Csf1R inhibition attenuates experimental autoimmune encephalomyelitis and promotes recovery

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

Residual Microglia Following Short-term PLX5622 Treatment in 5xFAD Mice Exhibit Diminished NLRP3 Inflammasome and mTOR Signaling, and Enhanced Autophagy

Chronic neuroinflammation represents a prominent hallmark of Alzheimer's disease (AD). While moderately activated microglia are pivotal in clearing amyloid beta (Aβ), hyperactivated microglia perpetuate neuroinflammation. Prior investigations have indicated that the elimination of ∼80% of microglia through a month-long inhibition of the colony-stimulating factor 1 receptor (CSF1R) during the advanced stage of neuroinflammation in 5xFamilial AD (5xFAD) mice mitigates synapse loss and neurodegeneration without impacting Aβ levels. Furthermore, prolonged CSF1R inhibition diminished the development of parenchymal plaques. Nonetheless, the immediate effects of short-term CSF1R inhibition during the early stages of neuroinflammation on residual microglial phenotype or metabolic fitness are unknown. Therefore, we investigated the effects of 10-day CSF1R inhibition in three-month-old female 5xFAD mice, a stage characterized by the onset of neuroinflammation and minimal Aβ plaques. We observed ∼65% microglia depletion in the hippocampus and cerebral cortex. The leftover microglia demonstrated a noninflammatory phenotype, with highly branched and ramified processes and reduced NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome complexes. Moreover, plaque-associated microglia were reduced in number with diminished Clec7a (dectin-1) expression. Additionally, both microglia and neurons displayed reduced mechanistic target of rapamycin (mTOR) signaling and autophagy. Biochemical assays validated the inhibition of NLRP3 inflammasome activation, decreased mTOR signaling, and enhanced autophagy. However, short-term CSF1R inhibition did not influence Aβ plaques, soluble Aβ-42 levels, or hippocampal neurogenesis. Thus, short-term CSF1R inhibition during the early stages of neuroinflammation in 5xFAD mice promotes the retention of homeostatic microglia with diminished inflammasome activation and mTOR signaling, alongside increased autophagy.

The interplay of inflammation and remyelination: rethinking MS treatment with a focus on oligodendrocyte progenitor cells

BACKGROUND

Multiple sclerosis (MS) therapeutic goals have traditionally been dichotomized into two distinct avenues: immune-modulatory-centric interventions and pro-regenerative strategies. Oligodendrocyte progenitor cells (OPCs) were regarded for many years solely in concern to their potential to generate oligodendrocytes and myelin in the central nervous system (CNS). However, accumulating data elucidate the multifaceted roles of OPCs, including their immunomodulatory functions, positioning them as cardinal constituents of the CNS's immune landscape.

MAIN BODY

In this review, we will discuss how the two therapeutic approaches converge. We present a model by which (1) an inflammation is required for the appropriate pro-myelinating immune function of OPCs in the chronically inflamed CNS, and (2) the immune function of OPCs is crucial for their ability to differentiate and promote remyelination. This model highlights the reciprocal interactions between OPCs' pro-myelinating and immune-modulating functions. Additionally, we review the specific effects of anti- and pro-inflammatory interventions on OPCs, suggesting that immunosuppression adversely affects OPCs' differentiation and immune functions.

CONCLUSION

We suggest a multi-systemic therapeutic approach, which necessitates not a unidimensional focus but a harmonious balance between OPCs' pro-myelinating and immune-modulatory functions.

Anti-PD-1 cancer immunotherapy induces central nervous system immune-related adverse events by microglia activation

Cancer treatment with anti-PD-1 immunotherapy can cause central nervous system immune-related adverse events (CNS-irAEs). The role of microglia in anti-PD-1 immunotherapy-induced CNS-irAEs is unclear. We found that anti-PD-1 treatment of mice caused morphological signs of activation and major histocompatibility complex (MHC) class II up-regulation on microglia. Functionally, anti-PD-1 treatment induced neurocognitive deficits in mice, independent of T cells, B cells, and natural killer cells. Instead, we found that microglia mediated these CNS-irAEs. Single-cell RNA sequencing revealed major transcriptional changes in microglia upon anti-PD-1 treatment. The anti-PD-1 effects were mediated by anti-PD-1 antibodies interacting directly with microglia and were not secondary to peripheral T cell activation. Using a proteomics approach, we identified spleen tyrosine kinase (Syk) as a potential target in activated microglia upon anti-PD-1 treatment. Syk inhibition reduced microglia activation and improved neurocognitive function without impairing anti-melanoma effects. Moreover, we analyzed CNS tissue from a patient cohort that had received anti-PD-1 treatment. Imaging mass cytometry revealed that anti-PD-1 treatment of patients was associated with increased surface marker expression indicative of microglia activation. In summary, we identified a disease-promoting role for microglia in CNS-irAEs driven by Syk and provide an inhibitor-based approach to interfere with this complication after anti-PD-1 immunotherapy.

A novel in vitro model for investigating oligodendroglial maturation and myelin deposition under demyelinating and remyelinating conditions: Impact of microglial depletion and repopulation

Experimental models of multiple sclerosis (MS) have significantly contributed to our understanding of pathophysiology and the development of therapeutic interventions. Various in vivo animal models have successfully replicated key features of MS and associated pathophysiological processes, shedding light on the sequence of events leading to disease initiation, progression, and resolution. Nevertheless, these models often entail substantial costs and prolonged treatment periods. In contrast, in vitro models offer distinct advantages, including cost-effectiveness and precise control over experimental conditions, thereby facilitating more reproducible results. We have developed a novel in vitro model tailored to the study of oligodendroglial maturation and myelin deposition under demyelinating and remyelinating conditions, which encompasses all the cell types present in the central nervous system (CNS). Of note, our model enables the evaluation of microglial cell commitment through a protocol involving their depletion and subsequent repopulation. Given that the development and survival of microglia are critically reliant on colony-stimulating factor-1 receptor (CSF-1R) signaling, we have employed CSF-1R inhibition to effectively deplete microglia. This versatile model holds promise for the assessment of potential therapies aimed at promoting oligodendroglial differentiation to safeguard and repair myelin, hence mitigate neurodegenerative processes.

Myeloid cell replacement is neuroprotective in chronic experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an autoimmune disease characterized by demyelination of the central nervous system (CNS). Autologous hematopoietic cell transplantation (HCT) shows promising benefits for relapsing-remitting MS in open-label clinical studies, but the cellular mechanisms underlying its therapeutic effects remain unclear. Using single-nucleus RNA sequencing, we identify a reactive myeloid cell state in chronic experimental autoimmune encephalitis (EAE) associated with neuroprotection and immune suppression. HCT in EAE mice results in an increase of the neuroprotective myeloid state, improvement of neurological deficits, reduced number of demyelinated lesions, decreased number of effector T cells and amelioration of reactive astrogliosis. Enhancing myeloid cell incorporation after a modified HCT further improved these neuroprotective effects. These data suggest that myeloid cell manipulation or replacement may be an effective therapeutic strategy for chronic inflammatory conditions of the CNS.

