Neuronal hyperactivity recruits microglial processes via neuronal NMDA receptors and microglial P2Y12 receptors after status epilepticus

Effects of Remote Immune Activation on Performance in the 5-Choice Serial Reaction Time Task Following Mild Traumatic Brain Injury in Adolescence

In adult pre-clinical models, traumatic brain injury (TBI) has been shown to prime microglia, exaggerating the central inflammatory response to an acute immune challenge, worsening depressive-like behavior, and enhancing cognitive deficits. Whether this phenomenon exists following mTBI during adolescence has yet to be explored, with age at injury potentially altering the inflammatory response. Furthermore, to date, studies have predominantly examined hippocampal-dependent learning domains, although pre-frontal cortex-driven functions, including attention, motivation, and impulsivity, are significantly affected by both adolescent TBI and acute inflammatory stimuli. As such, the current study examined the effects of a single acute peripheral dose of LPS (0.33 mg/kg) given in adulthood following mTBI in mid-adolescence in male Sprague–Dawley rats on performance in the 5-choice serial reaction time task (5-CSRTT). Only previously injured animals given LPS showed an increase in omissions and reward collection latency on the 5-CSRTT, with no effect noted in sham animals given LPS. This is suggestive of impaired motivation and a prolonged central inflammatory response to LPS administration in these animals. Indeed, morphological analysis of myeloid cells within the pre-frontal cortex, via IBA1 immunohistochemistry, found that injured animals administered LPS had an increase in complexity in IBA1+ve cells, an effect that was seen to a lesser extent in sham animals. These findings suggest that there may be ongoing alterations in the effects of acute inflammatory stimuli that are driven, in part by increased reactivity of microglial cells.

Purinergic signaling in nervous system health and disease: Focus on pannexin 1

Purinergic signaling encompasses the cycle of adenosine 5' triphosphate (ATP) release and its metabolism into nucleotide and nucleoside derivatives, the direct release of nucleosides, and subsequent receptor-triggered downstream intracellular pathways. Since the discovery of nerve terminal and glial ATP release into the neuropil, purinergic signaling has been implicated in the modulation of nervous system development, function, and disease. In this review, we detail our current understanding of the roles of the pannexin 1 (PANX1) ATP-release channel in neuronal development and plasticity, glial signaling, and neuron-glial-immune interactions. We additionally provide an overview of PANX1 structure, activation, and permeability to orientate readers and highlight recent research developments. We identify areas of convergence between PANX1 and purinergic receptor actions. Additional highlights include data on PANX1's participation in the pathophysiology of nervous system developmental, degenerative, and inflammatory disorders. Our aim in combining this knowledge is to facilitate the movement of our current understanding of PANX1 in the context of other nervous system purinergic signaling mechanisms one step closer to clinical translation.

A distinct microglial subset at the tumor-stroma interface of glioma

The characterization of the tumor microenvironment (TME) in high grade gliomas (HGG) has generated significant interest in an effort to understand how neoplastic lesions in the central nervous system (CNS) are supported and to devise novel therapeutic targets. The TME of the CNS contains unique and specialized cells, including the resident myeloid cells, microglia. Myeloid involvement in HGG, such as glioblastoma, is associated with poor outcomes. Glioma-associated microglia and infiltrating monocytes/macrophages (GAM) accumulate within the neoplastic lesion where they facilitate tumor growth and drive immunosuppression. However, it has been difficult to differentiate whether microglia and macrophages have similar or distinct roles in pathology, and if the spatial organization of these cells informs outcomes. Here, we characterize the tumor-stroma border and identify peritumoral GAM (PGAM) as a unique subpopulation of GAM. Using data mining and analyses of samples derived from both murine and human sources we show that PGAM exhibit a pro-inflammatory and chemotactic phenotype that is associated with peripheral monocyte recruitment, and decreased overall survival. PGAM act as a unique subset of GAM at the tumor-stroma interface. We define a novel gene signature to identify these cells and suggest that PGAM constitute a cellular target of the TME.

Assessing Microglial Dynamics by Live Imaging

Microglia are highly dynamic in the brain in terms of their ability to migrate, proliferate, and phagocytose over the course of an individual's life. Real-time imaging is a useful tool to examine how microglial behavior is regulated and how it affects the surrounding environment. However, microglia are sensitive to environmental stimuli, so they possibly change their state during live imaging in vivo, mainly due to surgical damage, and in vitro due to various effects associated with culture conditions. Therefore, it is difficult to perform live imaging without compromising the properties of the microglia under physiological conditions. To overcome this barrier, various experimental conditions have been developed; recently, it has become possible to perform live imaging of so-called surveillant microglia in vivo, ex vivo, and in vitro, although there are various limitations. Now, we can choose in vivo, ex vivo, or in vitro live imaging systems according to the research objective. In this review, we discuss the advantages and disadvantages of each experimental system and outline the physiological significance and molecular mechanisms of microglial behavior that have been elucidated by live imaging.

Microglia regulate synaptic development and plasticity

Synapses are fundamental structures of neural circuits that transmit information between neurons. Thus, the process of neural circuit formation via proper synaptic connections shapes the basis of brain functions and animal behavior. Synapses continuously undergo repeated formation and elimination throughout the lifetime of an organism, reflecting the dynamics of neural circuit function. The structural transformation of synapses has been described mainly in relation to neural activity-dependent strengthening and weakening of synaptic functions, that is, functional plasticity of synapses. An increasing number of studies have unveiled the roles of microglia, brain-resident immune cells that survey the brain parenchyma with highly motile processes, in synapse formation and elimination as well as in regulating synaptic function. Over the past 15 years, the molecular mechanisms underlying microglia-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of microglia-dependent regulation causes synaptic dysfunction that leads to brain diseases. In this review, we will broadly introduce studies that report the roles of microglia in synaptic plasticity and the possible underlying molecular mechanisms.

Perineural high-mobility group box 1 induces mechanical hypersensitivity through activation of spinal microglia: Involvement of glutamate-NMDA receptor dependent mechanism in spinal dorsal horn

High mobility box 1 (HMGB1), a damage-associated molecular pattern, has crucial roles in induction of neuropathic pain. Upregulation of HMGB1 around the injured sciatic nerve contributes to mechanical hypersensitivity following partial sciatic nerve ligation (PSNL) of mice. However, central mechanisms mediating perineural HMGB1-induced nociceptive hypersensitivity, especially within the spinal dorsal horn, have not been determined. The current study shows that perineural treatment of naïve mice with recombinant HMGB1, which mimics increased HMGB1 around the injured sciatic nerve of PSNL mice, significantly induced activation of microglia, but not astrocytes, in the spinal dorsal horn. Intraperitoneal injection of minocycline, a microglial inhibitor, ameliorated perineural rHMGB1-induced mechanical hypersensitivity. In addition, blockade of spinal N-methyl-D-aspartate (NMDA) receptors significantly prevented perineural rHMGB1-induced mechanical hypersensitivity and microglial activation. In contrast, non-NMDA receptors, neurokinin 1 receptor, colony-stimulating factor 1 receptor and P2Y₁₂ receptor were not involved in perineural rHMGB1-induced mechanical hypersensitivity. Furthermore, repeated perineural treatment with an anti-HMGB1 antibody blocked activation of spinal microglia in PSNL mice. Collectively, the current findings demonstrate that increased HMGB1 around injured sciatic nerve might induce nociceptive hypersensitivity through activation of spinal microglia. Thus, HMGB1-dependent mechanisms between the injured sciatic nerve and spinal dorsal horn could be crucial in induction of neuropathic pain.

