Microglia-Astrocyte Crosstalk: An Intimate Molecular Conversation

The microglial component of amyotrophic lateral sclerosis

Microglia are the primary immune cells of the CNS, carrying out key homeostatic roles and undergoing context-dependent and temporally regulated changes in response to injury and neurodegenerative diseases. Microglia have been implicated in playing a role in amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by extensive motor neuron loss leading to paralysis and premature death. However, as the pathomechansims of ALS are increasingly recognized to involve a multitude of different cell types, it has been difficult to delineate the specific contribution of microglia to disease. Here, we review the literature of microglial involvement in ALS and discuss the evidence for the neurotoxic and neuroprotective pathways that have been attributed to microglia in this disease. We also discuss accumulating evidence for spatiotemporal regulation of microglial activation in this context. A deeper understanding of the role of microglia in the ‘cellular phase’ of ALS is crucial in the development of mechanistically rationalized therapies.

Diffusion Tensor Imaging Detects Acute Pathology-Specific Changes in the P301L Tauopathy Mouse Model Following Traumatic Brain Injury

Traumatic brain injury (TBI) has been linked with tauopathy. However, imaging methods that can non-invasively detect tau-protein abnormalities following TBI need further investigation. This study aimed to investigate the potential of diffusion tensor imaging (DTI) to detect tauopathy following TBI in P301L mutant-tau-transgenic-pR5-mice. A total of 24 9-month-old pR5 mice were randomly assigned to sham and TBI groups. Controlled cortical injuries/craniotomies were performed for TBI/sham groups followed by DTI data acquisition on days 1 and 7 post-injury. DTI data were analyzed by using voxelwise analysis and track-based spatial statistics for gray matter and white matter. Further, immunohistochemistry was performed for total-tau and phosphorylated-tau, astrocytes, and microglia. To detect the association of DTI with these pathological markers, a correlation analysis was performed between DTI and histology findings. At day 1 post-TBI, DTI revealed a widespread reduction in fractional anisotropy (FA) and axial diffusivity (AxD) in the TBI group compared to shams. On day 7, further reduction in FA, AxD, and mean diffusivity and increased radial diffusivity were observed. FA was significantly increased in the amygdala and cortex. Correlation results showed that in the ipsilateral hemisphere FA reduction was associated with increased phosphorylated-tau and glial-immunoreactivity, whereas in the contralateral regions, the FA increase was associated with increased immunostaining for astrocytes. This study is the first to exploit DTI to investigate the effect of TBI in tau-transgenic mice. We show that alterations in the DTI signal were associated with glial activity following TBI and would most likely reflect changes that co-occur with/without phosphorylated-tau. In addition, FA may be a promising measure to identify discrete pathological processes such as increased astroglia activation, tau-hyperphosphorylation or both in the brain following TBI.

Hypothalamic Microglial Heterogeneity and Signature under High Fat Diet-Induced Inflammation

Under high-fat feeding, the hypothalamus atypically undergoes pro-inflammatory signaling activation. Recent data from transcriptomic analysis of microglia from rodents and humans has allowed the identification of several microglial subpopulations throughout the brain. Numerous studies have clarified the roles of these cells in hypothalamic inflammation, but how each microglial subset plays its functions upon inflammatory stimuli remains unexplored. Fortunately, these data unveiling microglial heterogeneity have triggered the development of novel experimental models for studying the roles and characteristics of each microglial subtype. In this review, we explore microglial heterogeneity in the hypothalamus and their crosstalk with astrocytes under high fat diet-induced inflammation. We present novel currently available ex vivo and in vivo experimental models that can be useful when designing a new research project in this field of study. Last, we examine the transcriptomic data already published to identify how the hypothalamic microglial signature changes upon short-term and prolonged high-fat feeding.

Neurovascular Units and Neural-Glia Networks in Intracerebral Hemorrhage: from Mechanisms to Translation

Intracerebral hemorrhage (ICH), the most lethal type of stroke, often leads to poor outcomes in the clinic. Due to the complex mechanisms and cell-cell crosstalk during ICH, the neurovascular unit (NVU) was proposed to serve as a promising therapeutic target for ICH research. This review aims to summarize the development of pathophysiological shifts in the NVU and neural-glia networks after ICH. In addition, potential targets for ICH therapy are discussed in this review. Beyond cerebral blood flow, the NVU also plays an important role in protecting neurons, maintaining central nervous system (CNS) homeostasis, coordinating neuronal activity among supporting cells, forming and maintaining the blood-brain barrier (BBB), and regulating neuroimmune responses. During ICH, NVU dysfunction is induced, along with neuronal cell death, microglia and astrocyte activation, endothelial cell (EC) and tight junction (TJ) protein damage, and BBB disruption. In addition, it has been shown that certain targets and candidates can improve ICH-induced secondary brain injury based on an NVU and neural-glia framework. Moreover, therapeutic approaches and strategies for ICH are discussed.

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.

Cell-free oxidized hemoglobin drives reactive oxygen species production and pro-inflammation in an immature primary rat mixed glial cell culture

Background

Germinal matrix intraventricular hemorrhage (GM-IVH) is associated with deposition of redox active cell-free hemoglobin (Hb), derived from hemorrhagic cerebrospinal fluid (CSF), in the cerebrum and cerebellum. In a recent study, using a preterm rabbit pup model of IVH, intraventricularly administered haptoglobin (Hp), a cell-free Hb scavenger, partially reversed the damaging effects observed following IVH. Together, this suggests that cell-free Hb is central in the pathophysiology of the injury to the immature brain following GM-IVH. An increased understanding of the causal pathways and metabolites involved in eliciting the damaging response following hemorrhage is essential for the continued development and implementation of neuroprotective treatments of GM-IVH in preterm infant.

Methods

We exposed immature primary rat mixed glial cells to hemorrhagic CSF obtained from preterm human infants with IVH (containing a mixture of Hb-metabolites) or to a range of pure Hb-metabolites, incl. oxidized Hb (mainly metHb with iron in Fe³⁺), oxyHb (mainly Fe²⁺), or low equivalents of heme, with or without co-administration with human Hp (a mixture of isotype 2-2/2-1). Following exposure, cellular response, reactive oxygen species (ROS) generation, secretion and expression of pro-inflammatory cytokines and oxidative markers were evaluated.

Results

Exposure of the glial cells to hemorrhagic CSF as well as oxidized Hb, but not oxyHb, resulted in a significantly increased rate of ROS production that positively correlated with the rate of production of pro-inflammatory and oxidative markers. Congruently, exposure to oxidized Hb caused a disintegration of the polygonal cytoskeletal structure of the glial cells in addition to upregulation of F-actin proteins in microglial cells. Co-administration of Hp partially reversed the damaging response of hemorrhagic CSF and oxidized Hb.

Conclusion

Exposure of mixed glial cells to oxidized Hb initiates a pro-inflammatory and oxidative response with cytoskeletal disintegration. Early administration of Hp, aiming to minimize the spontaneous autoxidation of cell-free oxyHb and liberation of heme, may provide a therapeutic benefit in preterm infant with GM-IVH.

Toxoplasma Infection Induces Sustained Up-Regulation of Complement Factor B and C5a Receptor in the Mouse Brain via Microglial Activation: Implication for the Alternative Complement Pathway Activation and Anaphylatoxin Signaling in Cerebral Toxoplasmosis

Toxoplasma gondii is a neurotropic protozoan parasite, which is linked to neurological manifestations in immunocompromised individuals as well as severe neurodevelopmental sequelae in congenital toxoplasmosis. While the complement system is the first line of host defense that plays a significant role in the prevention of parasite dissemination, Toxoplasma artfully evades complement-mediated clearance via recruiting complement regulatory proteins to their surface. On the other hand, the details of Toxoplasma and the complement system interaction in the brain parenchyma remain elusive. In this study, infection-induced changes in the mRNA levels of complement components were analyzed by quantitative PCR using a murine Toxoplasma infection model in vivo and primary glial cells in vitro. In addition to the core components C3 and C1q, anaphylatoxin C3a and C5a receptors (C3aR and C5aR1), as well as alternative complement pathway components properdin (CFP) and factor B (CFB), were significantly upregulated 2 weeks after inoculation. Two months post-infection, CFB, C3, C3aR, and C5aR1 expression remained higher than in controls, while CFP upregulation was transient. Furthermore, Toxoplasma infection induced significant increase in CFP, CFB, C3, and C5aR1 in mixed glial culture, which was abrogated when microglial activation was inhibited by pre-treatment with minocycline. This study sheds new light on the roles for the complement system in the brain parenchyma during Toxoplasma infection, which may lead to the development of novel therapeutic approaches to Toxoplasma infection-induced neurological disorders.