CNS Resident Innate Immune Cells: Guardians of CNS Homeostasis

Although the CNS has been considered for a long time an immune-privileged organ, it is now well known that both the parenchyma and non-parenchymal tissue (meninges, perivascular space, and choroid plexus) are richly populated in resident immune cells. The advent of more powerful tools for multiplex immunophenotyping, such as single-cell RNA sequencing technique and upscale multiparametric flow and mass spectrometry, helped in discriminating between resident and infiltrating cells and, above all, the different spectrum of phenotypes distinguishing border-associated macrophages. Here, we focus our attention on resident innate immune players and their primary role in both CNS homeostasis and pathological neuroinflammation and neurodegeneration, two key interconnected aspects of the immunopathology of multiple sclerosis.

CSF1R antagonism results in increased supraspinal infiltration in EAE

BACKGROUND

Colony stimulating factor 1 receptor (CSF1R) signaling is crucial for the maintenance and function of various myeloid subsets. CSF1R antagonism was previously shown to mitigate clinical severity in experimental autoimmune encephalomyelitis (EAE). The associated mechanisms are still not well delineated.

METHODS

To assess the effect of CSF1R signaling, we employed the CSF1R antagonist PLX5622 formulated in chow (PLX5622 diet, PD) and its control chow (control diet, CD). We examined the effect of PD in steady state and EAE by analyzing cells isolated from peripheral immune organs and from the CNS via flow cytometry. We determined CNS infiltration sites and assessed the extent of demyelination using immunohistochemistry of cerebella and spinal cords. Transcripts of genes associated with neuroinflammation were also analyzed in these tissues.

RESULTS

In addition to microglial depletion, PD treatment reduced dendritic cells and macrophages in peripheral immune organs, both during steady state and during EAE. Furthermore, CSF1R antagonism modulated numbers and relative frequencies of T effector cells both in the periphery and in the CNS during the early stages of the disease. Classical neurological symptoms were milder in PD compared to CD mice. Interestingly, a subset of PD mice developed atypical EAE symptoms. Unlike previous studies, we observed that the CNS of PD mice was infiltrated by increased numbers of peripheral immune cells compared to that of CD mice. Immunohistochemical analysis showed that CNS infiltrates in PD mice were mainly localized in the cerebellum while in CD mice infiltrates were primarily localized in the spinal cords during the onset of neurological deficits. Accordingly, during the same timepoint, cerebella of PD but not of CD mice had extensive demyelinating lesions, while spinal cords of CD but not of PD mice were heavily demyelinated.

CONCLUSIONS

Our findings suggest that CSF1R activity modulates the cellular composition of immune cells both in the periphery and within the CNS, and affects lesion localization during the early EAE stages.

Modulation of central synapse remodeling after remote peripheral injuries by the CCL2-CCR2 axis and microglia

Microglia-mediated synaptic plasticity after CNS injury varies depending on injury severity, but the mechanisms that adjust synaptic plasticity according to injury differences are largely unknown. This study investigates differential actions of microglia on essential spinal motor synaptic circuits following different kinds of nerve injuries. Following nerve transection, microglia and C-C chemokine receptor type 2 signaling permanently remove Ia axons and synapses from the ventral horn, degrading proprioceptive feedback during motor actions and abolishing stretch reflexes. However, Ia synapses and reflexes recover after milder injuries (nerve crush). These different outcomes are related to the length of microglia activation, being longer after nerve cuts, with slower motor-axon regeneration and extended expression of colony-stimulating factor type 1 in injured motoneurons. Prolonged microglia activation induces CCL2 expression, and Ia synapses recover after ccl2 is deleted from microglia. Thus, microglia Ia synapse removal requires the induction of specific microglia phenotypes modulated by nerve regeneration efficiencies. However, synapse preservation was not sufficient to restore the stretch-reflex function.

Mesenchymal stromal cells suppress microglial activation and tumor necrosis factor production

BACKGROUND AIMS

White matter diseases are commonly associated with microglial activation and neuroinflammation. Mesenchymal stromal cells (MSCs) have immunomodulatory properties and thus have the potential to be developed as cell therapy for white matter disease. MSCs interact with resident macrophages to alter the trajectory of inflammation; however, the impact MSCs have on central nervous system macrophages and the effect this has on the progression of white matter disease are unclear.

METHODS

In this study, we utilized numerous assays of varying complexity to model different aspects of white matter disease. These assays ranged from an in vivo spinal cord acute demyelination model to a simple microglial cell line activation assay. Our goal was to investigate the influence of human umbilical cord tissue MSCs on the activation of microglia.

RESULTS

MSCs reduced the production of tumor necrosis factor (TNF) by microglia and decreased demyelinated lesions in the spinal cord after acute focal injury. To determine if MSCs could directly suppress the activation of microglia and to develop an efficient potency assay, we utilized isolated primary microglia from mouse brains and the Immortalized MicroGlial Cell Line (IMG). MSCs suppressed the activation of microglia and the release of TNF after stimulation with lipopolysaccharide, a toll-like receptor agonist.

CONCLUSIONS

In this study, we demonstrated that MSCs altered the immune response after acute injury in the spinal cord. In numerous assays, MSCs suppressed activation of microglia and release of the pro-inflammatory cytokine TNF. Of these assays, IMG could be standardized and used as an effective potency assay to determine the efficacy of MSCs for treating white matter disease or other neuroinflammatory conditions associated with microglial activation.

Chronic psychosocial stress triggers microglial-/macrophage-induced inflammatory responses leading to neuronal dysfunction and depressive-related behavior

Chronic environmental stress and traumatic social experiences induce maladaptive behavioral changes and is a risk factor for major depressive disorder (MDD) and various anxiety-related psychiatric disorders. Clinical studies and animal models of chronic stress have reported that symptom severity is correlated with innate immune responses and upregulation of neuroinflammatory cytokine signaling in brain areas implicated in mood regulation (mPFC; medial Prefrontal Cortex). Despite increasing evidence implicating impairments of neuroplasticity and synaptic signaling deficits into the pathophysiology of stress-related mental disorders, how microglia may modulate neuronal homeostasis in response to chronic stress has not been defined. Here, using the repeated social defeat stress (RSDS) mouse model we demonstrate that microglial-induced inflammatory responses are regulating neuronal plasticity associated with psychosocial stress. Specifically, we show that chronic stress induces a rapid activation and proliferation of microglia as well as macrophage infiltration in the mPFC, and these processes are spatially related to neuronal activation. Moreover, we report a significant association of microglial inflammatory responses with susceptibility or resilience to chronic stress. In addition, we find that exposure to chronic stress exacerbates phagocytosis of synaptic elements and deficits in neuronal plasticity. Importantly, by utilizing two different CSF1R inhibitors (the brain penetrant PLX5622 and the non-penetrant PLX73086) we highlight a crucial role for microglia (and secondarily macrophages) in catalyzing the pathological manifestations linked to psychosocial stress in the mPFC and the resulting behavioral deficits usually associated with depression.