The Emerging Role of Microglia in Neuromyelitis Optica

Neuromyelitis optica (NMO) is an autoantibody-triggered neuro-inflammatory disease which preferentially attacks the spinal cord and optic nerve. Its defining autoantibody is specific for the water channel protein, aquaporin-4 (AQP4), which primarily is localized at the end-feet of astrocytes. Histopathology studies of early NMO lesions demonstrated prominent activation of microglia, the resident immune sentinels of the central nervous system (CNS). Significant microglial reactivity is also observed in NMO animal models induced by introducing AQP4-IgG into the CNS. Here we review the potential roles for microglial activation in human NMO patients as well as different animal models of NMO. We will focus primarily on the molecular mechanisms underlying microglial function and microglia-astrocyte interaction in NMO pathogenesis. Understanding the role of microglia in NMO pathology may yield novel therapeutic approaches for this disease.

Microglia Fighting for Neurological and Mental Health: On the Central Nervous System Frontline of COVID-19 Pandemic

Coronavirus disease 2019 (COVID-19) is marked by cardio-respiratory alterations, with increasing reports also indicating neurological and psychiatric symptoms in infected individuals. During COVID-19 pathology, the central nervous system (CNS) is possibly affected by direct severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) invasion, exaggerated systemic inflammatory responses, or hypoxia. Psychosocial stress imposed by the pandemic further affects the CNS of COVID-19 patients, but also the non-infected population, potentially contributing to the emergence or exacerbation of various neurological or mental health disorders. Microglia are central players of the CNS homeostasis maintenance and inflammatory response that exert their crucial functions in coordination with other CNS cells. During homeostatic challenges to the brain parenchyma, microglia modify their density, morphology, and molecular signature, resulting in the adjustment of their functions. In this review, we discuss how microglia may be involved in the neuroprotective and neurotoxic responses against CNS insults deriving from COVID-19. We examine how these responses may explain, at least partially, the neurological and psychiatric manifestations reported in COVID-19 patients and the general population. Furthermore, we consider how microglia might contribute to increased CNS vulnerability in certain groups, such as aged individuals and people with pre-existing conditions.

Microglial remodeling of actin network by Tau oligomers, via G protein-coupled purinergic receptor, P2Y12R-driven chemotaxis

Alzheimer's disease (AD) is associated with age-related neurodegeneration, synaptic deformation and chronic inflammation mediated by microglia and infiltrated macrophages in the brain. Tau oligomers can be released from damaged neurons via various mechanisms such as exosomes, neurotransmitter, membrane leakage etc. Microglia sense the extracellular Tau through several cell-surface receptors and mediate chemotaxis and phagocytosis. The purinergic receptor P2Y12R recently gained interest in neurodegeneration for neuro-glial communication and microglial chemotaxis towards the site of plaque deposition. To understand the effect of extracellular Tau oligomers in microglial migration, the P2Y12R-mediated actin remodeling, reorientation of tubulin network and rate of migration were studied in the presence of ATP. The extracellular Tau species directly interacted with P2Y12R and also induced this purinoceptor expression in microglia. Microglial P2Y12R colocalized with remodeled membrane-associated actin network as a component of migration in response to Tau oligomers. As an inducer of P2Y12R, ATP facilitated the localization of P2Y12R in lamellipodia and filopodia during accelerated microglial migration. The direct interaction of extracellular Tau oligomers with microglial P2Y12R would facilitate the signal transduction in both way, directional chemotaxis and receptor-mediated phagocytosis. These unprecedented findings emphasize that microglia can modulate the membrane-associated actin structure and incorporate P2Y12R to perceive the axis and rate of chemotaxis in Tauopathy.

ATP-evoked intracellular Ca2+ transients shape the ionic permeability of human microglia from epileptic temporal cortex

Background

Intracellular Ca²⁺ modulates several microglial activities, such as proliferation, migration, phagocytosis, and inflammatory mediator secretion. Extracellular ATP, the levels of which significantly change during epileptic seizures, activates specific receptors leading to an increase of intracellular free Ca²⁺ concentration ([Ca²⁺]_i). Here, we aimed to functionally characterize human microglia obtained from cortices of subjects with temporal lobe epilepsy, focusing on the Ca²⁺-mediated response triggered by purinergic signaling.

Methods

Fura-2 based fluorescence microscopy was used to measure [Ca²⁺]_i in primary cultures of human microglial cells obtained from surgical specimens. The perforated patch-clamp technique, which preserves the cytoplasmic milieu, was used to measure ATP-evoked Ca²⁺-dependent whole-cell currents.

Results

In human microglia extracellular ATP evoked [Ca²⁺]_i increases depend on Ca²⁺ entry from the extracellular space and on Ca²⁺ mobilization from intracellular compartments. Extracellular ATP also induced a transient fivefold potentiation of the total transmembrane current, which was completely abolished when [Ca²⁺]_i increases were prevented by removing external Ca²⁺ and using an intracellular Ca²⁺ chelator. TRAM-34, a selective K_Ca3.1 blocker, significantly reduced the ATP-induced current potentiation but did not abolish it. The removal of external Cl⁻ in the presence of TRAM-34 further lowered the ATP-evoked effect. A direct comparison between the ATP-evoked mean current potentiation and mean Ca²⁺ transient amplitude revealed a linear correlation. Treatment of microglial cells with LPS for 48 h did not prevent the ATP-induced Ca²⁺ mobilization but completely abolished the ATP-mediated current potentiation. The absence of the Ca²⁺-evoked K⁺ current led to a less sustained ATP-evoked Ca²⁺ entry, as shown by the faster Ca²⁺ transient kinetics observed in LPS-treated microglia.

Conclusions

Our study confirms a functional role for K_Ca3.1 channels in human microglia, linking ATP-evoked Ca²⁺ transients to changes in membrane conductance, with an inflammation-dependent mechanism, and suggests that during brain inflammation the K_Ca3.1-mediated microglial response to purinergic signaling may be reduced.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02096-0.