The effect of aged microglia on synaptic impairment and its relevance in neurodegenerative diseases

Microglia serve key functions in the central nervous system (CNS), participating in the establishment and regulation of synapses and the neuronal network, and regulating activity-dependent plastic changes. As the neuroimmune system, they respond to endogenous and exogenous signals to protect the CNS. In aging, one of the main changes is the establishment of inflamm-aging, a mild chronic inflammation that reduces microglial response to stressors. Neuroinflammation depends mainly on the increased activation of microglia. Microglia over-activation may result in a reduced capacity for performing normal functions related to migration, clearance, and the adoption of an anti-inflammatory state, contributing to an increased susceptibility for neurodegeneration. Oxidative stress contributes both to aging and to the progression of neurodegenerative diseases. Increased production of reactive oxygen species (ROS) and neuroinflammation associated with age- and disease-dependent mechanisms affect synaptic activity and neurotransmission, leading to cognitive dysfunction. Astrocytes prevent microglial cell cytotoxicity by mechanisms mediated by transforming growth factor β1 (TGFβ1). However, TGFβ1-Smad3 pathway is impaired in aging, and the age-related impairment of TGFβ signaling can reduce protective activation while facilitating cytotoxic activation of microglia. A critical analysis on the effect of aging microglia on neuronal function is relevant for the understanding of age-related changes on neuronal function. Here, we present evidence in the context of the "microglial dysregulation hypothesis", which leads to the reduction of the protective functions and increased cytotoxicity of microglia, to discuss the mechanisms involved in neurodegenerative changes and Alzheimer's disease.

Müller glia-myeloid cell crosstalk accelerates optic nerve regeneration in the adult zebrafish

Neurodegenerative disorders, characterized by progressive neuronal loss, eventually lead to functional impairment in the adult mammalian central nervous system (CNS). Importantly, these deteriorations are irreversible, due to the very limited regenerative potential of these CNS neurons. Stimulating and redirecting neuroinflammation was recently put forward as an important approach to induce axonal regeneration, but it remains elusive how inflammatory processes and CNS repair are intertwined. To gain more insight into these interactions, we investigated how immunomodulation affects the regenerative outcome after optic nerve crush (ONC) in the spontaneously regenerating zebrafish. First, inducing intraocular inflammation using zymosan resulted in an acute inflammatory response, characterized by an increased infiltration and proliferation of innate blood-borne immune cells, reactivation of Müller glia, and altered retinal cytokine expression. Strikingly, inflammatory stimulation also accelerated axonal regrowth after optic nerve injury. Second, we demonstrated that acute depletion of both microglia and macrophages in the retina, using pharmacological treatments with both the CSF1R inhibitor PLX3397 and clodronate liposomes, compromised optic nerve regeneration. Moreover, we observed that csf1ra/b double mutant fish, lacking microglia in both retina and brain, displayed accelerated RGC axonal regrowth after ONC, which was accompanied with unusual Müller glia proliferative gliosis. Altogether, our results highlight the importance of altered glial cell interactions in the axonal regeneration process after ONC in adult zebrafish. Unraveling the relative contribution of the different cell types, as well as the signaling pathways involved, may pinpoint new targets to stimulate repair in the vertebrate CNS.

Common genes and pathways involved in the response to stressful stimuli by astrocytes: A meta-analysis of genome-wide expression studies

Astrocytes play pivotal roles in the brain and they become reactive under stress conditions. Here, we carried out, for the first time, an integrative meta-analysis of genome-wide expression profiling of astrocytes from human and mouse exposed to different stressful stimuli (hypoxia, infections by virus and bacteria, cytokines, ethanol, among others). We identified common differentially expressed genes and pathways in human and murine astrocytes. Our results showed that astrocytes induce expression of genes associated with stress response and immune system regulation when they are exposed to stressful stimuli, whereas genes related to neurogenesis are found as downregulated. Several of the identified genes showed to be important hubs in the protein-protein interaction analysis (TRAF2, CDC37 and PAX6). This work demonstrates that despite astrocytes are highly heterogeneous and complex, there are common gene expression signatures that can be triggered under distinct detrimental stimuli, which opens an opportunity for exploring other possible markers of reactivity.

The NFκB Antagonist CDGSH Iron-Sulfur Domain 2 Is a Promising Target for the Treatment of Neurodegenerative Diseases

Proinflammatory response and mitochondrial dysfunction are related to the pathogenesis of neurodegenerative diseases (NDs). Nuclear factor κB (NFκB) activation has been shown to exaggerate proinflammation and mitochondrial dysfunction, which underlies NDs. CDGSH iron-sulfur domain 2 (CISD2) has been shown to be associated with peroxisome proliferator-activated receptor-β (PPAR-β) to compete for NFκB and antagonize the two aforementioned NFκB-provoked pathogeneses. Therefore, CISD2-based strategies hold promise in the treatment of NDs. CISD2 protein belongs to the human NEET protein family and is encoded by the CISD2 gene (located at 4q24 in humans). In CISD2, the [2Fe-2S] cluster, through coordinates of 3-cysteine-1-histidine on the CDGSH domain, acts as a homeostasis regulator under environmental stress through the transfer of electrons or iron-sulfur clusters. Here, we have summarized the features of CISD2 in genetics and clinics, briefly outlined the role of CISD2 as a key physiological regulator, and presented modalities to increase CISD2 activity, including biomedical engineering or pharmacological management. Strategies to increase CISD2 activity can be beneficial for the prevention of inflammation and mitochondrial dysfunction, and thus, they can be applied in the management of NDs.

Delayed Onset Muscle Soreness (DOMS): The Repeated Bout Effect and Chemotherapy-Induced Axonopathy May Help Explain the Dying-Back Mechanism in Amyotrophic Lateral Sclerosis and Other Neurodegenerative Diseases

Delayed onset muscle soreness (DOMS) is hypothesized to be caused by glutamate excitotoxicity-induced acute compression axonopathy of the sensory afferents in the muscle spindle. Degeneration of the same sensory afferents is implicated in the disease onset and progression of amyotrophic lateral sclerosis (ALS). A series of “silent” acute compression proprioceptive axonopathies with underlying genetic/environmental factors, damaging eccentric contractions and the non-resolving neuroinflammatory process of aging could lead to ALS disease progression. Since the sensory terminals in the muscle spindle could not regenerate from the micro-damage in ALS, unlike in DOMS, the induced protective microcircuits and their long-term functional plasticity (the equivalent of the repeated bout effect in DOMS) will be dysfunctional. The acute stress invoking osteocalcin, bradykinin, COX1, COX2, GDNF, PGE2, NGF, glutamate and N-methyl-D-aspartate (NMDA) receptors are suggested to be the critical signalers of this theory. The repeated bout effect of DOMS and the dysfunctional microcircuits in ALS are suggested to involve several dimensions of memory and learning, like pain memory, inflammation, working and episodic memory. The spatial encoding of these memory dimensions is compromised in ALS due to blunt position sense from the degenerating proprioceptive axon terminals of the affected muscle spindles. Dysfunctional microcircuits progressively and irreversibly interfere with postural control, with motor command and locomotor circuits, deplete the neuroenergetic system, and ultimately interfere with life-sustaining central pattern generators in ALS. The activated NMDA receptor is suggested to serve the “gate control” function in DOMS and ALS in line with the gate control theory of pain. Circumvention of muscle spindle-loading could be a choice of exercise therapy in muscle spindle-affected neurodegenerative diseases.

Glial Metabolic Reprogramming in Amyotrophic Lateral Sclerosis

ALS is a human neurodegenerative disorder that induces a progressive paralysis of voluntary muscles due to motor neuron loss. The causes are unknown, and there is no curative treatment available. Mitochondrial dysfunction is a hallmark of ALS pathology; however, it is currently unknown whether it is a cause or a consequence of disease progression. Recent evidence indicates that glial mitochondrial function changes to cope with energy demands and critically influences neuronal death and disease progression. Aberrant glial cells detected in the spinal cord of diseased animals are characterized by increased proliferation rate and reduced mitochondrial bioenergetics. These features can be compared with cancer cell behavior of adapting to nutrient microenvironment by altering energy metabolism, a concept known as metabolic reprogramming. We focus on data that suggest that aberrant glial cells in ALS undergo metabolic reprogramming and profound changes in glial mitochondrial activity, which are associated with motor neuron death in ALS. This review article emphasizes on the association between metabolic reprogramming and glial reactivity, bringing new paradigms from the area of cancer research into neurodegenerative diseases. Targeting glial mitochondrial function and metabolic reprogramming may result in promising therapeutic strategies for ALS.