Peripherally derived myeloid cells induce disease-dependent phenotypic changes in microglia

In central nervous system (CNS) injury and disease, peripherally derived myeloid cells infiltrate the CNS parenchyma and interact with resident cells, propagating the neuroinflammatory response. Because peripheral myeloid populations differ profoundly depending on the type and phase of injury, their crosstalk with CNS resident cells, particularly microglia, will lead to different functional outcomes. Thus, understanding how peripheral myeloid cells affect the phenotype and function of microglia in different disease conditions and phases may lead to a better understanding of disease-specific targetable pathways for neuroprotection and neurorepair. To this end, we set out to develop an in vitro system to investigate the communication between peripheral myeloid cells and microglia, with the goal of uncovering potential differences due to disease type and timing. We isolated peripheral myeloid cells from mice undergoing experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis, or acute cerebral ischemia by permanent middle cerebral artery occlusion (pMCAO) at different times after disease and probed their ability to change the phenotype of primary microglia isolated from the brain of adult mice. We identified changes not only dependent on the disease model, but also on the timepoint after disease onset from which the myeloid cells were isolated. Peripheral myeloid cells from acute EAE induced morphological changes in microglia, followed by increases in expression of genes involved in inflammatory signaling. Conversely, it was the peripheral myeloid cells from the chronic phase of pMCAO that induced gene expression changes in genes involved in inflammatory signaling and phagocytosis, which was not followed by a change in morphology. This underscores the importance of understanding the role of infiltrating myeloid cells in different disease contexts and phases. Furthermore, we showed that our assay is a valuable tool for investigating myeloid cell interactions in a range of CNS neuroinflammatory conditions.

Neuroglial components of brain lesions may provide new therapeutic strategies for multiple sclerosis

Multiple sclerosis (MS) is a chronic autoimmune and demyelinating disease of the central nervous system (CNS) which leads to focal demyelinated lesions in the brain and spinal cord. Failure of remyelination contributes to chronic disability in young adults. Characterization of events occurring during the demyelination and remyelination processes and those of which subsequently limit remyelination or contribute to demyelination can provide the possibility of new therapies development for MS. Most of the currently available therapies and investigations modulate immune responses and mediators. Since most therapeutic strategies have unsatisfied outcomes, developing new therapies that enhance brain lesion repair is a priority. A close look at cellular and chemical components of MS lesions will pave the way to a better understanding of lesions pathology and will provide possible opportunities for repair strategies and targeted pharmacotherapy. This review summarizes the lesion components and features, particularly the detrimental elements, and discusses the possibility of suggesting new potential targets as therapies for demyelinating diseases like MS.

B cell depletion modulates glial responses and enhances blood vessel integrity in a model of multiple sclerosis

Multiple sclerosis (MS) is characterized by a compromised blood-brain barrier (BBB) resulting in central nervous system (CNS) entry of peripheral lymphocytes, including T cells and B cells. While T cells have largely been considered the main contributors to neuroinflammation in MS, the success of B cell depletion therapies suggests an important role for B cells in MS pathology. Glial cells in the CNS are essential components in both disease progression and recovery, raising the possibility that they represent targets for B cell functions. Here, we examine astrocyte and microglia responses to B cell depleting treatments in an animal model of MS, experimental autoimmune encephalomyelitis (EAE). B cell depleted EAE animals had markedly reduced disease severity and myelin damage accompanied by reduced microglia and astrocyte reactivity 20 days after symptom onset. To identify potential initial mechanisms mediating functional changes following B cell depletion, astrocyte and microglia transcriptomes were analyzed 3 days following B cell depletion. In control EAE animals, transcriptomic analysis revealed astrocytic inflammatory pathways were activated and microglial influence on neuronal function were inhibited. Following B cell depletion, initial functional recovery was associated with an activation of astrocytic pathways linked with restoration of neurovascular integrity and of microglial pathways associated with neuronal function. These studies reveal an important role for B cell depletion in influencing glial function and CNS vasculature in an animal model of MS.

How myeloid cells shape experimental autoimmune encephalomyelitis: At the crossroads of outside-in immunity

Experimental autoimmune encephalomyelitis (EAE) is an animal model of central nervous system (CNS) autoimmunity. It is most commonly used to mimic aspects of multiple sclerosis (MS), a demyelinating disorder of the human brain and spinal cord. The innate immune response displays one of the core pathophysiological features linked to both the acute and chronic stages of MS. Hence, understanding and targeting the innate immune response is essential. Microglia and other CNS resident MUs, as well as infiltrating myeloid cells, diverge substantially in terms of both their biology and their roles in EAE. Recent advances in the field show that antigen presentation, as well as disease-propagating and regulatory interactions with lymphocytes, can be attributed to specific myeloid cell types and cell states in EAE lesions, following a distinct temporal pattern during disease initiation, propagation and recovery. Furthermore, single-cell techniques enable the assessment of characteristic proinflammatory as well as beneficial cell states, and identification of potential treatment targets. Here, we discuss the principles of EAE induction and protocols for varying experimental paradigms, the composition of the myeloid compartment of the CNS during health and disease, and systematically review effects on myeloid cells for therapeutic approaches in EAE.

Ablation of microglia following infection of the central nervous system with a neurotropic murine coronavirus infection leads to increased demyelination and impaired remyelination

Intracranial inoculation of susceptible mice with a glial-tropic strain of mouse hepatitis virus (JHMV), a murine coronavirus, results in an acute encephalomyelitis followed by viral persistence in white matter tracts accompanied by chronic neuroinflammation and demyelination. Microglia are the resident immune cell of the central nervous system (CNS) and are considered important in regulating events associated with neuroinflammation as well as influencing both white matter damage and remyelination. To better understand mechanisms by which microglia contribute to these immune-mediated events, JHMV-infected mice with established demyelination were treated with the small molecular inhibitor of colony stimulating factor 1 receptor (CSF1R), PLX5622, to deplete microglia. Treatment with PLX5622 did not affect viral replication within the CNS yet the severity of demyelination was increased and remyelination impaired compared to control mice. Gene expression analysis revealed that targeting microglia resulted in altered expression of genes associated with immune cell activation and phagocytosis of myelin debris. These findings indicate that microglia are not critical in viral surveillance in persistently JHMV-infected mice yet restrict white matter damage and remyelination, in part, by influencing phagocytosis of myelin debris.

