease. Therefore, we hypothesized that damage to the BBB caused by microelectrode implantation could amplify dysregulation of the microbiome-gut-brain axis. Here, we report that bacteria, including those commonly found in the gut, enter the brain following intracortical microelectrode implantation in mice implanted with single-shank silicon microelectrodes. Systemic antibiotic treatment of mice implanted with microelectrodes to suppress bacteria resulted in differential expression of bacteria in the brain tissue and a reduced acute inflammatory response compared to untreated controls, correlating with temporary improvements in microelectrode recording performance. Long-term antibiotic treatment resulted in worsening microelectrode recording performance and dysregulation of neurodegenerative pathways. Fecal microbiome composition was similar between implanted mice and an implanted human, suggesting translational findings. However, a significant portion of invading bacteria was not resident in the brain or gut. Together, the current study established a paradigm-shifting mechanism that may contribute to chronic intracortical microelectrode recording performance and affect overall brain health following intracortical microelectrode implantation.

Microglial activation in the lateral amygdala promotes anxiety-like behaviors in mice with chronic moderate noise exposure

BACKGROUND

Long-term non-traumatic noise exposure, such as heavy traffic noise, can elicit emotional disorders in humans. However, the underlying neural substrate is still poorly understood.

METHODS

We exposed mice to moderate white noise for 28 days to induce anxiety-like behaviors, measured by open-field, elevated plus maze, and light-dark box tests. In vivo multi-electrode recordings in awake mice were used to examine neuronal activity. Chemogenetics were used to silence specific brain regions. Viral tracing, immunofluorescence, and confocal imaging were applied to define the neural circuit and characterize the morphology of microglia.

RESULTS

Exposure to moderate noise for 28 days at an 85-dB sound pressure level resulted in anxiety-like behaviors in open-field, elevated plus maze, and light-dark box tests. Viral tracing revealed that fibers projecting from the auditory cortex and auditory thalamus terminate in the lateral amygdala (LA). A noise-induced increase in spontaneous firing rates of the LA and blockade of noise-evoked anxiety-like behaviors by chemogenetic inhibition of LA glutamatergic neurons together confirmed that the LA plays a critical role in noise-induced anxiety. Noise-exposed animals were more vulnerable to anxiety induced by acute noise stressors than control mice. In addition to these behavioral abnormalities, ionized calcium-binding adaptor molecule 1 (Iba-1)-positive microglia in the LA underwent corresponding morphological modifications, including reduced process length and branching and increased soma size following noise exposure. Treatment with minocycline to suppress microglia inhibited noise-associated changes in microglial morphology, neuronal electrophysiological activity, and behavioral changes. Furthermore, microglia-mediated synaptic phagocytosis favored inhibitory synapses, which can cause an imbalance between excitation and inhibition, leading to anxiety-like behaviors.

CONCLUSIONS

Our study identifies LA microglial activation as a critical mediator of noise-induced anxiety-like behaviors, leading to neuronal and behavioral changes through selective synapse phagocytosis. Our results highlight the pivotal but previously unrecognized roles of LA microglia in chronic moderate noise-induced behavioral changes.

Multi-Omics Profiling Identifies Microglial Annexin A2 as a Key Mediator of NF-κB Pro-inflammatory Signaling in Ischemic Reperfusion Injury

Cerebral stroke is one of the leading causes of mortality and disability worldwide. Restoring the cerebral circulation following a period of occlusion and subsequent tissue oxygenation leads to reperfusion injury. Cerebral ischemic reperfusion (I/R) injury triggers immune and inflammatory responses, apoptosis, neuronal damage, and even death. However, the cellular function and molecular mechanisms underlying cerebral I/R-induced neuronal injury are incompletely understood. By integrating proteomic, phosphoproteomic, and transcriptomic profiling in mouse hippocampi after cerebral I/R, we revealed that the differentially expressed genes and proteins mainly fall into several immune inflammatory response-related pathways. We identified that Annexin 2 (Anxa2) was exclusively upregulated in microglial cells in response to cerebral I/R in vivo and oxygen-glucose deprivation and reoxygenation (OGD/R) in vitro. RNA-seq analysis revealed a critical role of Anxa2 in the expression of inflammation-related genes in microglia via the NF-κB signaling. Mechanistically, microglial Anxa2 is required for nuclear translocation of the p65 subunit of NF-κB and its transcriptional activity upon OGD/R in BV2 microglial cells. Anxa2 knockdown inhibited the OGD/R-induced microglia activation and markedly reduced the expression of pro-inflammatory factors, including TNF-α, IL-1β, and IL-6. Interestingly, conditional medium derived from Anxa2-depleted BV2 cell cultures with OGD/R treatment alleviated neuronal death in vitro. Altogether, our findings revealed that microglia Anxa2 plays a critical role in I/R injury by regulating NF-κB inflammatory responses in a non-cell-autonomous manner, which might be a potential target for the neuroprotection against cerebral I/R injury.