Brain astrocytes and microglia express functional MR1 molecules that present microbial antigens to mucosal-associated invariant T (MAIT) cells

It is unknown whether brain astrocytes and microglia have the capacity to present microbial antigens via the innate immune MR1/MAIT cell axis. We have detected MAIT cells in the normal mouse brain and found that both astrocytes and microglia are MR1⁺. When we stimulated brain astrocytes and microglia with E. coli, and then co-cultured them with MAIT cells, MR1 surface expression was upregulated and MAIT cells were activated in an antigen-dependent manner. Considering the association of MAIT cells with inflammatory conditions, including those in the CNS, the MR1/MAIT cell axis could be a novel therapeutic target in neuroinflammatory disorders.

Spinal neuron-glia-immune interaction in cross-organ sensitization

Inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), historically considered as regional gastrointestinal disorders with heightened colonic sensitivity, are increasingly recognized to have concurrent dysfunction of other visceral and somatic organs, such as urinary bladder hyperactivity, leg pain, and skin hypersensitivity. The interorgan sensory cross talk is, at large, termed "cross-organ sensitization." These organs, anatomically distant from one another, physiologically interlock through projecting their sensory information into dorsal root ganglia (DRG) and then the spinal cord for integrative processing. The fundamental question of how sensitization of colonic afferent neurons conveys nociceptive information to activate primary afferents that innervate distant organs remains ambiguous. In DRG, primary afferent neurons are surrounded by satellite glial cells (SGCs) and macrophage accumulation in response to signals of injury to form a neuron-glia-macrophage triad. Astrocytes and microglia are major resident nonneuronal cells in the spinal cord to interact, physically and chemically, with sensory synapses. Cumulative evidence gathered so far indicate the indispensable roles of paracrine/autocrine interactions among neurons, glial cells, and immune cells in sensory cross-activation. Dichotomizing afferents, sensory convergency in the spinal cord, spinal nerve comingling, and extensive sprouting of central axons of primary afferents each has significant roles in the process of cross-organ sensitization; however, more results are required to explain their functional contributions. DRG that are located outside the blood-brain barrier and reside upstream in the cascade of sensory flow from one organ to the other in cross-organ sensitization could be safer therapeutic targets to produce less central adverse effects.

Astrocytes as Guardians of Neuronal Excitability: Mechanisms Underlying Epileptogenesis

Astrocytes are key homeostatic regulators in the central nervous system and play important roles in physiology. After brain damage caused by e.g., status epilepticus, traumatic brain injury, or stroke, astrocytes may adopt a reactive phenotype. This process of reactive astrogliosis is important to restore brain homeostasis. However, persistent reactive astrogliosis can be detrimental for the brain and contributes to the development of epilepsy. In this review, we will focus on physiological functions of astrocytes in the normal brain as well as pathophysiological functions in the epileptogenic brain, with a focus on acquired epilepsy. We will discuss the role of astrocyte-related processes in epileptogenesis, including reactive astrogliosis, disturbances in energy supply and metabolism, gliotransmission, and extracellular ion concentrations, as well as blood-brain barrier dysfunction and dysregulation of blood flow. Since dysfunction of astrocytes can contribute to epilepsy, we will also discuss their role as potential targets for new therapeutic strategies.

Astrocyte-Oligodendrocyte-Microglia Crosstalk in Astrocytopathies

Defective astrocyte function due to a genetic mutation can have major consequences for microglia and oligodendrocyte physiology, which in turn affects the white matter integrity of the brain. This review addresses the current knowledge on shared and unique pathophysiological mechanisms of astrocytopathies, including vanishing white matter, Alexander disease, megalencephalic leukoencephalopathy with subcortical cysts, Aicardi-Goutières syndrome, and oculodentodigital dysplasia. The mechanisms of disease include protein accumulation, unbalanced secretion of extracellular matrix proteins, pro- and anti-inflammatory molecules, cytokines and chemokines by astrocytes, as well as an altered gap junctional network and a changed ionic and nutrient homeostasis. Interestingly, the extent to which astrogliosis and microgliosis are present in these astrocytopathies is highly variable. An improved understanding of astrocyte-microglia-oligodendrocyte crosstalk might ultimately lead to the identification of druggable targets for these, currently untreatable, severe conditions.

IL-1β Induced Cytokine Expression by Spinal Astrocytes Can Play a Role in the Maintenance of Chronic Inflammatory Pain

It is now widely accepted that the glial cells of the central nervous system (CNS) are key players in many processes, especially when they are activated via neuron-glia or glia-glia interactions. In turn, many of the glia-derived pro-inflammatory cytokines contribute to central sensitization during inflammation or nerve injury-evoked pathological pain conditions. The prototype of pro-inflammatory cytokines is interleukin-1beta (IL-1β) which has widespread functions in inflammatory processes. Our earlier findings showed that in the spinal cord (besides neurons) astrocytes express the ligand binding interleukin-1 receptor type 1 (IL-1R1) subunit of the IL-1 receptor in the spinal dorsal horn in the chronic phase of inflammatory pain. Interestingly, spinal astrocytes are also the main source of the IL-1β itself which in turn acts on its neuronal and astrocytic IL-1R1 leading to cell-type specific responses. In the initial experiments we measured the IL-1β concentration in the spinal cord of C57BL/6 mice during the course of complete Freund adjuvant (CFA)-induced inflammatory pain and observed a peak of IL-1β level at the time of highest mechanical sensitivity. In order to further study astrocytic activation, primary astrocyte cultures from spinal cords of C57BL/6 wild type and IL-1R1 deficient mice were exposed to IL-1β in concentrations corresponding to the spinal levels in the CFA-induced pain model. By using cytokine array method we observed significant increase in the expressional level of three cytokines: interleukin-6 (IL-6), granulocyte-macrophage colony stimulating factor (GM-CSF) and chemokine (C-C motif) ligand 5 (CCL5 or RANTES). We also observed that the secretion of the three cytokines is mediated by the NFkB signaling pathway. Our data completes the picture of the IL-1β-triggered cytokine cascade in spinal astrocytes, which may lead to enhanced activation of the local cells (neurons and glia as well) and can lead to the prolonged maintenance of chronic pain. All these cytokines and the NFkB pathway can be possible targets of pain therapy.

Inflammatory factors and amyloid β-induced microglial polarization promote inflammatory crosstalk with astrocytes

The immunological responses are a key pathological factor in Alzheimer's disease (AD). We hypothesized that microglial polarization alters microglia-astrocyte immune interactions in AD. M1 and M2 microglia were isolated from primary rat microglia and were confirmed to secrete pro-inflammatory and anti-inflammatory factors, respectively. Primary rat astrocytes were co-cultured with M1 or M2 microglial medium. M1 microglial medium increased astrocyte production of pro-inflammatory factors (interleukin [IL]-1β, tumor necrosis factor α and IL-6), while M2 microglial medium enhanced astrocyte production of anti-inflammatory factors (IL-4 and IL-10). To analyze the crosstalk between microglia and astrocytes after microglial polarization specifically in AD, we co-cultured astrocytes with medium from microglia treated with amyloid-β (Aβ) alone or in combination with other inflammatory substances. Aβ alone and Aβ combined with lipopolysaccharide/interferon-γ induced pro-inflammatory activity in M1 microglia and astrocytes, whereas IL-4/IL-13 inhibited Aβ-induced pro-inflammatory activity. Nuclear factor κB p65 was upregulated in M1 microglia and pro-inflammatory astrocytes, while Stat6 was upregulated in M2 microglia and anti-inflammatory astrocytes. These results provide direct evidence that microglial polarization governs communication between microglia and astrocytes, and that AD debris alters this crosstalk.