Roles and regulation of microglia activity in multiple sclerosis: insights from animal models

As resident macrophages of the CNS, microglia are critical immune effectors of inflammatory lesions and associated neural dysfunctions. In multiple sclerosis (MS) and its animal models, chronic microglial inflammatory activity damages myelin and disrupts axonal and synaptic activity. In contrast to these detrimental effects, the potent phagocytic and tissue-remodelling capabilities of microglia support critical endogenous repair mechanisms. Although these opposing capabilities have long been appreciated, a precise understanding of their underlying molecular effectors is only beginning to emerge. Here, we review recent advances in our understanding of the roles of microglia in animal models of MS and demyelinating lesions and the mechanisms that underlie their damaging and repairing activities. We also discuss how the structured organization and regulation of the genome enables complex transcriptional heterogeneity within the microglial cell population at demyelinating lesions.

Strategies for Manipulating Microglia to Determine Their Role in the Healthy and Diseased Brain

Microglia are the specialized macrophages of the central nervous system and play an important role in neural circuit development, modulating neurotransmission, and maintaining brain homeostasis. Microglia in normal brain is quiescent and show ramified morphology with numerous branching processes. They constantly survey their surrounding microenvironment through the extension and retraction of their processes and interact with neurons, astrocytes, and blood vessels using these processes. Microglia respond quickly to any pathological event in the brain by assuming ameboid morphology devoid of branching processes and restore homeostasis. However, when there is chronic inflammation, microglia may lose their homeostatic functions and secrete various proinflammatory cytokines and mediators that initiate neural dysfunction and neurodegeneration. In this article, we review the role of microglia in the normal brain and in various pathological brain conditions, such as Alzheimer's disease and multiple sclerosis. We describe strategies to manipulate microglia, focusing on depletion, repopulation, and replacement, and we discuss their therapeutic potential.

Microglia depletion prevents lactation by inhibition of prolactin secretion

Microglial cells were eliminated from the brain with sustained 3-4 weeks long inhibition of colony stimulating factor 1 receptor by Pexidartinib 3397 (PLX3397). The prepartum treated mice mothers did not feed their pups after parturition. The pups of mothers treated orally only in the postpartum period starting immediately after parturition showed reduced body weight by 15.5 ± 0.22 postnatal days as the treatment progressed without the mothers showing altered caring behaviors. The apparent weight gain of foster pups during a suckling bout was reduced in mother mice fed by PLX3397-containing diet and also in rat dams following sustained intracerebroventricular infusion of PLX3397 in a separate experiment suggesting that lactation was affected by the reduced number of microglia. Prolactin secretion and signaling were markedly reduced in PLX3397-treated mothers. The results suggest that microglial cells are required for prolactin secretion and lactation whereas maternal motivation may not be directly affected by microglia.

Augmentation of a neuroprotective myeloid state by hematopoietic cell transplantation

Multiple sclerosis (MS) is an autoimmune disease associated with inflammatory demyelination in the central nervous system (CNS). Autologous hematopoietic cell transplantation (HCT) is under investigation as a promising therapy for treatment-refractory MS. Here we identify a reactive myeloid state in chronic experimental autoimmune encephalitis (EAE) mice and MS patients that is surprisingly associated with neuroprotection and immune suppression. HCT in EAE mice leads to an enhancement of this myeloid state, as well as clinical improvement, reduction of demyelinated lesions, suppression of cytotoxic T cells, and amelioration of reactive astrogliosis reflected in reduced expression of EAE-associated gene signatures in oligodendrocytes and astrocytes. Further enhancement of myeloid cell incorporation into the CNS following a modified HCT protocol results in an even more consistent therapeutic effect corroborated by additional amplification of HCT-induced transcriptional changes, underlining myeloid-derived beneficial effects in the chronic phase of EAE. Replacement or manipulation of CNS myeloid cells thus represents an intriguing therapeutic direction for inflammatory demyelinating disease.

Characterizing microglial gene expression in a model of secondary progressive multiple sclerosis

Multiple sclerosis (MS) is the most common inflammatory, demyelinating and neurodegenerative disease of the central nervous system in young adults. Chronic-relapsing experimental autoimmune encephalomyelitis (crEAE) in Biozzi ABH mice is an experimental model of MS. This crEAE model is characterized by an acute phase with severe neurological disability, followed by remission of disease, relapse of neurological disease and remission that eventually results in a chronic progressive phase that mimics the secondary progressive phase (SPEAE) of MS. In both MS and SPEAE, the role of microglia is poorly defined. We used a crEAE model to characterize microglia in the different phases of crEAE phases using morphometric and RNA sequencing analyses. At the initial, acute inflammation phase, microglia acquired a pro-inflammatory phenotype. At the remission phase, expression of standard immune activation genes was decreased while expression of genes associated with lipid metabolism and tissue remodeling were increased. Chronic phase microglia partially regain inflammatory gene sets and increase expression of genes associated with proliferation. Together, the data presented here indicate that microglia obtain different features at different stages of crEAE and a particularly mixed phenotype in the chronic stage. Understanding the properties of microglia that are present at the chronic phase of EAE will help to understand the role of microglia in secondary progressive MS, to better aid the development of therapies for this phase of the disease.

The Pathological Activation of Microglia Is Modulated by Sexually Dimorphic Pathways

Microglia are the primary immunocompetent cells of the central nervous system (CNS). Their ability to survey, assess and respond to perturbations in their local environment is critical in their role of maintaining CNS homeostasis in health and disease. Microglia also have the capability of functioning in a heterogeneous manner depending on the nature of their local cues, as they can become activated on a spectrum from pro-inflammatory neurotoxic responses to anti-inflammatory protective responses. This review seeks to define the developmental and environmental cues that support microglial polarization towards these phenotypes, as well as discuss sexually dimorphic factors that can influence this process. Further, we describe a variety of CNS disorders including autoimmune disease, infection, and cancer that demonstrate disparities in disease severity or diagnosis rates between males and females, and posit that microglial sexual dimorphism underlies these differences. Understanding the mechanism behind differential CNS disease outcomes between men and women is crucial in the development of more effective targeted therapies.