Loss of P2Y12 Has Behavioral Effects in the Adult Mouse

While microglia have been established as critical mediators of synaptic plasticity, the molecular signals underlying this process are still being uncovered. Increasing evidence suggests that microglia utilize these signals in a temporally and regionally heterogeneous manner. Subsequently, it is necessary to understand the conditions under which different molecular signals are employed by microglia to mediate the physiological process of synaptic remodeling in development and adulthood. While the microglial purinergic receptor P2Y12 is required for ocular dominance plasticity, an adolescent form of experience-dependent plasticity, it remains unknown whether P2Y12 functions in other forms of plasticity at different developmental time points or in different brain regions. Using a combination of ex vivo characterization and behavioral testing, we examined how the loss of P2Y12 affects developmental processes and behavioral performance in adulthood in mice. We found P2Y12 was not required for an early form of plasticity in the developing visual thalamus and did not affect microglial migration into barrels in the developing somatosensory cortex. In adult mice, however, the loss of P2Y12 resulted in alterations in recognition and social memory, as well as anxiety-like behaviors, suggesting that while P2Y12 is not a universal regulator of synaptic plasticity, the loss of P2Y12 is sufficient to cause functional defects.

Deficiency of Microglial Autophagy Increases the Density of Oligodendrocytes and Susceptibility to Severe Forms of Seizures

Excessive activation of mTOR in microglia impairs CNS homeostasis and causes severe epilepsy. Autophagy constitutes an important part of mTOR signaling. The contribution of microglial autophagy to CNS homeostasis and epilepsy remains to be determined. Here, we report that ATG7KO mice deficient for autophagy in microglia display a marked increase of myelination markers, a higher density of mature oligodendrocytes (ODCs), and altered lengths of the nodes of Ranvier. Moreover, we found that deficiency of microglial autophagy (ATG7KO) leads to increased seizure susceptibility in three seizure models (pilocarpine, kainic acid, and amygdala kindling). We demonstrated that ATG7KO mice develop severe generalized seizures and display nearly 100% mortality to convulsions induced by pilocarpine and kainic acid. In the amygdala kindling model, we observed significant facilitation of contralateral propagation of seizures, a process underlying the development of generalized seizures. Taken together, our results reveal impaired microglial autophagy as a novel mechanism underlying altered homeostasis of ODCs and increased susceptibility to severe and fatal generalized seizures.

Microglia and Neuroinflammation: What Place for P2RY12?

Microglia are immune brain cells involved in neuroinflammation. They express a lot of proteins on their surface such as receptors that can be activated by mediators released in the microglial environment. Among these receptors, purinergic receptor expression could be modified depending on the activation status of microglia. In this review, we focus on P2Y receptors and more specifically on P2RY12 that is involved in microglial motility and migration, the first step of neuroinflammation process. We describe the purinergic receptor families, P2RY12 structure, expression and physiological functions. The pharmacological and genetic tools for studying this receptor are detailed thereafter. Last but not least, we report the contribution of microglial P2RY12 to neuroinflammation in acute and chronic brain pathologies in order to better understand P2RY12 microglial role.

Endocannabinoid Signaling for GABAergic-Microglia (Mis)Communication in the Brain Aging

The aging brain seems to be characterized by neuronal loss leading to cognitive decline and progressively worsening symptoms related to neurodegeneration. Also, pro-inflammatory states, if prolonged, may increase neuronal vulnerability via excessive activation of microglia and their pro-inflammatory by-products, which is seen as individuals increase in age. Consequently, microglial activity is tightly regulated by neuron-microglia communications. The endocannabinoid system (ECS) is emerging as a regulator of microglia and the neuronal-microglia communication system. Recently, it has been demonstrated that cannabinoid 1 (CB1) receptor signaling on GABAergic interneurons plays a crucial role in regulating microglial activity. Interestingly, if endocannabinoid signaling on GABAergic neurons are disturbed, the phenotypes mimic central nervous system insult models by activating microglia and leading to accelerated brain aging. Investigating the endocannabinoid receptors, ligands, and genetic deletions yields the potential to understand the communication system and mechanism by which the ECS regulates glial cells and aspects of aging. While there remains much to discover with the ECS, the information gathered and identified already could lead to the development of cell-specific therapeutic interventions that help in reducing the effects of age-related pro-inflammatory states and neurodegeneration.

Engulfed by Glia: Glial Pruning in Development, Function, and Injury across Species

Phagocytic activity of glial cells is essential for proper nervous system sculpting, maintenance of circuitry, and long-term brain health. Glial engulfment of apoptotic cells and superfluous connections ensures that neuronal connections are appropriately refined, while clearance of damaged projections and neurotoxic proteins in the mature brain protects against inflammatory insults. Comparative work across species and cell types in recent years highlights the striking conservation of pathways that govern glial engulfment. Many signaling cascades used during developmental pruning are re-employed in the mature brain to "fine tune" synaptic architecture and even clear neuronal debris following traumatic events. Moreover, the neuron-glia signaling events required to trigger and perform phagocytic responses are impressively conserved between invertebrates and vertebrates. This review offers a compare-and-contrast portrayal of recent findings that underscore the value of investigating glial engulfment mechanisms in a wide range of species and contexts.

Astrocyte-microglia interaction drives evolving neuromyelitis optica lesion

Neuromyelitis optica (NMO) is a severe inflammatory autoimmune CNS disorder triggered by binding of an IgG autoantibody to the aquaporin 4 (AQP4) water channel on astrocytes. Activation of cytolytic complement has been implicated as the major effector of tissue destruction that secondarily involves myelin. We investigated early precytolytic events in the evolving pathophysiology of NMO in mice by continuously infusing IgG (NMO patient serum-derived or AQP4-specific mouse monoclonal), without exogenous complement, into the spinal subarachnoid space. Motor impairment and sublytic NMO-compatible immunopathology were IgG dose dependent, AQP4 dependent, and, unexpectedly, microglia dependent. In vivo spinal cord imaging revealed a striking physical interaction between microglia and astrocytes that required signaling from astrocytes by the C3a fragment of their upregulated complement C3 protein. Astrocytes remained viable but lost AQP4. Previously unappreciated crosstalk between astrocytes and microglia involving early-activated CNS-intrinsic complement components and microglial C3a receptor signaling appears to be a critical driver of the precytolytic phase in the evolving NMO lesion, including initial motor impairment. Our results indicate that microglia merit consideration as a potential target for NMO therapeutic intervention.

Chemogenetic manipulation of microglia inhibits neuroinflammation and neuropathic pain in mice

Microglia play an important role in the central sensitization and chronic pain. However, a direct connection between microglial function and pain development in vivo remains incompletely understood. To address this issue, we applied chemogenetic approach by using CX₃CR1^creER/+:R26^LSL-hM4Di/+ transgenic mice to enable expression of inhibitory Designer Receptors Exclusively Activated by Designer Drugs (Gi DREADD) in microglia. We found that microglial Gi DREADD activation inhibited spinal nerve transection (SNT)-induced microglial reactivity as well as chronic pain in both male and female mice. Gi DREADD activation downregulated the transcription factor interferon regulatory factor 8 (IRF8) and its downstream target pro-inflammatory cytokine interleukin 1 beta (IL-1β). Using in vivo spinal cord recording, we found that activation of microglial Gi DREADD attenuated synaptic transmission following SNT. Our results demonstrate that microglial Gi DREADD reduces neuroinflammation, synaptic function and neuropathic pain after SNT. Thus, chemogenetic approaches provide a potential opportunity for interrogating microglial function and neuropathic pain treatment.