Opioids, microglia, and temporal lobe epilepsy

A lack of treatment options for temporal lobe epilepsy (TLE) demands an urgent quest for new therapies to recover neuronal damage and reduce seizures, potentially interrupting the neurotoxic cascades that fuel hyper-excitability. Endogenous opioids, along with their respective receptors, particularly dynorphin and kappa-opioid-receptor, present as attractive candidates for controlling neuronal excitability and therapeutics in epilepsy. We perform a critical review of the literature to evaluate the role of opioids in modulating microglial function and morphology in epilepsy. We find that, in accordance with anticonvulsant effects, acute opioid receptor activation has unique abilities to modulate microglial activation through toll-like 4 receptors, regulating downstream secretion of cytokines. Abnormal activation of microglia is a dominant feature of neuroinflammation, and inflammatory cytokines are found to aggravate TLE, inspiring the challenge to alter microglial activation by opioids to suppress seizures. We further evaluate how opioids can modulate microglial activation in epilepsy to enhance neuroprotection and reduce seizures. With controlled application, opioids may interrupt inflammatory cycles in epilepsy, to protect neuronal function and reduce seizures. Research on opioid-microglia interactions has important implications for epilepsy and healthcare approaches. However, preclinical research on opioid modulation of microglia supports a new therapeutic pathway for TLE.

Impaired microglial phagocytosis promotes seizure development

In the central nervous system, triggering receptor expressed on myeloid cells 2 (TREM2) is exclusively expressed by microglia and is critical for microglial proliferation, migration, and phagocytosis. TREM2 plays an important role in neurodegenerative diseases, such as Alzheimer's disease and amyotrophic lateral sclerosis. However, little is known about the role TREM2 plays in epileptogenesis. To investigate this, we utilized TREM2 knockout (KO) mice within the murine intra-amygdala kainic acid seizure model. Electroencephalographic analysis, immunocytochemistry, and RNA sequencing revealed that TREM2 deficiency significantly promoted seizure-induced pathology. We found that TREM2 KO increased both acute status epilepticus and spontaneous recurrent seizures characteristic of chronic focal epilepsy. Mechanistically, phagocytic clearance of damaged neurons by microglia was impaired in TREM2 KO mice and the reduced phagocytic capacity correlated with increased spontaneous seizures. Analysis of human tissue from patients who underwent surgical resection for drug resistant temporal lobe epilepsy also showed a negative correlation between microglial phagocytic activity and focal to bilateral tonic-clonic generalized seizure history. These results indicate that microglial TREM2 and phagocytic activity may be important to epileptogenesis and the progression of focal temporal lobe epilepsy.

Spi1 regulates the microglial/macrophage inflammatory response via the PI3K/AKT/mTOR signaling pathway after intracerebral hemorrhage

Preclinical and clinical studies have shown that microglia and macrophages participate in a multiphasic brain damage repair process following intracerebral hemorrhage. The E26 transformation-specific sequence-related transcription factor Spi1 regulates microglial/macrophage commitment and maturation. However, the effect of Spi1 on intracerebral hemorrhage remains unclear. In this study, we found that Spi1 may regulate recovery from the neuroinflammation and neurofunctional damage caused by intracerebral hemorrhage by modulating the microglial/macrophage transcriptome. We showed that high Spi1 expression in microglia/macrophages after intracerebral hemorrhage is associated with the activation of many pathways that promote phagocytosis, glycolysis, and autophagy, as well as debris clearance and sustained remyelination. Notably, microglia with higher levels of Spi1 expression were characterized by activation of pathways associated with a variety of hemorrhage-related cellular processes, such as complement activation, angiogenesis, and coagulation. In conclusion, our results suggest that Spi1 plays a vital role in the microglial/macrophage inflammatory response following intracerebral hemorrhage. This new insight into the regulation of Spi1 and its target genes may advance our understanding of neuroinflammation in intracerebral hemorrhage and provide therapeutic targets for patients with intracerebral hemorrhage.