Women in the field of multiple sclerosis: How they contributed to paradigm shifts

History is full of women who made enormous contributions to science. While there is little to no imbalance at the early career stage, a decreasing proportion of women is found as seniority increases. In the multiple sclerosis (MS) field, 44% of first authors and only 35% of senior authors were female. So, in this review, we highlight ground-breaking research done by women in the field of MS, focusing mostly on their work as principal investigators. MS is an autoimmune disorder of the central nervous system (CNS), with evident paradigm shifts in the understating of its pathophysiology. It is known that the immune system becomes overactivated and attacks myelin sheath surrounding axons. The resulting demyelination disrupts the communication signals to and from the CNS, which causes unpredictable symptoms, depending on the neurons that are affected. Classically, MS was reported to cause mostly physical and motor disabilities. However, it is now recognized that cognitive impairment affects more than 50% of the MS patients. Another shifting paradigm was the involvement of gray matter in MS pathology, formerly considered to be a white matter disease. Additionally, the identification of different T cell immune subsets and the mechanisms underlying the involvement of B cells and peripheral macrophages provided a better understanding of the immunopathophysiological processes present in MS. Relevantly, the gut-brain axis, recognized as a bi-directional communication system between the CNS and the gut, was found to be crucial in MS. Indeed, gut microbiota influences not only different susceptibilities to MS pathology, but it can also be modulated in order to positively act in MS course. Also, after the identification of the first microRNA in 1993, the role of microRNAs has been investigated in MS, either as potential biomarkers or therapeutic agents. Finally, concerning MS therapeutical approaches, remyelination-based studies have arisen on the spotlight aiming to repair myelin loss/neuronal connectivity. Altogether, here we emphasize the new insights of remarkable women that have voiced the impact of cognitive impairment, white and gray matter pathology, immune response, and that of the CNS-peripheral interplay on MS diagnosis, progression, and/or therapy efficacy, leading to huge breakthroughs in the MS field.

Microglia and meningeal macrophages depletion delays the onset of experimental autoimmune encephalomyelitis

In multiple sclerosis and the experimental autoimmune encephalomyelitis (EAE) model, both resident microglia and infiltrating macrophages contribute to demyelination as well as spontaneous remyelination. Nevertheless, the specific roles of microglia versus macrophages are unknown. We investigated the influence of microglia in EAE using the colony stimulating factor 1 receptor (CSF-1R) inhibitor, PLX5622, to deplete microglial population and Ccr2^RFP/+ fms^EGFP/+ mice, to distinguish blood-derived macrophages from microglia. PLX5622 treatment depleted microglia and meningeal macrophages, and provoked a massive infiltration of CCR2⁺ macrophages into demyelinating lesions and spinal cord parenchyma, albeit it did not alter EAE chronic phase. In contrast, microglia and meningeal macrophages depletion reduced the expression of major histocompatibility complex II and CD80 co-stimulatory molecule in dendritic cells, macrophages and microglia. In addition, it diminished T cell reactivation and proliferation in the spinal cord parenchyma, inducing a significant delay in EAE onset. Altogether, these data point to a specific role of CNS microglia and meningeal macrophages in antigen presentation and T cell reactivation at initial stages of EAE.

Microglia depletion as a therapeutic strategy: friend or foe in multiple sclerosis models?

Multiple sclerosis is a chronic central nervous system demyelinating disease whose onset and progression are driven by a combination of immune dysregulation, genetic predisposition, and environmental factors. The activation of microglia and astrocytes is a key player in multiple sclerosis immunopathology, playing specific roles associated with anatomical location and phase of the disease and controlling demyelination and neurodegeneration. Even though reactive microglia can damage tissue and heighten deleterious effects and neurodegeneration, activated microglia also perform neuroprotective functions such as debris phagocytosis and growth factor secretion. Astrocytes can be activated into pro-inflammatory phenotype A1 through a mechanism mediated by activated neuroinflammatory microglia, which could also mediate neurodegeneration. This A1 phenotype inhibits oligodendrocyte proliferation and differentiation and is toxic to both oligodendrocytes and neurons. However, astroglial activation into phenotype A2 may also take place in response to neurodegeneration and as a protective mechanism. A variety of animal models mimicking specific multiple sclerosis features and the associated pathophysiological processes have helped establish the cascades of events that lead to the initiation, progression, and resolution of the disease. The colony-stimulating factor-1 receptor is expressed by myeloid lineage cells such as peripheral monocytes and macrophages and central nervous system microglia. Importantly, as microglia development and survival critically rely on colony-stimulating factor-1 receptor signaling, colony-stimulating factor-1 receptor inhibition can almost completely eliminate microglia from the brain. In this context, the present review discusses the impact of microglial depletion through colony-stimulating factor-1 receptor inhibition on demyelination, neurodegeneration, astroglial activation, and behavior in different multiple sclerosis models, highlighting the diversity of microglial effects on the progression of demyelinating diseases and the strengths and weaknesses of microglial modulation in therapy design.

Targeting CSF-1R represents an effective strategy in modulating inflammatory diseases

Colony-stimulating factor-1 receptor (CSF-1R), also known as FMS kinase, is a type I single transmembrane protein mainly expressed in myeloid cells, such as monocytes, macrophages, glial cells, and osteoclasts. The endogenous ligands, colony-stimulating factor-1 (CSF-1) and Interleukin-34 (IL-34), activate CSF-1R and downstream signaling pathways including PI3K-AKT, JAK-STATs, and MAPKs, and modulate the proliferation, differentiation, migration, and activation of target immune cells. Over the past decades, the promising therapeutic potential of CSF-1R signaling inhibition has been widely studied for decreasing immune suppression and escape in tumors, owing to depletion and reprogramming of tumor-associated macrophages. In addition, the excessive activation of CSF-1R in inflammatory diseases is consecutively uncovered in recent years, which may result in inflammation in bone, kidney, lung, liver and central nervous system. Agents against CSF-1R signaling have been increasingly investigated in preclinical or clinical studies for inflammatory diseases treatment. However, the pathological mechanism of CSF-1R in inflammation is indistinct and whether CSF-1R signaling can be identified as biomarkers remains controversial. With the background information aforementioned, this review focus on the dialectical roles of CSF-1R and its ligands in regulating innate immune cells and highlights various therapeutic implications of blocking CSF-1R signaling in inflammatory diseases.

Modulation of the Microglial Nogo-A/NgR Signaling Pathway as a Therapeutic Target for Multiple Sclerosis

Current therapeutics targeting chronic phases of multiple sclerosis (MS) are considerably limited in reversing the neural damage resulting from repeated inflammation and demyelination insults in the multi-focal lesions. This inflammation is propagated by the activation of microglia, the endogenous immune cell aiding in the central nervous system homeostasis. Activated microglia may transition into polarized phenotypes; namely, the classically activated proinflammatory phenotype (previously categorized as M1) and the alternatively activated anti-inflammatory phenotype (previously, M2). These transitional microglial phenotypes are dynamic states, existing as a continuum. Shifting microglial polarization to an anti-inflammatory status may be a potential therapeutic strategy that can be harnessed to limit neuroinflammation and further neurodegeneration in MS. Our research has observed that the obstruction of signaling by inhibitory myelin proteins such as myelin-associated inhibitory factor, Nogo-A, with its receptor (NgR), can regulate microglial cell function and activity in pre-clinical animal studies. Our review explores the microglial role and polarization in MS pathology. Additionally, the potential therapeutics of targeting Nogo-A/NgR cellular mechanisms on microglia migration, polarization and phagocytosis for neurorepair in MS and other demyelination diseases will be discussed.