Changing Functional Signatures of Microglia along the Axis of Brain Aging

Microglia, the innate immune cells of the brain, are commonly perceived as resident macrophages of the central nervous system (CNS). This definition, however, requires further specification, as under healthy homeostatic conditions, neither morphological nor functional properties of microglia mirror those of classical macrophages. Indeed, microglia adapt exceptionally well to their microenvironment, becoming a legitimate member of the cellular brain architecture. The ramified or surveillant microglia in the young adult brain are characterized by specific morphology (small cell body and long, thin motile processes) and physiology (a unique pattern of Ca²⁺ signaling, responsiveness to various neurotransmitters and hormones, in addition to classic “immune” stimuli). Their numerous physiological functions far exceed and complement their immune capabilities. As the brain ages, the respective changes in the microglial microenvironment impact the functional properties of microglia, triggering further rounds of adaptation. In this review, we discuss the recent data showing how functional properties of microglia adapt to age-related changes in brain parenchyma in a sex-specific manner, with a specific focus on early changes occurring at middle age as well as some strategies counteracting the aging of microglia.

Clinical evaluation of neuroinflammation in child-onset focal epilepsy: a translocator protein PET study

Background

Neuroinflammation is associated with various chronic neurological diseases, including epilepsy; however, neuroimaging approaches for visualizing neuroinflammation have not been used in the clinical routine yet. In this study, we used the translocator protein positron emission tomography (PET) with [¹¹C] DPA713 to investigate neuroinflammation in the epileptogenic zone in patients with child-onset focal epilepsy.

Methods

Patients with intractable focal epilepsy were recruited at the Epilepsy Center of Osaka University; those who were taking any immunosuppressants or steroids were excluded. PET images were acquired for 60 min after intravenous administration of [¹¹C] DPA713. The PET image of [¹¹C] DPA713 was co-registered to individual’s magnetic resonance imaging (MRI), and the standardized uptake value ratio (SUVr) in regions of interest, which were created in non-lesions and lesions, was calculated using the cerebellum as a pseudo-reference region. In the case of epilepsy surgery, the correlation between SUVr in lesions and pathological findings was analyzed.

Results

Twenty-seven patients (mean age: 11.3 ± 6.2 years, male/female: 17/10) were included in this study. Of these, 85.1% showed increased uptake of [¹¹C] DPA713 in the focal epileptic lesion. Three patients showed epileptic spasms, suggesting partial seizure onset, and all 18 patients with abnormal lesions on MRI were similarly highlighted by significant uptake of [¹¹C] DPA713. DPA713-positive patients had a broad range of etiologies, including focal cortical dysplasia, tumors, infarction, and hippocampal sclerosis. Five out of nine MRI-negative patients showed abnormal [¹¹C] DPA713 uptake. The SUVr of [¹¹C] DPA713 in lesions was significantly higher than that in non-lesions. In seven patients who underwent epilepsy surgery, increased [¹¹C] DPA713 uptake was associated with microglial activation.

Conclusions

This study indicates that [¹¹C] DPA713 uptake has valuable sensitivity in the identification of epileptic foci in child-onset focal epilepsy, and inflammation is implicated in the pathophysiology in the epileptic foci caused by various etiologies. Further research is required to establish diagnostic tools for identifying focal epileptogenic zones.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-020-02055-1.

Modulation of microglia activation and Alzheimer's disease: CX3 chemokine ligand 1/CX3CR and P2X7R signaling

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by cognitive deficits. Two hallmarks of AD that cause chronic inflammation and lead to neuronal dysfunction and damage are tau tangles and amyloid plaques. Microglial cells, the primary immune cells of the central nervous system, maintain a homeostatic active/inactive state via a bidirectional, dynamic communication with neurons. Several studies have revealed that dysregulated microglial activation leads to AD pathology. Therefore, we reviewed the relationship between AD and two important signaling complexes, CX3 chemokine ligand 1 (CX3CL1)/CX3CR1 and ATP/P2X₇R, that play critical roles in the regulation of microglial activation. CX3CL1/CX3CR1 is one important signaling which controls the microglia function. Altering this pathway can have opposite effects on amyloid and tau pathology in AD. Another important molecule is P2X₇R which involves in the activation of microglia. Over activation of P2X₇R is evident in AD pathogenesis. In this review, we discuss influence of the two signaling pathways at different stages of AD pathology as well as the drug candidates that can modulate CX3CL1/CX3CR1 and ATP/P2X₇R.

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.

How microglia sense and regulate neuronal activity

Microglia are innate immune cells of the central nervous system that sense extracellular cues. Brain injuries, inflammation, and pathology evoke dynamic structural responses in microglia, altering their morphology and motility. The dynamic motility of microglia is hypothesized to be a critical first step in sensing local alterations and engaging in pattern-specific responses. Alongside their pathological responses, microglia also sense and regulate neuronal activity. In this review, we consider the extracellular molecules, receptors, and mechanisms that allow microglia to sense neuronal activity changes under both hypoactivity and hyperactivity. We also highlight emerging in vivo evidence that microglia regulate neuronal activity, ranging from physiological to pathophysiological conditions. In addition, we discuss the emerging role of calcium signaling in microglial responses to the extracellular environment. The dynamic function of microglia in monitoring and influencing neuronal activity may be critical for brain homeostasis and circuit modification in health and disease.

Microglia dynamics in sleep/wake states and in response to sleep loss

Sleep has an essential role for optimal brain function, but the cellular substrates for sleep regulation are not fully understood. Microglia, the immune cells of the brain, have gained increasingly more attention over the last two decades for their important roles in various brain functions that extend beyond their well-known immune function, including brain development, neuronal protection, and synaptic plasticity. Here we review recent advances in understanding: i) morphological and phenotypic dynamics of microglia including process motility/growth and gene/protein expression, and ii) microglia-neuron interactions including phagocytosis and contact at synapses which alters neuronal circuit activity, both under physiological state in the adult brain. We discuss how the microglia-neuron interactions particularly at synapses could influence microglia and neuronal activities across circadian cycles and sleep/wake states. We also review recent findings on how microglia respond to sleep loss. We conclude by pointing out key questions and proposing suggestions for future research to better understand the role of microglia in sleep regulation, sleep homeostasis, and the function of sleep.

Inflammation, ictogenesis, and epileptogenesis: An exploration through human disease

Epilepsy is seen historically as a disease of aberrant neuronal signaling manifesting as seizures. With the discovery of numerous auto-antibodies and the subsequent growth in understanding of autoimmune encephalitis, there has been an increasing emphasis on the contribution of the innate and adaptive immune system to ictogenesis and epileptogenesis. Pathogenic antibodies, complement activation, CD8+ cytotoxic T cells, and microglial activation are seen, to various degrees, in different seizure-associated neuroinflammatory and autoimmune conditions. These aberrant immune responses are thought to cause disruptions in neuronal signaling, generation of acute symptomatic seizures, and, in some cases, the development of long-term autoimmune epilepsy. Although early treatment with immunomodulatory therapies improves outcomes in autoimmune encephalitides and autoimmune epilepsies, patient identification and treatment selection are not always clear-cut. This review examines the role of the different components of the immune system in various forms of seizure disorders including autoimmune encephalitis, autoimmune epilepsy, Rasmussen encephalitis, febrile infection-related epilepsy syndrome (FIRES), and new-onset refractory status epilepticus (NORSE). In particular, the pathophysiology and unique cytokine profiles seen in these disorders and their links with diagnosis, prognosis, and treatment decision-making are discussed.