Neuroinflammation, oxidative stress and their interplay in neuropathic pain: Focus on specialized pro-resolving mediators and NADPH oxidase inhibitors as potential therapeutic strategies

Neuropathic pain (NP) is a chronic condition that results from a lesion or disease of the nervous system, greatly impacting patients' quality of life. Current pharmacotherapy options deliver inadequate and/or insufficient responses and thus a significant unmet clinical need remains for alternative treatments in NP. Neuroinflammation, oxidative stress and their reciprocal relationship are critically involved in NP pathophysiology. In this context, new pharmacological approaches, aiming at enhancing the resolution phase of inflammation and/or restoring redox balance by targeting specific reactive oxygen species (ROS) sources, are emerging as potential therapeutic strategies for NP, with improved efficacy and safety profiles. Several reports have demonstrated that administration of exogenous specialized pro-resolving mediators (SPMs) ameliorates NP pathophysiology. Likewise, deletion or inhibition of the ROS-generating enzyme NADPH oxidase (NOX), particularly its isoforms 2 and 4, results in beneficial effects in NP models. Notably, SPMs also modulate oxidative stress and NOX also regulates neuroinflammation. By targeting neuroinflammatory and oxidative pathways, both SPMs analogues and isoform-specific NOX inhibitors are promising therapeutic strategies for NP.

Non-neuronal crosstalk promotes an inflammatory response in nodose ganglia cultures after exposure to byproducts from gram positive, high-fat-diet-associated gut bacteria

Vagal afferent neurons (VAN) projecting to the lamina propria of the digestive tract are the primary source of gut-originating signals to the central nervous system (CNS). VAN cell bodies are found in the nodose ganglia (NG). Responsiveness of VAN to gut-originating signals is altered by feeding status with sensitivity to satiety signals such as cholecystokinin (CCK) increasing in the fed state. Chronic high-fat (HF) feeding results in inflammation at the level of the NG associated with a loss of VAN ability to switch phenotype from the fasted to the fed state. HF feeding also leads to compositional changes in the gut microbiota. HF diet consumption notably drives increased Firmicutes to Bacteroidetes phyla ratio and increased members of the Actinobacteria phylum. Firmicutes and Actinobacteria are largely gram positive (GP). In this study, we aimed to determine if byproducts from GP bacteria can induce an inflammatory response in cultured NG and to characterize the mechanism and cell types involved in the response. NG were collected from male Wistar rats and cultured for a total of 72 hours. At 48-68 hours after plating, cultures were treated with neuronal culture media in which Serinicoccus chungangensis had been grown and removed (SUP), lipoteichoic acid (LTA), or meso-diaminopimelic acid (meso-DAP). Some treatments included the glial inhibitors minocycline (MINO) and/or fluorocitrate (FC). The responses were evaluated using immunocytochemistry, qPCR, and electrochemiluminescence. We found that SUP induced an inflammatory response characterized by increased interleukin (IL)-6 staining and increased expression of genes for IL-6, interferon (IFN)γ, and tumor necrosis factor (TNF)α along with genes associated with cell-to-cell communication such as C-C motif chemokine ligand-2 (CCL2). Inclusion of inhibitors attenuated some responses but failed to completely normalize all indications of response, highlighting the role of immunocompetent cellular crosstalk in regulating the inflammatory response. LTA and meso-DAP produced responses that shared characteristics with SUP but were not identical. Our results support a role for HF associated GP bacterial byproducts' ability to contribute to vagal inflammation and to engage signaling from nonneuronal cells.

Microglia-Astrocyte Cross Talk through IL-18/IL-18R Signaling Modulates Migraine-like Behavior in Experimental Models of Migraine

Interleukin-18 (IL-18) is an important regulator of innate and immune responses, and is involved in the pain process, including neuropathic and cancer pain. The current study demonstrated that inflammatory soup (IS) dural infusions elicited the activation of microglia and astrocytes. In comparison, IS dural infusions induced the upregulation of IL-18 and IL-18R in microglia and astrocytes, respectively. Blocking the IL-18 signaling pathway attenuated nociceptive behavior. In comparison, blocking IL-18 signaling also suppressed the activation of astrocytes and nuclear factor-kappa B (NF-κB). IL-18 dural infusions induced nociceptive behavior and glia activation. IL-18 is a product of the activation of microglial toll-like receptor 4 (TLR4), and it acted on IL-18R expressed in astrocytes. Subsequently, it stimulated the activation of nuclear factor-kappa B (NF-κB), leading to the activation of astrocytes. In conclusion, IL-18-mediated microglia/astrocyte interactions in the medullary dorsal horn likely contribute to the development of hyperpathia or allodynia induced by migraines.

High-Resolution Transcriptomic and Proteomic Profiling of Heterogeneity of Brain-Derived Microglia in Multiple Sclerosis

Microglia are important for central nervous system (CNS) homeostasis and first to respond to tissue damage and perturbations. Microglia are heterogeneous cells; in case of pathology, microglia adopt a range of phenotypes with altered functions. However, how these different microglia subtypes are implicated in CNS disease is largely unresolved. Multiple sclerosis (MS) is a chronic demyelinating disease of the CNS, characterized by inflammation and axonal degeneration, ultimately leading to neurological decline. One way microglia are implicated in MS is through stimulation of remyelination. They facilitate efficient remyelination by phagocytosis of myelin debris. In addition, microglia recruit oligodendrocyte precursor cells (OPCs) to demyelinated areas and stimulate remyelination. The development of high-resolution technologies to profile individual cells has greatly contributed to our understanding of microglia heterogeneity and function under normal and pathological conditions. Gene expression profiling technologies have evolved from whole tissue RNA sequencing toward single-cell or nucleus sequencing. Single microglia proteomic profiles are also increasingly generated, offering another layer of high-resolution data. Here, we will review recent studies that have employed these technologies in the context of MS and their respective advantages and disadvantages. Moreover, recent developments that allow for (single) cell profiling while retaining spatial information and tissue context will be discussed.

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.

The Function of Astrocyte Mediated Extracellular Vesicles in Central Nervous System Diseases

Astrocyte activation plays an important role during disease-induced inflammatory response in the brain. Exosomes in the brain could be released from bone marrow (BM)-derived stem cells, neuro stem cells (NSC), mesenchymal stem cells (MSC), etc. We summarized that exosomes release and transport signaling to the target cells, and then produce function. Furthermore, we discussed the pathological interactions between astrocytes and other brain cells, which are related to brain diseases such as stroke, Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS) disease, multiple sclerosis (MS), psychiatric, traumatic brain injury (TBI), etc. We provide up-to-date, comprehensive and valuable information on the involvement of exosomes in brain diseases, which is beneficial for basic researchers and clinical physicians.

Cholinergic Modulation of Glial Function During Aging and Chronic Neuroinflammation

Aging is a complex biological process that increases the risk of age-related cognitive degenerative diseases such as dementia, including Alzheimer’s disease (AD), Lewy Body Dementia (LBD), and mild cognitive impairment (MCI). Even non-pathological aging of the brain can involve chronic oxidative and inflammatory stress, which disrupts the communication and balance between the brain and the immune system. There has been an increasingly strong connection found between chronic neuroinflammation and impaired memory, especially in AD. While microglia and astrocytes, the resident immune cells of the central nervous system (CNS), exerting beneficial effects during the acute inflammatory phase, during chronic neuroinflammation they can become more detrimental. Central cholinergic circuits are involved in maintaining normal cognitive function and regulating signaling within the entire cerebral cortex. While neuronal-glial cholinergic signaling is anti-inflammatory and anti-oxidative, central cholinergic neuronal degeneration is implicated in impaired learning, memory sleep regulation, and attention. Although there is evidence of cholinergic involvement in memory, fewer studies have linked the cholinergic anti-inflammatory and anti-oxidant pathways to memory processes during development, normal aging, and disease states. This review will summarize the current knowledge of cholinergic effects on microglia and astroglia, and their role in both anti-inflammatory and anti-oxidant mechanisms, concerning normal aging and chronic neuroinflammation. We provided details on how stimulation of α7 nicotinic acetylcholine (α7nACh) receptors can be neuroprotective by increasing amyloid-β phagocytosis, decreasing inflammation and reducing oxidative stress by promoting the nuclear factor erythroid 2-related factor 2 (Nrf2) pathways and decreasing the release of pro-inflammatory cytokines. There is also evidence for astroglial α7nACh receptor stimulation mediating anti-inflammatory and antioxidant effects by inhibiting the nuclear factor-κB (NF-κB) pathway and activating the Nrf2 pathway respectively. We conclude that targeting cholinergic glial interactions between neurons and glial cells via α7nACh receptors could regulate neuroinflammation and oxidative stress, relevant to the treatment of several neurodegenerative diseases.