A prebiotic diet modulates microglial states and motor deficits in α-synuclein overexpressing mice

Parkinson's disease (PD) is a movement disorder characterized by neuroinflammation, α-synuclein pathology, and neurodegeneration. Most cases of PD are non-hereditary, suggesting a strong role for environmental factors, and it has been speculated that disease may originate in peripheral tissues such as the gastrointestinal (GI) tract before affecting the brain. The gut microbiome is altered in PD and may impact motor and GI symptoms as indicated by animal studies, although mechanisms of gut-brain interactions remain incompletely defined. Intestinal bacteria ferment dietary fibers into short-chain fatty acids, with fecal levels of these molecules differing between PD and healthy controls and in mouse models. Among other effects, dietary microbial metabolites can modulate activation of microglia, brain-resident immune cells implicated in PD. We therefore investigated whether a fiber-rich diet influences microglial function in α-synuclein overexpressing (ASO) mice, a preclinical model with PD-like symptoms and pathology. Feeding a prebiotic high-fiber diet attenuates motor deficits and reduces α-synuclein aggregation in the substantia nigra of mice. Concomitantly, the gut microbiome of ASO mice adopts a profile correlated with health upon prebiotic treatment, which also reduces microglial activation. Single-cell RNA-seq analysis of microglia from the substantia nigra and striatum uncovers increased pro-inflammatory signaling and reduced homeostatic responses in ASO mice compared to wild-type counterparts on standard diets. However, prebiotic feeding reverses pathogenic microglial states in ASO mice and promotes expansion of protective disease-associated macrophage (DAM) subsets of microglia. Notably, depletion of microglia using a CSF1R inhibitor eliminates the beneficial effects of prebiotics by restoring motor deficits to ASO mice despite feeding a prebiotic diet. These studies uncover a novel microglia-dependent interaction between diet and motor symptoms in mice, findings that may have implications for neuroinflammation and PD.

What microglia depletion approaches tell us about the role of microglia on synaptic function and behavior

Microglia are dynamic cells, constantly surveying their surroundings and interacting with neurons and synapses. Indeed, a wealth of knowledge has revealed a critical role of microglia in modulating synaptic transmission and plasticity in the developing brain. In the past decade, novel pharmacological and genetic strategies have allowed the acute removal of microglia, opening the possibility to explore and understand the role of microglia also in the adult brain. In this review, we summarized and discussed the contribution of microglia depletion strategies to the current understanding of the role of microglia on synaptic function, learning and memory, and behavior both in physiological and pathological conditions. We first described the available microglia depletion methods highlighting their main strengths and weaknesses. We then reviewed the impact of microglia depletion on structural and functional synaptic plasticity. Next, we focused our analysis on the effects of microglia depletion on behavior, including general locomotor activity, sensory perception, motor function, sociability, learning and memory both in healthy animals and animal models of disease. Finally, we integrated the findings from the reviewed studies and discussed the emerging roles of microglia on the maintenance of synaptic function, learning, memory strength and forgetfulness, and the implications of microglia depletion in models of brain disease.

Microglia in multiple sclerosis: Protectors turn destroyers

Microglia are implicated in all stages of multiple sclerosis (MS). Microglia alterations are detected by positron emission tomography in people living with MS prior to the formation of structural lesions determined through magnetic resonance imaging. In histological specimens, clusters of microglia form in normal-appearing tissue likely predating the development of lesions. Features of degeneration-associated/pro-inflammatory states of microglia increase with chronicity of MS. However, microglia play many beneficial roles including the removal of neurotoxins and in fostering repair. The protector-gone-rogue microglia in MS is featured herein. We consider mechanisms of microglia neurotoxicity and discuss factors, including aging, osteopontin, and iron metabolism, that cause microglia to lose their protective states and become injurious. We evaluate medications to affect microglia in MS, such as the emerging class of Bruton's tyrosine kinase inhibitors. The framework of microglia-turned-destroyers may instigate new approaches to counter microglia-driven neurodegeneration in MS.

CSF1R-Mediated Myeloid Cell Depletion Prolongs Lifespan But Aggravates Distinct Motor Symptoms in a Model of Multiple System Atrophy

As the CNS-resident macrophages and member of the myeloid lineage, microglia fulfill manifold functions important for brain development and homeostasis. In the context of neurodegenerative diseases, they have been implicated in degenerative and regenerative processes. The discovery of distinct activation patterns, including increased phagocytosis, indicated a damaging role of myeloid cells in multiple system atrophy (MSA), a devastating, rapidly progressing atypical parkinsonian disorder. Here, we analyzed the gene expression profile of microglia in a mouse model of MSA (MBP29-hα-syn) and identified a disease-associated expression profile and upregulation of the colony-stimulating factor 1 (Csf1). Thus, we hypothesized that CSF1 receptor-mediated depletion of myeloid cells using PLX5622 modifies the disease progression and neuropathological phenotype in this mouse model. Intriguingly, sex-balanced analysis of myeloid cell depletion in MBP29-hα-syn mice revealed a two-faced outcome comprising an improved survival rate accompanied by a delayed onset of neurological symptoms in contrast to severely impaired motor functions. Furthermore, PLX5622 reversed gene expression profiles related to myeloid cell activation but reduced gene expression associated with transsynaptic signaling and signal release. While transcriptional changes were accompanied by a reduction of dopaminergic neurons in the SNpc, striatal neuritic density was increased upon myeloid cell depletion in MBP29-hα-syn mice. Together, our findings provide insight into the complex, two-faced role of myeloid cells in the context of MSA emphasizing the importance to carefully balance the beneficial and adverse effects of CSF1R inhibition in different models of neurodegenerative disorders before its clinical translation.SIGNIFICANCE STATEMENT Myeloid cells have been implicated as detrimental in the disease pathogenesis of multiple system atrophy. However, long-term CSF1R-dependent depletion of these cells in a mouse model of multiple system atrophy demonstrates a two-faced effect involving an improved survival associated with a delayed onset of disease and reduced inflammation which was contrasted by severely impaired motor functions, synaptic signaling, and neuronal circuitries. Thus, this study unraveled a complex role of myeloid cells in multiple system atrophy, which indicates important functions beyond the previously described disease-associated, destructive phenotype and emphasized the need of further investigation to carefully and individually fine-tune immunologic processes in different neurodegenerative diseases.