Neuroinflammation in epileptogenesis: from pathophysiology to therapeutic strategies

Epilepsy is a group of enduring neurological disorder characterized by spontaneous and recurrent seizures with heterogeneous etiology, clinical expression, severity, and prognosis. Growing body of research investigates that epileptic seizures are originated from neuronal synchronized and excessive electrical activity. However, the underlying molecular mechanisms of epileptogenesis have not yet been fully elucidated and 30% of epileptic patients still are resistant to the currently available pharmacological treatments with recurrent seizures throughout life. Over the past two decades years accumulated evidences provide strong support to the hypothesis that neuroinflammation, including microglia and astrocytes activation, a cascade of inflammatory mediator releasing, and peripheral immune cells infiltration from blood into brain, is associated with epileptogenesis. Meanwhile, an increasing body of preclinical researches reveal that the anti-inflammatory therapeutics targeting crucial inflammatory components are effective and promising in the treatment of epilepsy. The aim of the present study is to highlight the current understanding of the potential neuroinflammatory mechanisms in epileptogenesis and the potential therapeutic targets against epileptic seizures.

DHA and EPA Alleviate Epileptic Depression in PTZ-Treated Young Mice Model by Inhibiting Neuroinflammation through Regulating Microglial M2 Polarization and Improving Mitochondrial Metabolism

Depression is the most common complication of childhood epilepsy, leading to a poor prognosis for seizure control and poor quality of life. However, the molecular mechanisms underlying epileptic depression have not been completely elucidated. Increasing evidence suggests that oxidative stress and neuroinflammation are major contributors to depression. The positive effects of dietary supplementation with docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) on depression have been previously reported. However, knowledge regarding the effects of EPA and DHA in managing depressive symptoms in pediatric patients with epilepsy is limited. Therefore, this study aims to investigate the effects of EPA and DHA on epileptic depression in a pentylenetetrazole (PTZ)-treated young mouse model. Three-week-old mice were fed a DHA- or EPA-enriched diet for 21 days and treated with PTZ (35 mg/kg, i.p.) every other day for a total of 10 times. EPA was more effective than DHA at alleviating PTZ-induced depressive symptoms. Pathological results revealed that DHA and EPA significantly improved neuronal degeneration in the hippocampus. Analysis of the mechanism revealed that DHA and EPA mitigated PTZ-induced myelin damage by increasing the protein levels of CNPase, Olig2, and MBP. Furthermore, both DHA and EPA reduced neuroinflammation by promoting microglial M2 polarization and suppressing the LCN2-NLRP3 inflammasome pathway. Notably, EPA polarized microglia towards the M2 phenotype. In addition, DHA and EPA decreased oxidative stress by inhibiting NOX2 and enhancing mitochondrial metabolism through the increased expression of mitochondrial respiratory chain complex I-V proteins. These findings suggest that DHA and EPA can be used as effective interventions to improve depression in children with epilepsy, with EPA being a particularly favorable option.

Translocator protein (TSPO) expression in neoplastic cells and tumor-associated macrophages in meningiomas

Meningiomas are the most common primary intracranial tumors and show extensive infiltration of macrophages. The mitochondrial membrane protein translocator protein (TSPO) has been used as an in vivo marker of microglia and macrophage activation to visualize neuroinflammation. However, it is unknown which cell types express TSPO in meningiomas. Immunohistochemistry of 38 WHO grade 1-3 meningiomas was subjected to segmentation and deep learning classification of TSPO expression to either Iba1-positive tumor-associated macrophages (TAMs) or all other (mainly neoplastic) cells. A possible association between clinical data and TSPO expression intensities was also investigated. TAMs accounted for 15.9%-26% of all cells in the meningioma tissue. Mean fluorescence intensity of TSPO was significantly higher in TAMs (p < 0.0001), but the mass of neoplastic cells in the tumors exceeded that of TAMs. Thus, the summed fluorescence intensity of TSPO in meningioma cells was 64.1% higher than in TAMs (p = 0.0003). We observed no correlation between TSPO expression intensity and WHO grade. These results indicate that both macrophage-lineage and neoplastic cells in meningiomas express TSPO and that the SPECT-TSPO signal in meningiomas mainly reflects the latter; TSPO is expressed equally in parenchymal activated and resting macrophage/microglia lineage cells.