The immune system on the TRAIL of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, characterized by progressive degeneration and loss of neurons in specific regions of the central nervous system. Chronic activation of the immune cells resident in the brain, peripheral immune cell trafficking across the blood-brain barrier, and release of inflammatory and neurotoxic factors, appear critical contributors of the neuroinflammatory response that drives the progression of neurodegenerative processes in AD. As the neuro-immune network is impaired in course of AD, this review is aimed to point out the essential supportive role of innate and adaptive immune response either in normal brain as well as in brain recovery from injury. Since a fine-tuning of the immune response appears crucial to ensure proper nervous system functioning, we focused on the role of the TNF superfamily member, TNF-related apoptosis-inducing ligand (TRAIL), which modulates both the innate and adaptive immune response in the pathogenesis of several immunological disorders and, in particular, in AD-related neuroinflammation. We here summarized mounting evidence of potential involvement of TRAIL signaling in AD pathogenesis, with the aim to provide clearer insights about potential novel therapeutic approaches in AD.

Gamma ray-induced glial activation and neuronal loss occur before the delayed onset of brain necrosis

Radiotherapy is one of the most effective treatments for head and neck tumors. However, delayed radiation-induced brain necrosis (RN) remains a serious issue due to the lack of satisfying prevention and effective treatment. The pathological role of radiation in the delayed onset of brain necrosis is still largely unknown, and the traditional animal model of whole brain irradiation, although being widely used, does not produce reliable and localized brain necrosis mimicking clinical features of RN. In this study, we demonstrated a successful RN mouse model using optimized gamma knife irradiation in male C57BL/6 mice. On the premise that brain necrosis started to appear at 6 weeks postirradiation in our RN model, as confirmed by both MRI and histopathological examinations, we systematically examined different time points before the onset of RN for the histopathological changes and biochemical indicators. Our initial results demonstrated that in the ipsilateral hemisphere of the irradiated brains, a significant decrease in neuronal numbers that occurred at 4 weeks and a sustained increase in TNF-α, iNOS, and other inflammatory cytokines beginning at 1-week postirradiation. Changes of cell morphology and cell numbers of both microglia and astrocytes occurred as early as 1-week postirradiation, and intervention by bevacizumab administration resulted in reduced microglia activation and reduction of radiation-induced lesion volume, indicating that chronic glial activation may result in subsequent elevation of inflammatory factors, which led to the delayed onset of neuronal loss and brain necrosis. Since C57BL/6 is the most widely used strain of genetic engineered mouse model, our data provide an invaluable platform for the mechanistic study of RN pathogenesis, identification of potential imaging and biological biomarkers, and the development of therapeutic treatment for the disease.

Shaping Microglial Phenotypes Through Estrogen Receptors: Relevance to Sex-Specific Neuroinflammatory Responses to Brain Injury and Disease

In mammals, 17β-estradiol (E2), the primary endogenous estrogen, maintains normal central nervous system (CNS) function throughout life and influences brain responses to injury and disease. Estradiol-induced cellular changes are mediated through the activation of nuclear and extranuclear estrogen receptors (ERs), which include ERα, ERβ, and the G-protein coupled receptor, GPER1. ERs are widely expressed throughout the brain, acting as transcriptional effectors or rapidly initiating membrane and cytoplasmic signaling cascades in practically all brain cells including microglia, the resident immune cells of the CNS. Activation of ERs by E2 exerts potent anti-inflammatory effects through mechanisms involving the modification of microglial cell responses to acute or chronic brain injury. Recent studies suggest that microglial maturation is influenced by the internal gonadal hormone milieu and that their functions in the normal and diseased brain are sex specific. However, the role that each ER subtype plays in microglial development or in determining microglial function as the primary cellular defense mechanism against pathogens and injury remains unclear. This is partly due to the fact that most studies investigating the mechanisms by which E2-ER signaling modifies microglial cellular phenotypes have been restricted to one sex or age. This review examines the different in vivo and in vitro models used to study the direct and indirect regulation of microglial cell function by E2 through ERs. Ischemic stroke will be used as an example of a neurologic disease in which activation of ERs shapes microglial phenotypes in response to injury in a sex- and age-specific fashion. SIGNIFICANCE STATEMENT: As the primary immune sensors of central nervous system damage, microglia are important potential therapeutic targets. Estrogen receptor signaling modulates microglial responses to brain injury and disease in a sex- and age-specific fashion. Hence, investigating the molecular mechanisms by which estrogen receptors regulate and shape microglial functions throughout life may result in novel and effective therapeutic opportunities that are tailored for each sex and age.

TNF Production and Release from Microglia via Extracellular Vesicles: Impact on Brain Functions

Tumor necrosis factor (TNF) is a pleiotropic cytokine powerfully influencing diverse processes of the central nervous system (CNS) under both physiological and pathological conditions. Here, we analyze current literature describing the molecular processes involved in TNF synthesis and release from microglia, the resident immune cells of the CNS and the main source of this cytokine both in brain development and neurodegenerative diseases. A special attention has been given to the unconventional vesicular pathway of TNF, based on the emerging role of microglia-derived extracellular vesicles (EVs) in the propagation of inflammatory signals and in mediating cell-to-cell communication. Moreover, we describe the contribution of microglial TNF in regulating important CNS functions, including the neuroinflammatory response following brain injury, the neuronal circuit formation and synaptic plasticity, and the processes of myelin damage and repair. Specifically, the available data on the functions mediated by microglial EVs carrying TNF have been scrutinized to gain insights on possible novel therapeutic strategies targeting TNF to foster CNS repair.

Tuberous Sclerosis Complex as Disease Model for Investigating mTOR-Related Gliopathy During Epileptogenesis

Tuberous sclerosis complex (TSC) represents the prototypic monogenic disorder of the mammalian target of rapamycin (mTOR) pathway dysregulation. It provides the rational mechanistic basis of a direct link between gene mutation and brain pathology (structural and functional abnormalities) associated with a complex clinical phenotype including epilepsy, autism, and intellectual disability. So far, research conducted in TSC has been largely neuron-oriented. However, the neuropathological hallmarks of TSC and other malformations of cortical development also include major morphological and functional changes in glial cells involving astrocytes, oligodendrocytes, NG2 glia, and microglia. These cells and their interglial crosstalk may offer new insights into the common neurobiological mechanisms underlying epilepsy and the complex cognitive and behavioral comorbidities that are characteristic of the spectrum of mTOR-associated neurodevelopmental disorders. This review will focus on the role of glial dysfunction, the interaction between glia related to mTOR hyperactivity, and its contribution to epileptogenesis in TSC. Moreover, we will discuss how understanding glial abnormalities in TSC might give valuable insight into the pathophysiological mechanisms that could help to develop novel therapeutic approaches for TSC or other pathologies characterized by glial dysfunction and acquired mTOR hyperactivation.

Neuroinflammation and progressive myoclonus epilepsies: from basic science to therapeutic opportunities

Progressive myoclonus epilepsies (PMEs) are a group of genetic neurological disorders characterised by the occurrence of epileptic seizures, myoclonus and progressive neurological deterioration including cerebellar involvement and dementia. The primary cause of PMEs is variable and alterations in the corresponding mutated genes determine the progression and severity of the disease. In most cases, they lead to the death of the patient after a period of prolonged disability. PMEs also share poor information on the pathophysiological bases and the lack of a specific treatment. Recent reports suggest that neuroinflammation is a common trait under all these conditions. Here, we review similarities and differences in neuroinflammatory response in several PMEs and discuss the window of opportunity of using anti-inflammatory drugs in the treatment of several of these conditions.

Deleterious Alteration of Glia in the Brain of Alzheimer's Disease

The deterioration of neurons in Alzheimer’s disease (AD) arises from genetic, immunologic, and cellular factors inside the cortex. The traditional consensus of the amyloid-beta (Aβ) paradigm as a singular cause of AD has been under revision, with the accumulation of exploding neurobiological evidence. Among the multifaceted casualties of AD, the involvement of glia gains significance for its dynamic contribution to neurons, either in a neuroprotective or neurotoxic fashion. Basically, microglia and astrocytes contribute to neuronal sustainability by releasing neuroprotective cytokines, maintaining an adequate amount of glutamate in the synapse, and pruning excessive synaptic terminals. Such beneficial effects divert to the other detrimental cascade in chronic neuroinflammatory conditions. In this change, there are new discoveries of specific cytokines, microRNAs, and complementary factors. Previously unknown mechanisms of ion channels such as Kv1.3, Kir2.1, and HCN are also elucidated in the activation of microglia. The activation of glia is responsible for the excitotoxicity through the overflow of glutamate transmitter via mGluRs expressed on the membrane, which can lead to synaptic malfunction and engulfment. The communication between microglia and astrocytes is mediated through exosomes as well as cytokines, where numerous pieces of genetic information are transferred in the form of microRNAs. The new findings tell us that the neuronal environment in the AD condition is a far more complicated and dynamically interacting space. The identification of each molecule in the milieu and cellular communication would contribute to a better understanding of AD in the neurobiological perspective, consequently suggesting a possible therapeutic clue.