Shaping Neuronal Fate: Functional Heterogeneity of Direct Microglia-Neuron Interactions

The functional contribution of microglia to normal brain development, healthy brain function, and neurological disorders is increasingly recognized. However, until recently, the nature of intercellular interactions mediating these effects remained largely unclear. Recent findings show microglia establishing direct contact with different compartments of neurons. Although communication between microglia and neurons involves intermediate cells and soluble factors, direct membrane contacts enable a more precisely regulated, dynamic, and highly effective form of interaction for fine-tuning neuronal responses and fate. Here, we summarize the known ultrastructural, molecular, and functional features of direct microglia-neuron interactions and their roles in brain disease.

Microglia alter the threshold of spreading depolarization and related potassium uptake in the mouse brain

Selective elimination of microglia from the brain was shown to dysregulate neuronal Ca²⁺ signaling and to reduce the incidence of spreading depolarization (SD) during cerebral ischemia. However, the mechanisms through which microglia interfere with SD remained unexplored. Here, we identify microglia as essential modulators of the induction and evolution of SD in the physiologically intact brain in vivo. Confocal- and super-resolution microscopy revealed that a series of SDs induced rapid morphological changes in microglia, facilitated microglial process recruitment to neurons and increased the density of P2Y12 receptors (P2Y12R) on recruited microglial processes. In line with this, depolarization and hyperpolarization during SD were microglia- and P2Y12R-dependent. An absence of microglia was associated with altered potassium uptake after SD and increased the number of c-fos-positive neurons, independently of P2Y12R. Thus, the presence of microglia is likely to be essential to maintain the electrical elicitation threshold and to support the full evolution of SD, conceivably by interfering with the extracellular potassium homeostasis of the brain through sustaining [K⁺]_e re-uptake mechanisms.

Brain cancer induces systemic immunosuppression through release of non-steroid soluble mediators

Immunosuppression of unknown aetiology is a hallmark feature of glioblastoma and is characterized by decreased CD4 T-cell counts and downregulation of major histocompatibility complex class II expression on peripheral blood monocytes in patients. This immunosuppression is a critical barrier to the successful development of immunotherapies for glioblastoma. We recapitulated the immunosuppression observed in glioblastoma patients in the C57BL/6 mouse and investigated the aetiology of low CD4 T-cell counts. We determined that thymic involution was a hallmark feature of immunosuppression in three distinct models of brain cancer, including mice harbouring GL261 glioma, B16 melanoma, and in a spontaneous model of diffuse intrinsic pontine glioma. In addition to thymic involution, we determined that tumour growth in the brain induced significant splenic involution, reductions in peripheral T cells, reduced MHC II expression on blood leucocytes, and a modest increase in bone marrow resident CD4 T cells. Using parabiosis we report that thymic involution, declines in peripheral T-cell counts, and reduced major histocompatibility complex class II expression levels were mediated through circulating blood-derived factors. Conversely, T-cell sequestration in the bone marrow was not governed through circulating factors. Serum isolated from glioma-bearing mice potently inhibited proliferation and functions of T cells both in vitro and in vivo. Interestingly, the factor responsible for immunosuppression in serum is non-steroidal and of high molecular weight. Through further analysis of neurological disease models, we determined that the immunosuppression was not unique to cancer itself, but rather occurs in response to brain injury. Non-cancerous acute neurological insults also induced significant thymic involution and rendered serum immunosuppressive. Both thymic involution and serum-derived immunosuppression were reversible upon clearance of brain insults. These findings demonstrate that brain cancers cause multifaceted immunosuppression and pinpoint circulating factors as a target of intervention to restore immunity.

[Risks of progression of cerebrovascular pathology associated with the activity of the brain purinergic system]

Until now, there is no understanding of the relationship between risk factors and the progression of cerebrovascular pathology. The review presents facts that confirm the involvement of various subtypes of purine P2 receptors in neuron activation, growth and myelination of axons, migration and microglia phagocytosis, astrogliosis, regulation of vascular tone, thrombosis and angiogenesis, neuroinflammation and immune responses. The data suggest the possibility of the activation of purinergic system of the brain during the development of main risk factors for cerebrovascular pathology (age, arterial hypertension, diabetes), as a stereotypical mechanism that can affect the homeostasis of the ensemble "neuron-glia-capillary". Purinergic P2 receptors may be a potential target for the development of pharmacological methods to limit the progression of cerebrovascular pathology.

Delta-gamma phase-amplitude coupling as a biomarker of postictal generalized EEG suppression

Postictal generalized EEG suppression is the state of suppression of electrical activity at the end of a seizure. Prolongation of this state has been associated with increased risk of sudden unexpected death in epilepsy, making characterization of underlying electrical rhythmic activity during postictal suppression an important step in improving epilepsy treatment. Phase-amplitude coupling in EEG reflects cognitive coding within brain networks and some of those codes highlight epileptic activity; therefore, we hypothesized that there are distinct phase-amplitude coupling features in the postictal suppression state that can provide an improved estimate of this state in the context of patient risk for sudden unexpected death in epilepsy. We used both intracranial and scalp EEG data from eleven patients (six male, five female; age range 21–41 years) containing 25 seizures, to identify frequency dynamics, both in the ictal and postictal EEG suppression states. Cross-frequency coupling analysis identified that during seizures there was a gradual decrease of phase frequency in the coupling between delta (0.5–4 Hz) and gamma (30+ Hz), which was followed by an increased coupling between the phase of 0.5–1.5 Hz signal and amplitude of 30–50 Hz signal in the postictal state as compared to the pre-seizure baseline. This marker was consistent across patients. Then, using these postictal-specific features, an unsupervised state classifier—a hidden Markov model—was able to reliably classify four distinct states of seizure episodes, including a postictal suppression state. Furthermore, a connectome analysis of the postictal suppression states showed increased information flow within the network during postictal suppression states as compared to the pre-seizure baseline, suggesting enhanced network communication. When the same tools were applied to the EEG of an epilepsy patient who died unexpectedly, ictal coupling dynamics disappeared and postictal phase-amplitude coupling remained constant throughout. Overall, our findings suggest that there are active postictal networks, as defined through coupling dynamics that can be used to objectively classify the postictal suppression state; furthermore, in a case study of sudden unexpected death in epilepsy, the network does not show ictal-like phase-amplitude coupling features despite the presence of convulsive seizures, and instead demonstrates activity similar to postictal. The postictal suppression state is a period of elevated network activity as compared to the baseline activity which can provide key insights into the epileptic pathology.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Phosphoinositide-3-Kinase γ Is Not a Predominant Regulator of ATP-Dependent Directed Microglial Process Motility or Experience-Dependent Ocular Dominance Plasticity