Research Progress on the Immune-Inflammatory Mechanisms of Posttraumatic Epilepsy

Posttraumatic epilepsy (PTE) is a severe complication arising from a traumatic brain injury caused by various violent actions on the brain. The underlying mechanisms for the pathogenesis of PTE are complex and have not been fully defined. Approximately, one-third of patients with PTE are resistant to antiepileptic therapy. Recent research evidence has shown that neuroinflammation is critical in the development of PTE. This article reviews the immune-inflammatory mechanisms regarding microglial activation, astrocyte proliferation, inflammatory signaling pathways, chronic neuroinflammation, and intestinal flora. These mechanisms offer novel insights into the pathophysiological mechanisms of PTE and have groundbreaking implications in the prevention and treatment of PTE. Immunoinflammatory cross-talk between glial cells and gut microbiota in posttraumatic epilepsy. This graphical abstract depicts the roles of microglia and astrocytes in posttraumatic epilepsy, highlighting the influence of the gut microbiota on their function. TBI traumatic brain injury, AQP4 aquaporin-4, Kir4.1 inward rectifying K channels.

Microglia and complement mediate early corticostriatal synapse loss and cognitive dysfunction in Huntington's disease

Huntington's disease (HD) is a devastating monogenic neurodegenerative disease characterized by early, selective pathology in the basal ganglia despite the ubiquitous expression of mutant huntingtin. The molecular mechanisms underlying this region-specific neuronal degeneration and how these relate to the development of early cognitive phenotypes are poorly understood. Here we show that there is selective loss of synaptic connections between the cortex and striatum in postmortem tissue from patients with HD that is associated with the increased activation and localization of complement proteins, innate immune molecules, to these synaptic elements. We also found that levels of these secreted innate immune molecules are elevated in the cerebrospinal fluid of premanifest HD patients and correlate with established measures of disease burden.In preclinical genetic models of HD, we show that complement proteins mediate the selective elimination of corticostriatal synapses at an early stage in disease pathogenesis, marking them for removal by microglia, the brain's resident macrophage population. This process requires mutant huntingtin to be expressed in both cortical and striatal neurons. Inhibition of this complement-dependent elimination mechanism through administration of a therapeutically relevant C1q function-blocking antibody or genetic ablation of a complement receptor on microglia prevented synapse loss, increased excitatory input to the striatum and rescued the early development of visual discrimination learning and cognitive flexibility deficits in these models. Together, our findings implicate microglia and the complement cascade in the selective, early degeneration of corticostriatal synapses and the development of cognitive deficits in presymptomatic HD; they also provide new preclinical data to support complement as a therapeutic target for early intervention.

Activated protein C in epilepsy pathophysiology

Epilepsy is one of the most common neurologic disorders that is characterized by recurrent seizures, and depending on the type of seizure, it could lead to a severe outcome. Epilepsy's mechanism of development is not fully understood yet, but some of the common features of the disease are blood-brain barrier disruption, microglia activation, and neuroinflammation. Those are also targets of activated protein C (APC). In fact, by downregulating thrombin, known as a pro-inflammatory, APC acts as an anti-inflammatory. APC is also an anti-apoptotic protein, instance by blocking p53-mediated apoptosis. APC's neuroprotective effect could prevent blood-brain barrier dysfunction by acting on endothelial cells. Furthermore, through the downregulation of proapoptotic, and proinflammatory genes, APC's neuroprotection could reduce the effect or prevent epilepsy pathogenesis. APC's activity acts on blood-brain barrier disruption, inflammation, and apoptosis and causes neurogenesis, all hallmarks that could potentially treat or prevent epilepsy. Here we review both Activated Protein C and epilepsy mechanism, function, and the possible association between them.