Enteric Glia Modulate Macrophage Phenotype and Visceral Sensitivity following Inflammation

Mechanisms resulting in abdominal pain include altered neuro-immune interactions in the gastrointestinal tract, but the signaling processes that link immune activation with visceral hypersensitivity are unresolved. We hypothesized that enteric glia link the neural and immune systems of the gut and that communication between enteric glia and immune cells modulates the development of visceral hypersensitivity. To this end, we manipulated a major mechanism of glial intercellular communication that requires connexin-43 and assessed the effects on acute and chronic inflammation, visceral hypersensitivity, and immune responses. Deleting connexin-43 in glia protected against the development of visceral hypersensitivity following chronic colitis. Mechanistically, the protective effects of glial manipulation were mediated by disrupting the glial-mediated activation of macrophages through the macrophage colony-stimulating factor. Collectively, our data identified enteric glia as a critical link between gastrointestinal neural and immune systems that could be harnessed by therapies to ameliorate abdominal pain.

TTDA inhibited apoptosis by regulating the p53-Bax/Bcl2 axis in glioma

The trichothiodystrophy group A protein (TTDA) functions in nucleotide excision repair and basal transcription. TTDA plays a role in cancers and serves as a prognostic and predictive factor in high-grade serous ovarian cancer; however, its role in human glioma remains unknown. Here, we found that TTDA was overexpressed in glioma tissues. In vitro experiments revealed that TTDA overexpression inhibited apoptosis of glioma cells and promoted cell growth, whereas knockdown of TTDA had the opposite effect. Increased TTDA expression significantly decreased the Bax/Bcl2 ratio and the level of cleaved-caspase3. TTDA interacted with the p53 gene at the -1959 bp and -1530 bp region and regulated its transcription, leading to inhibition of the p53-Bax/Bcl2 mitochondrial apoptosis pathway in glioma cells. These results indicate that TTDA is an upstream regulator of p53-mediated apoptosis and acts as an oncogene, suggesting its value as a potential molecular target for the diagnosis and treatment of glioma.

The Role of Brain Cyclooxygenase-2 (Cox-2) Beyond Neuroinflammation: Neuronal Homeostasis in Memory and Anxiety

Cyclooxygenases are a group of heme-containing isozymes (namely Cox-1 and Cox-2) that catalyze the conversion of arachidonic acid to largely bioactive prostaglandins (PGs). Cox-1 is the ubiquitous housekeeping enzyme, and the mitogen-inducible Cox-2 is activated to cause inflammation. Interestingly, Cox-2 is constitutively expressed in the brain at the postsynaptic dendrites and excitatory terminals of the cortical and spinal cord neurons. Neuronal Cox-2 is activated in response to synaptic excitation to yield PGE₂, the predominant Cox-2 metabolite in the brain, which in turn stimulates the release of glutamate and neuronal firing in a retrograde fashion. Cox-2 is also engaged in the metabolism of new endocannabinoids from 2-arachidonoyl-glycerol to modulate their actions at presynaptic terminals. In addition to these interactions, the induction of neuronal Cox-2 is coupled to the trans-synaptic activation of the dopaminergic mesolimbic system and some serotoninergic receptors, which might contribute to the development of emotional behavior. Although much of the focus regarding the induction of Cox-2 in the brain has been centered on neuroinflammation-related neurodegenerative and psychiatric disorders, some evidence also suggests that Cox-2 release during neuronal signaling may be pivotal for the fine tuning of cortical networks to regulate behavior. This review compiles the evidence supporting the homeostatic role of neuronal Cox-2 in synaptic transmission and plasticity, since neuroinflammation is originally triggered by the induction of glial Cox-2 expression. The goal is to provide perspective on the roles of Cox-2 beyond neuroinflammation, such as those played in memory and anxiety, and whose evidence is still scant.

Oxyresveratrol Inhibits IL-1β-Induced Inflammation via Suppressing AKT and ERK1/2 Activation in Human Microglia, HMC3

Oxyresveratrol (OXY), a major phytochemical component derived from several plants, has been proved to have several pharmacological properties. However, the role of OXY in regulating neuroinflammation is still unclear. Here, we focused mainly on the anti-neuroinflammatory effects at the cellular level of OXY in the interleukin-1 beta (IL-1β)-stimulated HMC3 human microglial cell line. We demonstrated that OXY strongly decreased the release of IL-6 and MCP-1 from HMC3 cells stimulated with IL-1β. Nevertheless, IL-1β could not induce the secretion of TNF-α and CXCL10 in this specific cell line, and that OXY did not have any effects on reducing the basal level of these cytokines in the sample culture supernatants. The densitometry analysis of immunoreactive bands from Western blot clearly indicated that IL-1β does not trigger the nuclear factor-kappa B (NF-κB) signaling. We discovered that OXY exerted its anti-inflammatory role in IL-1β-induced HMC3 cells by suppressing IL-1β-induced activation of the PI3K/AKT/p70S6K pathway. Explicitly, the presence of OXY for only 4 h could strongly inhibit AKT phosphorylation. In addition, OXY had moderate effects on inhibiting the activation of ERK1/2. Results from immunofluorescence study further confirmed that OXY inhibited the phosphorylation of AKT and ERK1/2 MAPK upon IL-1β stimulation in individual cells. These findings suggest that the possible anti-inflammatory mechanisms of OXY in IL-1β-induced HMC3 cells are mainly through its ability to suppress the PI3K/AKT/p70S6K and ERK1/2 MAPK signal transduction cascades. In conclusion, our study provided accumulated data that OXY is able to suppress IL-1β stimulation signaling in human microglial cells, and we believe that OXY could be a probable pharmacologic agent for altering microglial function in the treatment of neuroinflammation.

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.

Reactive Astrocytes Display Pro-inflammatory Adaptability with Modulation of Notch-PI3K-AKT Signaling Pathway Under Inflammatory Stimulation

Astrocytes are major glial cells critical in assisting the function of the central nervous system (CNS), but the functional changes and regulation mechanism of reactive astrocytes are still poorly understood in CNS diseases. In this study, mouse primary astrocytes were cultured, and inflammatory insult was performed to observe functional changes in astrocytes and the involvement of Notch-PI3K-AKT signaling activation through immunofluorescence, PCR, Western blot, CCK-8, and inhibition experiments. Notch downstream signal Hes-1 was clearly observed in the astrocytes, and Notch signal inhibitor GSI dose-dependently decreased the cleaved Notch-l level without an influence on cell viability. Inflammatory insult of lipopolysaccharide plus interferon-γ (LPS+IFNγ) induced an increase in pro-inflammatory cytokines, that is, iNOS, IL-1β, IL-6, and TNF, at the protein and mRNA levels in activated astrocytes, which was reduced or blocked by GSI treatment. The cell viability of the astrocytes did not show significant differences among different groups. While an increase in MyD88, NF-кB, and phosphor-NF-кB was confirmed, upregulation of PI3K, AKT, and phosphor-AKT was observed in the activated astrocytes with LPS+IFNγ insult and was reduced by GSI treatment. Inhibitor experiments showed that inhibition of Notch-PI3K-AKT signaling activation reduced the pro-inflammatory cytokine production triggered by LPS+IFNγ inflammatory insult. This study showed that the reactive astrocytes displayed pro-inflammatory adaptability through Notch-PI3K-AKT signaling activation in response to inflammatory stimulation, suggesting that the Notch-PI3K-AKT pathway in reactive astrocytes may serve as a promising target against CNS inflammatory disorders.

Rising Stars: Astrocytes as a Therapeutic Target for ALS Disease

Amyotrophic lateral sclerosis (ALS) is a multifactorial disease, characterized by a progressive loss of motor neurons that eventually leads to paralysis and death. The current ALS-approved drugs modestly change the clinical course of the disease. The mechanism by which motor neurons progressively degenerate remains unclear but entails a non-cell autonomous process. Astrocytes impaired biological functionality were implicated in multiple neurodegenerative diseases, including ALS, frontotemporal dementia (FTD), Parkinson’s disease (PD), and Alzheimer disease (AD). In ALS disease patients, A1 reactive astrocytes were found to play a key role in the pathology of ALS disease and death of motor neurons, via loss or gain of function or acquired toxicity. The contribution of astrocytes to the maintenance of motor neurons by diverse mechanisms makes them a promising therapeutic candidate for the treatment of ALS. Therapeutic approaches targeting at modulating the function of endogenous astrocytes or replacing lost functionality by transplantation of healthy astrocytes, may contribute to the development of therapies which might slow down or even halt the progression ALS diseases. The proposed mechanisms by which astrocytes can potentially ameliorate ALS progression and the status of ALS clinical studies involving astrocytes are discussed.