Microglia Regulate Blood-Brain Barrier Integrity via MiR-126a-5p/MMP9 Axis during Inflammatory Demyelination

Blood-brain barrier (BBB) impairment is an early prevalent feature of multiple sclerosis (MS), and remains vital for MS progression. Microglial activation precedes BBB disruption and cellular infiltrates in the brain of MS patients. However, little is known about the function of microglia in BBB impairment. Here, microglia acts as an important modulator of BBB integrity in inflammatory demyelination. Microglial depletion profoundly ameliorates BBB impairment in experimental autoimmune encephalomyelitis (EAE). Specifically, miR-126a-5p in microglia is positively correlated with BBB integrity in four types of MS plaques. Mechanistically, microglial deletion of miR-126a-5p exacerbates BBB leakage and EAE severity. The protective effect of miR-126a-5p is mimicked and restored by specific inhibition of MMP9 in microglia. Importantly, Auranofin, an FDA-approved drug, is identified to protect BBB integrity and mitigate EAE progression via a microglial miR-126a-5p dependent mechanism. Taken together, microglia can be manipulated to protect BBB integrity and ameliorate inflammatory demyelination. Targeting microglia to regulate BBB permeability merits consideration in therapeutic interventions in MS.

Regulation of activated microglia and macrophages by systemically administered DNA/RNA heteroduplex oligonucleotides

Microglial activation followed by recruitment of blood-borne macrophages into the central nervous system (CNS) aggravates neuroinflammation. Specifically, in multiple sclerosis (MS) as well as in experimental autoimmune encephalomyelitis (EAE), a rodent model of MS, activated microglia and macrophages (Mg/Mφ) promote proinflammatory responses and expand demyelination in the CNS. However, a potent therapeutic approach through the systemic route for regulating their functions has not yet been developed. Here, we demonstrate that a systemically injected DNA/RNA heteroduplex oligonucleotide (HDO), composed of an antisense oligonucleotide (ASO) and its complementary RNA, conjugated to cholesterol (Chol-HDO) distributed more efficiently to demyelinating lesions of the spinal cord in EAE mice with significant gene silencing than the parent ASO. Importantly, systemic administration of Cd40-targeting Chol-HDO improved clinical signs of EAE with significant downregulation of Cd40 in Mg/Mφ. Furthermore, we successfully identify that macrophage scavenger receptor 1 (MSR1) is responsible for the uptake of Chol-HDO by Mg/Mφ of EAE mice. Overall, our findings demonstrate the therapeutic potency of systemically administered Chol-HDO to regulate activated Mg/Mφ in neuroinflammation.

Beyond Myelination: Possible Roles of the Immune Proteasome in Oligodendroglial Homeostasis and Dysfunction

Oligodendroglia play a critical role in CNS homeostasis by myelinating neuronal axons in their mature stages. Dysfunction in this lineage occurs when early stage OPCs are not able to differentiate to replace dying Mature Myelinating Oligodendrocytes. Many hypotheses exist as to why de- and hypo-myelinating disorders and diseases occur. In this review, we present data to show that oligodendroglia can adopt components of the immune proteasome under inflammatory conditions. The works reviewed further reflect that these immune-component expressing oligodendroglia can in fact function as antigen presenting cells, phagocytosing foreign entities and presenting them via MHC II to activate CD4+ T cells. Additionally, we hypothesize, based on the limited literature, that the adoption of immune components by oligodendroglia may contribute to their stalled differentiation in the context of these disorders and diseases. The present review will underline: (1) Mechanisms of neuroinflammation in diseases associated with Immune Oligodendroglia; (2) the first associations between the immune proteasome and oligodendroglia and the subtle distinctions between these works; (3) the suggested functionality of these cells as it is described by current literature; and (4) the hypothesized consequences on metabolism. In doing so we aim to shed light on this fairly under-explored cell type in hopes that study of their functionality may lead to further mechanistic understanding of hypo- and de-myelinating neuroinflammatory disorders and diseases.

CSF-1 maintains pathogenic but not homeostatic myeloid cells in the central nervous system during autoimmune neuroinflammation

The receptor for colony stimulating factor 1 (CSF-1R) is important for the survival and function of myeloid cells that mediate pathology during experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). CSF-1 and IL-34, the ligands of CSF-1R, have similar bioactivities but distinct tissue and context-dependent expression patterns, suggesting that they have different roles. This could be the case in EAE, given that CSF-1 expression is up-regulated in the CNS, while IL-34 remains constitutively expressed. We found that targeting CSF-1 with neutralizing antibody halted ongoing EAE, with efficacy superior to CSF-1R inhibitor BLZ945, whereas IL-34 neutralization had no effect, suggesting that pathogenic myeloid cells were maintained by CSF-1. Both anti–CSF-1 and BLZ945 treatment greatly reduced the number of monocyte-derived cells and microglia in the CNS. However, anti–CSF-1 selectively depleted inflammatory microglia and monocytes in inflamed CNS areas, whereas BLZ945 depleted virtually all myeloid cells, including quiescent microglia, throughout the CNS. Anti–CSF-1 treatment reduced the size of demyelinated lesions and microglial activation in the gray matter. Lastly, we found that bone marrow–derived immune cells were the major mediators of CSF-1R–dependent pathology, while microglia played a lesser role. Our findings suggest that targeting CSF-1 could be effective in ameliorating MS pathology, while preserving the homeostatic functions of myeloid cells, thereby minimizing risks associated with ablation of CSF-1R–dependent cells.

Inhibition of colony stimulating factor-1 receptor (CSF-1R) as a potential therapeutic strategy for neurodegenerative diseases: opportunities and challenges

Microglia are specialized dynamic immune cells in the central nervous system (CNS) that plays a crucial role in brain homeostasis and in disease states. Persistent neuroinflammation is considered a hallmark of many neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson's disease (PD), Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS) and primary progressive multiple sclerosis (MS). Colony stimulating factor 1-receptor (CSF-1R) is predominantly expressed on microglia and its expression is significantly increased in neurodegenerative diseases. Cumulative findings have indicated that CSF-1R inhibitors can have beneficial effects in preclinical neurodegenerative disease models. Research using CSF-1R inhibitors has now been extended into non-human primates and humans. This review article summarizes the most recent advances using CSF-1R inhibitors in different neurodegenerative conditions including AD, PD, HD, ALS and MS. Potential challenges for translating these findings into clinical practice are presented.