Microglia in epilepsy

Epilepsy is one of most common chronic neurological disorders, and the antiseizure medications developed by targeting neurocentric mechanisms have not effectively reduced the proportion of patients with drug-resistant epilepsy. Further exploration of the cellular or molecular mechanism of epilepsy is expected to provide new options for treatment. Recently, more and more researches focus on brain network components other than neurons, among which microglia have attracted much attention for their diverse biological functions. As the resident immune cells of the central nervous system, microglia have highly plastic transcription, morphology and functional characteristics, which can change dynamically in a context-dependent manner during the progression of epilepsy. In the pathogenesis of epilepsy, highly reactive microglia interact with other components in the epileptogenic network by performing crucial functions such as secretion of soluble factors and phagocytosis, thus continuously reshaping the landscape of the epileptic brain microenvironment. Indeed, microglia appear to be both pro-epileptic and anti-epileptic under the different spatiotemporal contexts of disease, rendering interventions targeting microglia biologically complex and challenging. This comprehensive review critically summarizes the pathophysiological role of microglia in epileptic brain homeostasis alterations and explores potential therapeutic or modulatory targets for epilepsy targeting microglia.

Can glial cells save neurons in epilepsy?

Epilepsy is a neurological disorder caused by the pathological hyper-synchronization of neuronal discharges. The fundamental research of epilepsy mechanisms and the targets of drug design options for its treatment have focused on neurons. However, approximately 30% of patients suffering from epilepsy show resistance to standard anti-epileptic chemotherapeutic agents while the symptoms of the remaining 70% of patients can be alleviated but not completely removed by the current medications. Thus, new strategies for the treatment of epilepsy are in urgent demand. Over the past decades, with the increase in knowledge on the role of glia in the genesis and development of epilepsy, glial cells are receiving renewed attention. In a normal brain, glial cells maintain neuronal health and in partnership with neurons regulate virtually every aspect of brain function. In epilepsy, however, the supportive roles of glial cells are compromised, and their interaction with neurons is altered, which disrupts brain function. In this review, we will focus on the role of glia-related processes in epileptogenesis and their contribution to abnormal neuronal activity, with the major focus on the dysfunction of astroglial potassium channels, water channels, gap junctions, glutamate transporters, purinergic signaling, synaptogenesis, on the roles of microglial inflammatory cytokines, microglia-astrocyte interactions in epilepsy, and on the oligodendroglial potassium channels and myelin abnormalities in the epileptic brain. These recent findings suggest that glia should be considered as the promising next-generation targets for designing anti-epileptic drugs that may improve epilepsy and drug-resistant epilepsy.

Age and sex drive differential behavioral and neuroimmune phenotypes during postoperative pain

Surgical procedures in the geriatric population are steadily increasing, driven by improved healthcare technologies and longer lifespans. However, effective postoperative pain treatments are lacking, and this diminishes quality of life and recovery. Here we present one of the first preclinical studies to pursue sex- and age-specific differences in postoperative neuroimmune phenotypes and pain. We found that aged males, but not females, had a delayed onset of mechanical hypersensitivity post-surgery and faster resolution than young counterparts. This sex-specific age effect was accompanied by decreased paw innervation and increased local inflammation. Additionally, we find evidence of an age-dependent decrease in hyperalgesic priming and perioperative changes in nociceptor populations and spinal microglia in the aged. These findings suggest that impaired neuronal function and maladaptive inflammatory mechanisms influence postoperative pain development in advanced age. Elucidation of these neuroimmune phenotypes across age and sex enables the development of novel therapies that can be tailored for improved pain relief.

Reduced epilepsy development in synapsin 2 knockout mice with autistic behavior following early systemic treatment with interleukin-6 receptor antibody

BACKGROUND

Individuals with autism spectrum disorder (ASD) have an increased risk of developing epilepsy. Both ASD and epilepsy have been associated with increased levels of immune factors in the blood, including the proinflammatory cytokine interleukin 6 (IL-6). Mice lacking the synapsin 2 gene (Syn2 KO) exhibit ASD-like behavior and develop epileptic seizures. Their brains display neuroinflammatory changes including elevated IL-6 levels. We aimed to investigate the effect of systemic IL-6 receptor antibody (IL-6R ab) treatment on seizure development and frequency in Syn2 KO mice.