A1 reactive astrocytes and a loss of TREM2 are associated with an early stage of pathology in a mouse model of cerebral amyloid angiopathy

Background

Cerebral amyloid angiopathy (CAA) is typified by the cerebrovascular deposition of amyloid. The mechanisms underlying the contribution of CAA to neurodegeneration are not currently understood. Although CAA is highly associated with the accumulation of amyloid beta (Aβ), other amyloids are known to associate with the vasculature. Alzheimer’s disease (AD) is characterized by parenchymal Aβ deposition, intracellular accumulation of tau, and significant neuroinflammation. CAA increases with age and is present in 85–95% of individuals with AD. A substantial amount of research has focused on understanding the connection between parenchymal amyloid and glial activation and neuroinflammation, while associations between vascular amyloid pathology and glial reactivity remain understudied.

Methods

Here, we dissect the glial and immune responses associated with early-stage CAA with histological, biochemical, and gene expression analyses in a mouse model of familial Danish dementia (FDD), a neurodegenerative disease characterized by the vascular accumulation of Danish amyloid (ADan). Findings observed in this CAA mouse model were complemented with primary culture assays.

Results

We demonstrate that early-stage CAA is associated with dysregulation in immune response networks and lipid processing, severe astrogliosis with an A1 astrocytic phenotype, and decreased levels of TREM2 with no reactive microgliosis. Our results also indicate how cholesterol accumulation and ApoE are associated with vascular amyloid deposits at the early stages of pathology. We also demonstrate A1 astrocytic mediation of TREM2 and microglia homeostasis.

Conclusion

The initial glial response associated with early-stage CAA is characterized by the upregulation of A1 astrocytes without significant microglial reactivity. Gene expression analysis revealed that several AD risk factors involved in immune response and lipid processing may also play a preponderant role in CAA. This study contributes to the increasing evidence that brain cholesterol metabolism, ApoE, and TREM2 signaling are major players in the pathogenesis of AD-related dementias, including CAA. Understanding the basis for possible differential effects of glial response, ApoE, and TREM2 signaling on parenchymal plaques versus vascular amyloid deposits provides important insight for developing future therapeutic interventions.

Neuron-derived exosomes-transmitted miR-124-3p protect traumatically injured spinal cord by suppressing the activation of neurotoxic microglia and astrocytes

Background

Spinal cord injury (SCI) is a catastrophic injury that can cause irreversible motor dysfunction with high disability. Exosomes participate in the transport of miRNAs and play an essential role in intercellular communication via transfer of genetic material. However, the miRNAs in exosomes which derived from neurons, and the underlying mechanisms by which they contribute to SCI remain unknown.

Methods

A contusive in vivo SCI model and a series of in vitro experiments were carried out to explore the therapeutic effects of exosomes. Then, a miRNA microarray analysis and rescue experiments were performed to confirm the role of neuron-derived exosomal miRNA in SCI. Western blot, luciferase activity assay, and RNA-ChIP were used to investigate the underlying mechanisms.

Results

The results indicated that neuron-derived exosomes promoted functional behavioral recovery by suppressing the activation of M1 microglia and A1 astrocytes in vivo and in vitro. A miRNA array showed miR-124-3p to be the most enriched in neuron-derived exosomes. MYH9 was identified as the target downstream gene of miR-124-3p. A series of experiments were used to confirm the miR-124-3p/MYH9 axis. Finally, it was found that PI3K/AKT/NF-κB signaling cascades may be involved in the modulation of microglia by exosomal miR-124-3p.

Conclusion

A combination of miRNAs and neuron-derived exosomes may be a promising, minimally invasive approach for the treatment of SCI.

Three-dimensional morphometric analysis reveals time-dependent structural changes in microglia and astrocytes in the central amygdala and hypothalamic paraventricular nucleus of heart failure rats

BACKGROUND

Cardiovascular diseases, including heart failure, are the most common cause of death globally. Recent studies support a high degree of comorbidity between heart failure and cognitive and mood disorders resulting in memory loss, depression, and anxiety. While neuroinflammation in the hypothalamic paraventricular nucleus contributes to autonomic and cardiovascular dysregulation in heart failure, mechanisms underlying cognitive and mood disorders in this disease remain elusive. The goal of this study was to quantitatively assess markers of neuroinflammation (glial morphology, cytokines, and A1 astrocyte markers) in the central amygdala, a critical forebrain region involved in emotion and cognition, and to determine its time course and correlation to disease severity during the progression of heart failure.

METHODS

We developed and implemented a comprehensive microglial/astrocyte profiler for precise three-dimensional morphometric analysis of individual microglia and astrocytes in specific brain nuclei at different time points during the progression of heart failure. To this end, we used a well-established ischemic heart failure rat model. Morphometric studies were complemented with quantification of various pro-inflammatory cytokines and A1/A2 astrocyte markers via qPCR.

RESULTS

We report structural remodeling of central amygdala microglia and astrocytes during heart failure that affected cell volume, surface area, filament length, and glial branches, resulting overall in somatic swelling and deramification, indicative of a change in glial state. These changes occurred in a time-dependent manner, correlated with the severity of heart failure, and were delayed compared to changes in the hypothalamic paraventricular nucleus. Morphometric changes correlated with elevated mRNA levels of pro-inflammatory cytokines and markers of reactive A1-type astrocytes in the paraventricular nucleus and central amygdala during heart failure.

CONCLUSION

We provide evidence that in addition to the previously described hypothalamic neuroinflammation implicated in sympathohumoral activation during heart failure, microglia, and astrocytes within the central amygdala also undergo structural remodeling indicative of glial shifts towards pro-inflammatory phenotypes. Thus, our studies suggest that neuroinflammation in the amygdala stands as a novel pathophysiological mechanism and potential therapeutic target that could be associated with emotional and cognitive deficits commonly observed at later stages during the course of heart failure.

Nuclear factor kappa B activation appears weaker in schizophrenia patients with high brain cytokines than in non-schizophrenic controls with high brain cytokines

Background

High inflammation status despite an absence of known infection characterizes a subpopulation of people with schizophrenia who suffer from more severe cognitive deficits, less cortical grey matter, and worse neuropathology. Transcripts encoding factors upstream of nuclear factor kappa B (NF-κB), a major transcriptional activator for the synthesis of pro-inflammatory cytokines, are increased in the frontal cortex in schizophrenia compared to controls. However, the extent to which these changes are disease-specific, restricted to those with schizophrenia and high-neuroinflammatory status, or caused by loss of a key NF-κB inhibitor (HIVEP2) found in schizophrenia brain, has not been tested.

Methods

Post-mortem prefrontal cortex samples were assessed in 141 human brains (69 controls and 72 schizophrenia) and 13 brains of wild-type mice and mice lacking HIVEP2 (6 wild-type, 7 knockout mice). Gene expression of pro-inflammatory cytokines and acute phase protein SERPINA3 was used to categorize high and low neuroinflammation biotype groups in human samples via cluster analysis. Expression of 18 canonical and non-canonical NF-κB pathway genes was assessed by qPCR in human and mouse tissue.

Results

In humans, we found non-canonical upstream activators of NF-κB were generally elevated in individuals with neuroinflammation regardless of diagnosis, supporting NF-κB activation in both controls and people with schizophrenia when cytokine mRNAs are high. However, high neuroinflammation schizophrenia patients had weaker (or absent) transcriptional increases of several canonical upstream activators of NF-κB as compared to the high neuroinflammation controls. HIVEP2 mRNA reduction was specific to patients with schizophrenia who also had high neuroinflammatory status, and we also found decreases in NF-κB transcripts typically induced by activated microglia in mice lacking HIVEP2.

Conclusions

Collectively, our results show that high cortical expression of pro-inflammatory cytokines and low cortical expression of HIVEP2 in a subset of people with schizophrenia is associated with a relatively weak NF-κB transcriptional signature compared to non-schizophrenic controls with high cytokine expression. We speculate that this comparatively milder NF-κB induction may reflect schizophrenia-specific suppression possibly related to HIVEP2 deficiency in the cortex.