Microglia are dynamic cells whose extensive interactions with neurons and glia during development allow them to regulate neuronal development and function. The microglial P2Y12 receptor is crucial for microglial responsiveness to extracellular ATP and mediates numerous microglial functions, including ATP-dependent directional motility, microglia-neuron interactions, and experience-dependent synaptic plasticity. However, little is known about the downstream signaling effectors that mediate these diverse actions of P2Y12. Phosphoinositide-3-kinase γ (PI3Kγ), a lipid kinase activated downstream of G_i-protein-coupled receptors such as P2Y12, could translate localized extracellular ATP signals into directed microglial action and serve as a broad effector of P2Y12-dependent signaling. Here, we used pharmacological and genetic methods to manipulate P2Y12 and PI3Kγ signaling to determine whether inhibiting PI3Kγ phenocopied the loss of P2Y12 signaling in mouse microglia. While pan-inhibition of all PI3K activity substantially affected P2Y12-dependent microglial responses, our results suggest that PI3Kγ specifically is only a minor part of the P2Y12 signaling pathway. PI3Kγ was not required to maintain homeostatic microglial morphology or their dynamic surveillance in vivo Further, PI3Kγ was not strictly required for P2Y12-dependent microglial responses ex vivo or in vivo, although we did observe subtle deficits in the recruitment of microglial process toward sources of ATP. Finally, PI3Kγ was not required for ocular dominance plasticity, a P2Y12-dependent form of experience-dependent synaptic plasticity that occurs in the developing visual cortex. Overall, our results demonstrate that PI3Kγ is not the major mediator of P2Y12 function in microglia, but may have a role in amplifying or fine-tuning the chemotactic response.

Microglia in depression: current perspectives

Major depressive disorder (MDD) is a prevalent psychiatric disease that involves malfunctions of different cell types in the brain. Accumulating studies started to reveal that microglia, the primary resident immune cells, play an important role in the development and progression of depression. Microglia respond to stress-triggered neuroinflammation, and through the release of proinflammatory cytokines and their metabolic products, microglia may modulate the function of neurons and astrocytes to regulate depression. In this review, we focused on the role of microglia in the etiology of depression. We discussed the dynamic states of microglia; the correlative and causal evidence of microglial abnormalities in depression; possible mechanisms of how microglia sense depression-related stress and modulate depression state; and how antidepressive therapies affect microglia. Understanding the role of microglia in depression may shed light on developing new treatment strategies to fight against this devastating mental illness.

Transitions between neocortical seizure and non-seizure-like states and their association with presynaptic glutamate release

The transition between seizure and non-seizure states in neocortical epileptic networks is governed by distinct underlying dynamical processes. Based on the gamma distribution of seizure and inter-seizure durations, over time, seizures are highly likely to self-terminate; whereas, inter-seizure durations have a low chance of transitioning back into a seizure state. Yet, the chance of a state transition could be formed by multiple overlapping, unknown synaptic mechanisms. To identify the relationship between the underlying synaptic mechanisms and the chance of seizure-state transitions, we analyzed the skewed histograms of seizure durations in human intracranial EEG and seizure-like events (SLEs) in local field potential activity from mouse neocortical slices, using an objective method for seizure state classification. While seizures and SLE durations were demonstrated to have a unimodal distribution (gamma distribution shape parameter >1), suggesting a high likelihood of terminating, inter-SLE intervals were shown to have an asymptotic exponential distribution (gamma distribution shape parameter <1), suggesting lower probability of cessation. Then, to test cellular mechanisms for these distributions, we studied the modulation of synaptic neurotransmission during, and between, the in vitro SLEs. Using simultaneous local field potential and whole-cell voltage clamp recordings, we found a suppression of presynaptic glutamate release at SLE termination, as demonstrated by electrically- and optogenetically-evoked excitatory postsynaptic currents (EPSCs), and focal hypertonic sucrose application. Adenosine A1 receptor blockade interfered with the suppression of this release, changing the inter-SLE shape parameter from asymptotic exponential to unimodal, altering the chance of state transition occurrence with time. These findings reveal a critical role for presynaptic glutamate release in determining the chance of neocortical seizure state transitions.

Microglial depletion aggravates the severity of acute and chronic seizures in mice

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, CX3CR1CreER/+:R26iDTA/+, CX3CR1CreER/+:R26iDTR/+, and CX3CR1CreER/+:Csf1rFlox/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.

Negative feedback control of neuronal activity by microglia

Microglia, the brain’s resident macrophages, help to regulate brain function by removing dying neurons, pruning non-functional synapses, and producing ligands that support neuronal survival¹. Here we show that microglia are also critical modulators of neuronal activity and associated behavioural responses in mice. Microglia respond to neuronal activation by suppressing neuronal activity, and ablation of microglia amplifies and synchronizes the activity of neurons, leading to seizures. Suppression of neuronal activation by microglia occurs in a highly region-specific fashion and depends on the ability of microglia to sense and catabolize extracellular ATP, which is released upon neuronal activation by neurons and astrocytes. ATP triggers the recruitment of microglial protrusions and is converted by the microglial ATP/ADP hydrolysing ectoenzyme CD39 into AMP; AMP is then converted into adenosine by CD73, which is expressed on microglia as well as other brain cells. Microglial sensing of ATP, the ensuing microglia-dependent production of adenosine, and the adenosine-mediated suppression of neuronal responses via the adenosine receptor A₁R are essential for the regulation of neuronal activity and animal behaviour. Our findings suggest that this microglia-driven negative feedback mechanism operates similarly to inhibitory neurons and is essential for protecting the brain from excessive activation in health and disease.

The role of inflammation in epileptogenesis

Epilepsy is a chronic neurological disorder that has an extensive impact on a patient’s life. Accumulating evidence has suggested that inflammation participates in the progression of spontaneous and recurrent seizures. Pro-convulsant incidences can stimulate immune cells, augment the release of pro-inflammatory cytokines, elicit neuronal excitation as well as blood-brain barrier (BBB) dysfunction, and finally trigger the generation or recurrence of seizures. Understanding the pathogenic roles of inflammatory mediators, including inflammatory cytokines, cells, and BBB, in epileptogenesis will be beneficial for the treatment of epilepsy. In this systematic review, we performed a literature search on the PubMed database using the following keywords: “epilepsy” or “seizures” or “epileptogenesis”, and “immunity” or “inflammation” or “neuroinflammation” or “damage-associated molecular patterns” or “cytokines” or “chemokines” or “adhesion molecules” or “microglia” or “astrocyte” or “blood-brain barrier”. We summarized the classic inflammatory mediators and their pathogenic effects in the pathogenesis of epilepsy, based on the most recent findings from both human and animal model studies.