MATERIAL AND METHODS

Weekly systemic (i.p.) injections of IL-6R ab or saline were given to Syn2 KO mice starting either early in life at 1 month of age, before seizure debut or at 3 months of age, directly after seizure debut and continued for 4 or 2 months, respectively. Seizures were provoked by handling the mice three times per week. The neuroinflammatory response and synaptic protein levels in the brain were determined by ELISA, immunohistochemistry and western blots. In an additional group of Syn2 KO mice, with IL-6R ab treatment early in life, ASD-related behavioral tests including social interaction and repetitive self-grooming, as well as cognitive memory and depressive-/anxiety-like tests, and actigraphy measurements of circadian sleep-awake rhythm were analyzed.

RESULTS

The IL-6R ab treatment reduced seizure development and frequency in Syn2 KO mice when initiated before, but not after, seizure debut. However, early treatment did not reverse the neuroinflammatory response or the imbalance in synaptic protein levels in the brain previously reported in Syn2 KO mice. The treatment did not affect social interaction, performance in memory, depressive-/anxiety-like tests or the sleep-awake rhythm of Syn2 KO mice.

CONCLUSION

These findings suggest the involvement of IL-6 receptor signaling during epilepsy development in Syn2 KO mice, without significant alterations of the immune reaction in the brain, and independently of cognitive performance, mood and circadian sleep-awake rhythm.

Reactive microglia are the major source of tumor necrosis factor alpha and contribute to astrocyte dysfunction and acute seizures in experimental temporal lobe epilepsy

Extensive microglia reactivity has been well described in human and experimental temporal lobe epilepsy (TLE). To date, however, it is not clear whether and based on which molecular mechanisms microglia contribute to the development and progression of focal epilepsy. Astroglial gap junction coupled networks play an important role in regulating neuronal activity and loss of interastrocytic coupling causally contributes to TLE. Here, we show in the unilateral intracortical kainate (KA) mouse model of TLE that reactive microglia are primary producers of tumor necrosis factor (TNF)α and contribute to astrocyte dysfunction and severity of status epilepticus (SE). Immunohistochemical analyses revealed pronounced and persistent microglia reactivity, which already started 4 h after KA-induced SE. Partial depletion of microglia using a colony stimulating factor 1 receptor inhibitor prevented early astrocyte uncoupling and attenuated the severity of SE, but increased the mortality of epileptic mice following surgery. Using microglia-specific inducible TNFα knockout mice we identified microglia as the major source of TNFα during early epileptogenesis. Importantly, microglia-specific TNFα knockout prevented SE-induced gap junction uncoupling in astrocytes. Continuous telemetric EEG recordings revealed that during the first 4 weeks after SE induction, microglial TNFα did not significantly contribute to spontaneous generalized seizure activity. Moreover, the absence of microglial TNFα did not affect the development of hippocampal sclerosis but attenuated gliosis. Taken together, these data implicate reactive microglia in astrocyte dysfunction and network hyperexcitability after an epileptogenic insult.

Downregulation of oxidative stress-mediated glial innate immune response suppresses seizures in a fly epilepsy model

Previous work in our laboratory has shown that mutations in prickle (pk) cause myoclonic-like seizures and ataxia in Drosophila, similar to what is observed in humans carrying mutations in orthologous PRICKLE genes. Here, we show that pk mutant brains show elevated, sustained neuronal cell death that correlates with increasing seizure penetrance, as well as an upregulation of mitochondrial oxidative stress and innate immune response (IIR) genes. Moreover, flies exhibiting more robust seizures show increased levels of IIR-associated target gene expression suggesting they may be linked. Genetic knockdown in glia of either arm of the IIR (Immune Deficiency [Imd] or Toll) leads to a reduction in neuronal death, which in turn suppresses seizure activity, with oxidative stress acting upstream of IIR. These data provide direct genetic evidence that oxidative stress in combination with glial-mediated IIR leads to progression of an epilepsy disorder.