The Role of MicroRNAs in Repair Processes in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disorder characterised by demyelination of central nervous system neurons with subsequent damage, cell death and disability. While mechanisms exist in the CNS to repair this damage, they are disrupted in MS and currently there are no treatments to address this deficit. In recent years, increasing attention has been paid to the influence of the small, non-coding RNA molecules, microRNAs (miRNAs), in autoimmune disorders, including MS. In this review, we examine the role of miRNAs in remyelination in the different cell types that contribute to MS. We focus on key miRNAs that have a central role in mediating the repair process, along with several more that play either secondary or inhibitory roles in one or more aspects. Finally, we consider the current state of miRNAs as therapeutic targets in MS, acknowledging current challenges and potential strategies to overcome them in developing effective novel therapeutics to enhance repair mechanisms in MS.

Dietary fat exacerbates postprandial hypothalamic inflammation involving glial fibrillary acidic protein-positive cells and microglia in male mice

In humans, obesity is associated with brain inflammation, glial reactivity, and immune cells infiltration. Studies in rodents have shown that glial reactivity occurs within 24 hr of high-fat diet (HFD) consumption, long before obesity development, and takes place mainly in the hypothalamus (HT), a crucial brain structure for controlling body weight. Here, we sought to characterize the postprandial HT inflammatory response to 1, 3, and 6 hr of exposure to either a standard diet (SD) or HFD. HFD exposure increased gene expression of astrocyte and microglial markers (glial fibrillary acidic protein [GFAP] and Iba1, respectively) compared to SD-treated mice and induced morphological modifications of microglial cells in HT. This remodeling was associated with higher expression of inflammatory genes and differential regulation of hypothalamic neuropeptides involved in energy balance regulation. DREADD and PLX5622 technologies, used to modulate GFAP-positive or microglial cells activity, respectively, showed that both glial cell types are involved in hypothalamic postprandial inflammation, with their own specific kinetics and reactiveness to ingested foods. Thus, recurrent exacerbated postprandial inflammation in the brain might promote obesity and needs to be characterized to address this worldwide crisis.

Mouse Astrocytes Promote Microglial Ramification by Releasing TGF-β and Forming Glial Fibers

The morphology of microglial cells is often closely related to their functions. The mechanisms that regulate microglial ramification are not well understood. Here we reveal the biological mechanisms by which astrocytes regulate microglial ramification. Morphological variation in mouse microglial cultures was measured in terms of cell area as well as branch number and length. Effects on microglial ramification were analyzed after microinjecting the toxin L-alpha-aminoadipic acid (L-AAA) in the mouse cortex or hippocampus to ablate astrocytes, and after culturing microglia on their own in an astrocyte-conditioned medium (ACM) or together with astrocytes in coculture. TGF-β expression was determined by Western blotting, immunohistochemistry, and ELISA. The TGF-β signaling pathway was blocked by the TGF-β antibody to assess the role of TGF-β on microglial ramification. The results showed that microglia had more and longer branches and smaller cell bodies in brain areas where astrocytes were abundant. In the mouse cortex and hippocampus, ablation of astrocytes by L-AAA decreased number and length of microglial branches and increased the size of cell bodies. Similar results were obtained with isolated microglia in culture. However, isolated microglia were able to maintain their multibranched structure for a long time when cultured on astrocyte monolayers. Ameboid microglia isolated from P0 to P3 mice showed increased ramification when cultured in ACM or on astrocyte monolayers. Microglia cultured on astrocyte monolayers showed more complex branching structures than those cultured in ACM. Blocking astrocyte-derived TGF-β decreased microglial ramification. Astrocytes induced the formation of protuberances on branches of microglia by forming glial fibers that increased traction. These experiments in mice suggest that astrocytes promote microglial ramification by forming glial fibers to create traction and by secreting soluble factors into the surroundings. For example, astrocyte-secreted TGF-β promotes microglia to generate primitive branches, whose ramification is refined by glial fibers.

Interaction of Microglia and Astrocytes in the Neurovascular Unit

The interaction between microglia and astrocytes significantly influences neuroinflammation. Microglia/astrocytes, part of the neurovascular unit (NVU), are activated by various brain insults. The local extracellular and intracellular signals determine their characteristics and switch of phenotypes. Microglia and astrocytes are activated into two polarization states: the pro-inflammatory phenotype (M1 and A1) and the anti-inflammatory phenotype (M2 and A2). During neuroinflammation, induced by stroke or lipopolysaccharides, microglia are more sensitive to pathogens, or damage; they are thus initially activated into the M1 phenotype and produce common inflammatory signals such as IL-1 and TNF-α to trigger reactive astrocytes into the A1 phenotype. These inflammatory signals can be amplified not only by the self-feedback loop of microglial activation but also by the unique anatomy structure of astrocytes. As the pathology further progresses, resulting in local environmental changes, M1-like microglia switch to the M2 phenotype, and M2 crosstalk with A2. While astrocytes communicate simultaneously with neurons and blood vessels to maintain the function of neurons and the blood-brain barrier (BBB), their subtle changes may be identified and responded by astrocytes, and possibly transferred to microglia. Although both microglia and astrocytes have different functional characteristics, they can achieve immune "optimization" through their mutual communication and cooperation in the NVU and build a cascaded immune network of amplification.

Fasudil inhibits the activation of microglia and astrocytes of transgenic Alzheimer's disease mice via the downregulation of TLR4/Myd88/NF-κB pathway

Emerging evidence suggests an association of Alzheimer's Disease (AD) with microglial and astrocytic dysregulation. Recent studies have proposed that activated microglia can transform astrocytes to a neurotoxic A1 phenotype, which has been shown to be involved in the promotion of neuronal damage in several neurodegenerative diseases, including AD. In the present study, we observed an obvious microglial activation and A1-specific astrocyte response in the brain tissue of APP/PS1 Tg mice. Fasudil treatment improved the cognitive deficits of APP/PS1 Tg mice, inhibited microglial activation and promoted their transformation to an anti-inflammatory phenotype, and further shifted astrocytes from an A1 to an A2 phenotype. Our experiments suggest Fasudil exerted these functions by inhibing the expression of TLR4, MyD88, and NF-κB, which are key mediators of inflammation. Using in vitro experiments, we further validated in vivo findings. Our cell experiments indicated that Fasudil induces a shift of inflammatory microglia towards an anti-inflammatory phenotype. LPS-induced microglia-conditioned medium promotes A1 astrocytic polarization, but Fasudil treatment resulted in a direct transformation of A1 astrocytes to A2. To summarize, our results show that Fasudil inhibits the neurotoxic activation of microglia and shifts astrocytes towards a neuroprotective A2 phenotype, representing a promising candidate for AD treatment.

Function of astrocyte MyD88 in high-fat-diet-induced hypothalamic inflammation

BACKGROUND

A growing body of evidence shows that hypothalamic inflammation is an important factor in the initiation of obesity. In particular, reactive gliosis accompanied by inflammatory responses in the hypothalamus are pivotal cellular events that elicit metabolic abnormalities. In this study, we examined whether MyD88 signaling in hypothalamic astrocytes controls reactive gliosis and inflammatory responses, thereby contributing to the pathogenesis of obesity.

METHODS

To analyze the role of astrocyte MyD88 in obesity pathogenesis, we used astrocyte-specific Myd88 knockout (KO) mice fed a high-fat diet (HFD) for 16 weeks or injected with saturated free fatty acids. Astrocyte-specific gene expression in the hypothalamus was determined using real-time PCR with mRNA purified by the Ribo-Tag system. Immunohistochemistry was used to detect the expression of glial fibrillary acidic protein, ionized calcium-binding adaptor molecule 1, phosphorylated signal transducer and activator of transcription 3, and α-melanocyte-stimulating hormone in the hypothalamus. Animals' energy expenditure was measured using an indirect calorimetry system.

RESULTS

The astrocyte-specific Myd88 KO mice displayed ameliorated hypothalamic reactive gliosis and inflammation induced by injections of saturated free fatty acids and a long-term HFD. Accordingly, the KO mice were resistant to long-term HFD-induced obesity and showed an improvement in HFD-induced leptin resistance.

CONCLUSIONS

These results suggest that MyD88 in hypothalamic astrocytes is a critical molecular unit for obesity pathogenesis that acts by mediating HFD signals for reactive gliosis and inflammation.