Microglia modulate the structure and function of the hippocampus after early-life seizures

The hippocampus is a brain region well-known to exhibit structural and functional changes in temporal lobe epilepsy. Studies analyzing the brains of patients with epilepsy and those from animal models of epilepsy have revealed that microglia are excessively activated, especially in the hippocampus. These findings suggest that microglia may contribute to the onset and aggravation of epilepsy; however, direct evidence for microglial involvement or the underlying mechanisms by which this occurs remain to be fully discovered. To date, neuron-microglia interactions have been vigorously studied in adult epilepsy models; such studies have clarified microglial responses to excessive synchronous firing of neurons. In contrast, the role of microglia in the postnatal brain of patients with epileptic seizures remain largely unclear. Some early-life seizures, such as complex febrile seizures, have been shown to cause structural and functional changes in the brain, which is a risk factor for future development of epilepsy. Because brain structure and function are actively modulated by microglia in both health and disease, it is essential to clarify the role of microglia in early-life seizures and its impact on epileptogenesis.

Actin-mediated Microglial Chemotaxis via G-Protein Coupled Purinergic Receptor in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease mainly associated with aging, oxidative stress and genetic mutations. There are two pathological proteins involved in AD; Amyloid-β peptide and microtubule-associated protein Tau (MAPT). The β- and γ-secretase enzyme cleaves the Amyloid precursor protein, which results in the formation of extracellular plaques in brain. While, Tau undergoes hyperphosphorylation and other post-translational modifications (PTMs), which eventually generates Tau oligomers, and intracellular neurofibrillary tangles (NFTs) in neurons. Moreover, the brain-resident glia and infiltrated macrophages elevate the level of CNS inflammation, which trigger the oxidative damage of neuronal circuits by reactive oxygen species (ROS) and Nitric oxide (NO). Microglia is the primary immune cell in the CNS, which is continuously surveilling the neuronal synapses and pathogen invasion. Microglia in the resting state is called 'Ramified', which possess long surveilling extensions with a small cell body. But, upon activation, microglia retracts the cellular extensions and transform into round migratory cells, called as 'Amoeboid' state. Activated microglia undergoes actin remodeling by forming lamellipodia and filopodia, which directs the migratory axis while podosomes formed are involved in extracellular matrix degradation for invasion. Protein-aggregates in malfunctioning synapses and in CNS milieu can be detected by microglia, which results in its activation and migration. Subsequently, the phagocytosis of synapses leads to the inflammatory burst and memory loss. The extracellular nucleotides released from damaged neurons and the cytokine-chemokine gradients allow the neighboring microglia and macrophages to migrate-infiltrate at the site of neuronal-damage. The ionotropic (P2XR) and metabotropic (P2YR) purinergic receptor recognize extracellular ATP/ADP, which propagates through the intracellular calcium signaling, chemotaxis, phagocytosis and inflammation. The P2Y receptors give 'find me' or 'eat me' signals to microglia to either migrate or phagocytose cellular debris. Further, the actin cytoskeleton helps microglia to mediate directed chemotaxis and neuronal repair during neurodegeneration. Hence, we aim to emphasize the connection between purinergic signaling and actin-driven mechanical movements of microglia for migration and inflammation in AD.

microRNA-421-3p prevents inflammatory response in cerebral ischemia/reperfusion injury through targeting m6A Reader YTHDF1 to inhibit p65 mRNA translation

OBJECTIVE

Ischemic stroke is one of the leading causes of death globally, and inflammation is considered as a vital contributor to the pathophysiology of ischemic stroke. Recently, microRNA-421-3p-derived macrophages is found to promote motor function recovery in spinal cord injury. Here, we explored whether microRNA-421-3p is involved in inflammation responses during cerebral ischemia/reperfusion (I/R) injury and its molecular mechanism.

METHODS

An in vivo experimental animal model of intraluminal middle cerebral artery occlusion/reperfusion (MCAO/R) and in vitro model of microglial subjected to oxygen-glucose deprivation and reoxygenation (OGD/R) were used. The effects of microRNA-421-3p on cerebral I/R injury and its underlying mechanism were detected by quantitative real-time PCR, western blotting, immunofluorescence staining, RNA immunoprecipitation, flow cytometry, luciferase reporter assay, and bioinformatics analysis.

RESULTS

We find that microRNA-421-3p is significantly decreased in cerebral I/R injury in vitro and in vivo. Furthermore, overexpression of microRNA-421-3p evidently suppresses pro-inflammatory factor expressions and inhibits NF-κB p65 protein expression and nuclear translocation in BV2 microglia cells treated with OGD/R. However, microRNA-421-3p neither promotes p65 mRNA expression, nor affects p65 mRNA or protein stability. Moreover, we find the m6A 'reader' protein YTH domain family protein 1 (YTHDF1) is the specific target of microRNA-421-3p, and YTHDF1 specifically binds to the m6a site of p65 mRNA to promote its translation.

CONCLUSION

microRNA-421-3p prevents inflammatory response in cerebral ischemia/reperfusion injury through targeting YTHDF1 to inhibit p65 mRNA translation. These findings provide novel insights into understanding the molecular pathogenesis of cerebral I/R injury.

Diazepam limits microglia-mediated neuronal remodeling in the prefrontal cortex and associated behavioral consequences following chronic unpredictable stress

Chronic stress induces neuronal atrophy and synaptic loss in the medial prefrontal cortex (PFC), and this leads to behavioral and cognitive impairments. Our recent findings indicate that microglia contribute to structural remodeling of neurons via increased colony-stimulating factor (CSF)-1 in the medial PFC. Other work shows that chronic stress induces aberrant neuronal activity in the medial PFC, and that neuronal hyperactivity increases CSF1 signaling and alters microglia function. Thus, the present studies were designed to examine the role of neuronal activity in stress-induced CSF1 signaling and microglia-mediated neuronal remodeling in the medial PFC. Additional analyses probed stress effects on the dorsal hippocampus (HPC), basolateral amygdala (BLA), and somatosensory cortex (SSCTX). Mice were exposed to chronic unpredictable stress (CUS) or handled intermittently as controls, and received daily injection of vehicle or diazepam (1 mg/kg). As anticipated, diazepam attenuated CUS-induced behavioral despair and cognitive impairments. Further studies showed that diazepam normalized Csf1 and C3 mRNA in the PFC, and prevented increases in Csf1r and Cd11b in frontal cortex microglia following CUS. Stress had no effect on neuroimmune gene expression in the HPC. Confocal imaging in Thy1-GFP(M) mice demonstrated that diazepam limited microglial engulfment of neuronal elements and blocked CUS-induced dendritic spine loss in the medial PFC. Altogether, these findings indicate that modulation of chronic stress-induced neuronal activity limits microglia-mediated neuronal remodeling in the medial PFC, and subsequent behavioral and cognitive consequences.