Regionally restricted modulation of Sam68 expression and Arhgef9 alternative splicing in the hippocampus of a murine model of multiple sclerosis

Multiple sclerosis (MS) and its preclinical models are characterized by marked changes in neuroplasticity, including excitatory/inhibitory imbalance and synaptic dysfunction that are believed to underlie the progressive cognitive impairment (CI), which represents a significant clinical hallmark of the disease. In this study, we investigated several parameters of neuroplasticity in the hippocampus of the experimental autoimmune encephalomyelitis (EAE) SJL/J mouse model, characterized by rostral inflammatory and demyelinating lesions similar to Relapsing-Remitting MS. By combining morphological and molecular analyses, we found that the hippocampus undergoes extensive inflammation in EAE-mice, more pronounced in the CA3 and dentate gyrus (DG) subfields than in the CA1, associated with changes in GABAergic circuitry, as indicated by the increased expression of the interneuron marker Parvalbumin selectively in CA3. By laser-microdissection, we investigated the impact of EAE on the alternative splicing of Arhgef9, a gene encoding a post-synaptic protein playing an essential role in GABAergic synapses and whose mutations have been related to CI and epilepsy. Our results indicate that EAE induces a specific increase in inclusion of the alternative exon 11a only in the CA3 and DG subfields, in line with the higher local levels of inflammation. Consistently, we found a region-specific downregulation of Sam68, a splicing-factor that represses this splicing event. Collectively, our findings confirm a regionalized distribution of inflammation in the hippocampus of EAE-mice. Moreover, since neuronal circuit rearrangement and dynamic remodeling of structural components of the synapse are key processes that contribute to neuroplasticity, our study suggests potential new molecular players involved in EAE-induced hippocampal dysfunction.

CYFIP2 p.Arg87Cys Causes Neurological Defects and Degradation of CYFIP2

Here, we report the generation and comprehensive characterization of a knockin mouse model for the hotspot p.Arg87Cys variant of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) gene, which was recently identified in individuals diagnosed with West syndrome, a developmental and epileptic encephalopathy. The Cyfip2^+/R87C mice recapitulated many neurological and neurobehavioral phenotypes of the patients, including spasmlike movements, microcephaly, and impaired social communication. Age-progressive cytoarchitectural disorganization and gliosis were also identified in the hippocampus of Cyfip2^+/R87C mice. Beyond identifying a decrease in CYFIP2 protein levels in the Cyfip2^+/R87C brains, we demonstrated that the p.Arg87Cys variant enhances ubiquitination and proteasomal degradation of CYFIP2. ANN NEUROL 2023;93:155-163.

Spine morphogenesis and synapse formation in tubular sclerosis complex models

Tuberous sclerosis complex (TSC) is caused by mutations in the Tsc1 or Tsc2 genes, whose products form a complex and inactivate the small G-protein Rheb1. The activation of Rheb1 may cause refractory epilepsy, intellectual disability, and autism, which are the major neuropsychiatric manifestations of TSC. Abnormalities in dendritic spines and altered synaptic structure are hallmarks of epilepsy, intellectual disability, and autism. In addition, spine dysmorphology and aberrant synapse formation are observed in TSC animal models. Therefore, it is important to investigate the molecular mechanism underlying the regulation of spine morphology and synapse formation in neurons to identify therapeutic targets for TSC. In this review, we focus on the representative proteins regulated by Rheb1 activity, mTORC1 and syntenin, which are pivotal downstream factors of Rheb1 in the alteration of spine formation and synapse function in TSC neurons.

Implications of fractalkine on glial function, ablation and glial proteins/receptors/markers—understanding its therapeutic usefulness in neurological settings: a narrative review

Background

Fractalkine (CX3CL1) is a chemokine predominantly released by neurons. As a signaling molecule, CX3CL1 facilitates talk between neurons and glia. CX3CL1 is considered as a potential target which could alleviate neuroinflammation. However, certain controversial results and ambiguous role of CX3CL1 make it inexorable to decipher the overall effects of CX3CL1 on the physiopathology of glial cells.

Main body of the abstract

Implications of cross-talk between CX3CL1 and different glial proteins/receptors/markers will give a bird eye view of the therapeutic significance of CX3CL1. Keeping with the need, this review identifies the effects of CX3CL1 on glial physiopathology, glial ablation, and gives a wide coverage on the effects of CX3CL1 on certain glial proteins/receptors/markers.