Microglial mTOR is Neuronal Protective and Antiepileptogenic in the Pilocarpine Model of Temporal Lobe Epilepsy

Excessive activation of mammalian target of rapamycin (mTOR) signaling is epileptogenic in genetic epilepsy. However, the exact role of microglial mTOR in acquired epilepsy remains to be clarified. In the present study, we found that mTOR is strongly activated in microglia following excitatory injury elicited by status epilepticus. To determine the role of microglial mTOR signaling in excitatory injury and epileptogenesis, we generated mice with restrictive deletion of mTOR in microglia. Both male and female mice were used in the present study. We found that mTOR-deficient microglia lost their typical proliferative and inflammatory responses to excitatory injury, whereas the proliferation of astrocytes was preserved. In addition, mTOR-deficient microglia did not effectively engulf injured/dying neurons. More importantly, microglial mTOR-deficient mice displayed increased neuronal loss and developed more severe spontaneous seizures. These findings suggest that microglial mTOR plays a protective role in mitigating neuronal loss and attenuating epileptogenesis in the excitatory injury model of epilepsy.SIGNIFICANCE STATEMENT The mammalian target of rapamycin (mTOR) pathway is strongly implicated in epilepsy. However, the effect of mTOR inhibitors in preclinical models of acquired epilepsy is inconsistent. The broad presence of mTOR signaling in various brain cells could prevent mTOR inhibitors from achieving a net therapeutic effect. This conundrum has spurred further investigation of the cell type-specific effects of mTOR signaling in the CNS. We found that activation of microglial mTOR is antiepileptogenic. Thus, microglial mTOR activation represents a novel antiepileptogenic route that appears to parallel the proepileptogenic route of neuronal mTOR activation. This may explain why the net effect of mTOR inhibitors is paradoxical in the acquired models of epilepsy. Our findings could better guide the use of mTOR inhibitors in preventing acquired epilepsy.

Microglia as therapeutic target in central nervous system disorders

Chronic microglial activation is associated with the pathogenesis of several CNS disorders. Microglia show phenotypic diversity and functional complexity in diseased CNS. Thus, understanding the pathology-specific heterogeneity of microglial behavior is crucial for the future development of microglia-modulating therapy for variety of CNS disorders. This review summarizes up-to-date knowledge on how microglia contribute to CNS homeostasis during development and throughout adulthood. We discuss the heterogeneity of microglial phenotypes in the context of CNS disorders with an emphasis on neurodegenerative diseases, demyelinating diseases, CNS trauma, and epilepsy. We conclude this review with a discussion about the disease-specific heterogeneity of microglial function and how it could be exploited for therapeutic intervention.

Microglia depletion exacerbates acute seizures and hippocampal neuronal degeneration in mouse models of epilepsy

Epileptic seizures are the manifestation of hypersynchronous and excessive neuronal excitation. While the glutamatergic and GABAergic neurons play major roles in shaping fast neuronal excitation/inhibition homeostasis, it is well illustrated that astrocytes profoundly regulate neuronal excitation by controlling glutamate, GABA, cannabinoids, adenosine, and concentration of K⁺ around neurons. However, little is known about whether microglia take part in the regulation of acute neuronal excitation and ongoing epileptic behaviors. We proposed that if microglia are innately ready to respond to epileptic overexcitation, depletion of microglia might alter neuronal excitability and severity of acute epileptic seizures. We found that microglia depletion by plx3397, an inhibitor of CSF1R, exacerbates seizure severity and excitotoxicity-induced neuronal degeneration, indicating that microglia are rapidly responsive to the change of excitation/inhibition homeostasis and participate in the protection of neurons from overexcitation.

Role of glia in optic nerve

Glial cells are critically important for maintenance of neuronal activity in the central nervous system (CNS), including the optic nerve (ON). However, the ON has several unique characteristics, such as an extremely high myelination level of retinal ganglion cell (RGC) axons throughout the length of the nerve (with virtually all fibers myelinated by 7 months of age in humans), lack of synapses and very narrow geometry. Moreover, the optic nerve head (ONH) - a region where the RGC axons exit the eye - represents an interesting area that is morphologically distinct in different species. In many cases of multiple sclerosis (demyelinating disease of the CNS) vision problems are the first manifestation of the disease, suggesting that RGCs and/or glia in the ON are more sensitive to pathological conditions than cells in other parts of the CNS. Here, we summarize current knowledge on glial organization and function in the ON, focusing on glial support of RGCs. We cover both well-established concepts on the important role of glial cells in ON health and new findings, including novel insights into mechanisms of remyelination, microglia/NG2 cell-cell interaction, astrocyte reactivity and the regulation of reactive astrogliosis by mitochondrial fragmentation in microglia.

Ketamine/xylazine and barbiturates modulate microglial morphology and motility differently in a mouse model

Microglia, the resident immune cells of the brain, are highly ramified and motile and their morphology is strongly linked to their function. Microglia constantly monitor the brain parenchyma and are crucial for maintaining brain homeostasis and fine-tuning neuronal networks. Besides affecting neurons, anesthetics may have wide-ranging effects mediated by non-neuronal cells and in particular microglia. We thus examined the effect of two commonly used anesthetic agents, ketamine/xylazine and barbiturates, on microglial motility and morphology. A combination of two-photon in vivo imaging and electroencephalography (EEG) recordings in unanesthetized and anesthetized mice as well as automated analysis of ex vivo sections were used to assess morphology and dynamics of microglia. We found that administration of ketamine/xylazine and pentobarbital anesthesia resulted in quite distinct EEG profiles. Both anesthetics reduced microglial motility, but only ketamine/xylazine administration led to reduction of microglial complexity in vivo. The change of cellular dynamics in vivo was associated with a region-dependent reduction of several features of microglial cells ex vivo, such as the complexity index and the ramification length, whereas thiopental altered the size of the cytoplasm. Our results show that anesthetics have considerable effects on neuronal activity and microglial morphodynamics and that barbiturates may be a preferred anesthetic agent for the study of microglial morphology. These findings will undoubtedly raise compelling questions about the functional relevance of anesthetics on microglial cells in neuronal physiology and anesthesia-induced neurotoxicity.

Nanoscale Surveillance of the Brain by Microglia via cAMP-Regulated Filopodia

Microglia, the brain's immune cells, maintain homeostasis and sense pathological changes by continuously surveying the parenchyma with highly motile large processes. Here, we demonstrate that microglia also use thin actin-dependent filopodia that allow fast nanoscale sensing within discrete regions. Filopodia are distinct from large processes by their size, speed, and regulation mechanism. Increasing cyclic AMP (cAMP) by activating norepinephrine G_s-coupled receptors, applying nitric oxide, or inhibiting phosphodiesterases rapidly increases filopodia but collapses large processes. Alternatively, G_i-coupled P2Y12 receptor activation collapses filopodia but triggers large processes extension with bulbous tips. Similar control of cytoskeletal dynamics and microglial morphology by cAMP is observed in ramified primary microglia, suggesting that filopodia are intrinsically generated sensing structures. Therefore, nanoscale surveillance of brain parenchyma by microglia requires localized cAMP increases that drive filopodia formation. Shifting intracellular cAMP levels controls the polarity of microglial responses to changes in brain homeostasis and alters the scale of immunosurveillance.