Short conclusion

Pinpoint prediction of the therapeutic effect of CX3CL1 on neuroinflammation needs further research. This is owing to certain obscure roles and implications of CX3CL1 on different glial proteins/receptors/markers, which are crucial under neurological settings. Further challenges are imposed due to the dichotomous roles played by CX3CL1. The age-old chemokine shows many newer scopes of research in near future. Thus, overall assessment of the effect of CX3CL1 becomes crucial prior to its administration in neuroinflammation.

Microglial diversity along the hippocampal longitudinal axis impacts synaptic plasticity in adult male mice under homeostatic conditions

The hippocampus is a plastic brain area that shows functional segregation along its longitudinal axis, reflected by a higher level of long-term potentiation (LTP) in the CA1 region of the dorsal hippocampus (DH) compared to the ventral hippocampus (VH), but the mechanisms underlying this difference remain elusive. Numerous studies have highlighted the importance of microglia-neuronal communication in modulating synaptic transmission and hippocampal plasticity, although its role in physiological contexts is still largely unknown. We characterized in depth the features of microglia in the two hippocampal poles and investigated their contribution to CA1 plasticity under physiological conditions. We unveiled the influence of microglia in differentially modulating the amplitude of LTP in the DH and VH, showing that minocycline or PLX5622 treatment reduced LTP amplitude in the DH, while increasing it in the VH. This was recapitulated in Cx3cr1 knockout mice, indicating that microglia have a key role in setting the conditions for plasticity processes in a region-specific manner, and that the CX3CL1-CX3CR1 pathway is a key element in determining the basal level of CA1 LTP in the two regions. The observed LTP differences at the two poles were associated with transcriptional changes in the expression of genes encoding for Il-1, Tnf-α, Il-6, and Bdnf, essential players of neuronal plasticity. Furthermore, microglia in the CA1 SR region showed an increase in soma and a more extensive arborization, an increased prevalence of immature lysosomes accompanied by an elevation in mRNA expression of phagocytic markers Mertk and Cd68 and a surge in the expression of microglial outward K⁺ currents in the VH compared to DH, suggesting a distinct basal phenotypic state of microglia across the two hippocampal poles. Overall, we characterized the molecular, morphological, ultrastructural, and functional profile of microglia at the two poles, suggesting that modifications in hippocampal subregions related to different microglial statuses can contribute to dissect the phenotypical aspects of many diseases in which microglia are known to be involved.

The appearance of phagocytic microglia in the postnatal brain of Niemann Pick type C mice is developmentally regulated and underscores shortfalls in fine odor discrimination

The loss of NPC1 or NPC2 function results in cholesterol and sphingolipid dyshomeostasis that impairs developmental trajectories, predisposing the postnatal brain to the appearance of pathological signs, including progressive and stereotyped Purkinje cell loss and microgliosis. Despite increasing evidence reporting the activation of pro-inflammatory microglia as a cardinal event of NPC1 disease progression at symptomatic stages both in patients and preclinical models, how microglia cells respond to altered neurodevelopmental dynamics remains not completely understood. To gain an insight on this issue, we have characterized patterns of microglia activation in the early postnatal cerebellum and young adult olfactory bulb of the hypomorphic Npc1^nmf164 mouse model. Previous evidence has shown that both these areas display a number of anomalies affecting neuron and glial cell proliferation and differentiation, which largely anticipate cellular changes and clinical signs, raising our interest on how microglia interplay to these changes. Even so, to separate the contribution of cues provided by the dysfunctional microenvironment we have also studied microglia isolated from mice of increasing ages and cultured in vitro for 1 week. Our findings show that microglia of both cerebellum and olfactory bulb of Npc1^nmf164 mice adopt an activated phenotype, characterized by increased cell proliferation, enlarged soma size and de-ramified processes, as well as a robust phagocytic activity, in a time- and space-specific manner. Enhanced phagocytosis associates with a profound remodeling of gene expression signatures towards gene products involved in chemotaxis, cell recognition and engulfment, including Cd68 and Trem2. These early changes in microglia morphology and activities are induced by region-specific developmental anomalies that likely anticipate alterations in neuronal connectivity. As a proof of concept, we show that microglia activation within the granule cell layer and glomerular layer of the olfactory bulb of Npc1^nmf164 mice is associated with shortfalls in fine odor discrimination.