Colony-stimulating factor 1 receptor signaling in the central nervous system and the potential of its pharmacological inhibitors to halt the progression of neurological disorders

Colony Stimulating Factor-1 (CSF-1)/Colony Stimulating Factor-1 Receptor (CSF-1R) signaling axis plays an essential role in the development, maintenance, and proliferation of macrophage lineage cells. Within the central nervous system, CSF-1R signaling primarily maintains microglial homeostasis. Microglia, being the resident macrophage and first responder to any neurological insults, plays critical importance in overall health of the human brain. Aberrant and sustained activation of microglia along with continued proliferation and release of neurotoxic proinflammatory cytokines have been reported in various neurological and neurodegenerative diseases. Therefore, halting the neuroinflammatory pathway via targeting microglial proliferation, which depends on CSF-1R signaling, has emerged as a potential therapeutic target for neurological disorders. However, apart from regulating the microglial function, recently it has been discovered that CSF-1R has much broader role in central nervous system. These findings limit the therapeutic utility of CSF-1R inhibitors but also highlight the need for a complete understanding of CSF-1R function within the central nervous system. Moreover, it has been found that selective inhibitors of CSF-1R may be more efficient in avoiding non-specific targeting and associated side effects. Short-term depletion of microglial population in diseased conditions have also been found to be beneficial; however, the dose and therapeutic window for optimum effects may need to be standardized further.This review summarizes the present understanding of CSF-1R function within the central nervous system. We discuss the CSF-1R signaling in the context of microglia function, crosstalk between microglia and astroglia, and regulation of neuronal cell function. We also discuss a few of the neurological disorders with a focus on the utility of CSF-1R inhibitors as potential therapeutic strategy for halting the progression of neurological diseases.

Microglia as therapeutic targets for central nervous system remyelination

Failed remyelination underpins neurodegeneration and central nervous system (CNS) dysfunction with aging and progression of neurological diseases, such as multiple sclerosis and Alzheimer's disease. Existing therapies have shown limited efficacy in halting disease progression in humans, highlighting the need to identify pro-remyelination treatments. Microglia are CNS-resident macrophages with critical roles in the regulation of remyelination, representing a promising therapeutic target. However, there are currently no therapeutics which specifically target microglia. Recent studies have revealed that microglia are a heterogenous population with distinct transcriptional states in health and disease conditions, including during remyelination, suggesting functional differences between states. Here, we discuss the potential contributions of different microglia states to degenerative and regenerative processes, examine the potential to target microglia in a state-specific manner to promote remyelination and consider the key issues to be addressed before such therapies can be clinically applied.

The role of glial cells in multiple sclerosis disease progression

Despite the development of highly effective treatments for relapsing-remitting multiple sclerosis (MS), limited progress has been made in addressing primary progressive or secondary progressive MS, both of which lead to loss of oligodendrocytes and neurons and axons, and to irreversible accumulation of disability. Neuroinflammation is central to all forms of MS. The current effective therapies for relapsing-remitting MS target the peripheral immune system; these treatments, however, have repeatedly failed in progressive MS. Greater understanding of inflammation driven by CNS-resident cells - including astrocytes and microglia - is, therefore, required to identify novel potential therapeutic opportunities. Advances in imaging, biomarker analysis and genomics suggest that microglia and astrocytes have central roles in the progressive disease process. In this Review, we provide an overview of the involvement of astrocytes and microglia at major sites of pathology in progressive MS. We discuss current and future therapeutic approaches to directly target glial cells, either to inhibit pathogenic functions or to restore homeostatic functions lost during the course of the disease. We also discuss how bidirectional communication between astrocytes and microglia needs to be considered, as therapeutic targeting of one is likely to alter the functions of the other.

The roles of microglia and astrocytes in phagocytosis and myelination: Insights from the cuprizone model of multiple sclerosis

In human demyelinating diseases such as multiple sclerosis (MS), an imbalance between demyelination and remyelination can trigger progressive degenerative processes. The clearance of myelin debris (phagocytosis) from the site of demyelination by microglia is critically important to achieve adequate remyelination and to slow the progression of the disease. However, how microglia phagocytose the myelin debris, and why clearance is impaired in MS, is not fully known; likewise, the role of the microglia in remyelination remains unclear. Recent studies using cuprizone (CPZ) as an animal model of central nervous system demyelination revealed that the up‐regulation of signaling proteins in microglia facilitates effective phagocytosis of myelin debris. Moreover, during demyelination, protective mediators are released from activated microglia, resulting in the acceleration of remyelination in the CPZ model. In contrast, inadequate microglial activation or recruitment to the site of demyelination, and the production of toxic mediators, impairs remyelination resulting in progressive demyelination. In addition to the microglia‐mediated phagocytosis, astrocytes play an important role in the phagocytic process by recruiting microglia to the site of demyelination and producing regenerative mediators. The current review is an update of these emerging findings from the CPZ animal model, discussing the roles of microglia and astrocytes in phagocytosis and myelination.

Microglia and monocytes in inflammatory CNS disease: integrating phenotype and function

In neurological diseases, the actions of microglia, the resident myeloid cells of the CNS parenchyma, may diverge from, or intersect with, those of recruited monocytes to drive immune-mediated pathology. However, defining the precise roles of each cell type has historically been impeded by the lack of discriminating markers and experimental systems capable of accurately identifying them. Our ability to distinguish microglia from monocytes in neuroinflammation has advanced with single-cell technologies, new markers and drugs that identify and deplete them, respectively. Nevertheless, the focus of individual studies on particular cell types, diseases or experimental approaches has limited our ability to connect phenotype and function more widely and across diverse CNS pathologies. Here, we critically review, tabulate and integrate the disease-specific functions and immune profiles of microglia and monocytes to provide a comprehensive atlas of myeloid responses in viral encephalitis, demyelination, neurodegeneration and ischemic injury. In emphasizing the differential roles of microglia and monocytes in the severe neuroinflammatory disease of viral encephalitis, we connect inflammatory pathways common to equally incapacitating diseases with less severe inflammation. We examine these findings in the context of human studies and highlight the benefits and inherent limitations of animal models that may impede or facilitate clinical translation. This enables us to highlight common and contrasting, non-redundant and often opposing roles of microglia and monocytes in disease that could be targeted therapeutically.

ASK1 signaling regulates phase-specific glial interactions during neuroinflammation

Neuroinflammation is associated with many neurodegenerative diseases such as Alzheimer’s disease and multiple sclerosis (MS). Thus, decreasing neuroinflammation may be a promising treatment for these diseases. Apoptosis signal-related kinase 1 (ASK1) has been shown to cause neuroinflammation in neurodegenerative disease models, but its mechanism of action has been unclear. Here, we generated conditional knockout mice that lack ASK1 in T cells, dendritic cells, microglia/macrophages, microglia, or astrocytes, to assess the roles of ASK1 during experimental autoimmune encephalomyelitis (EAE), a mouse model of MS. We propose that ASK1 is required in microglia and astrocytes to cause and maintain neuroinflammation by a feedback loop between these two cell types.

Neuroinflammation is well known to be associated with neurodegenerative diseases. Apoptosis signal-regulating kinase 1 (ASK1) is a mitogen-activated protein kinase kinase kinase that has been implicated in neuroinflammation, but its precise cellular and molecular mechanisms remain unknown. In this study, we generated conditional knockout (CKO) mice that lack ASK1 in T cells, dendritic cells, microglia/macrophages, microglia, or astrocytes, to assess the roles of ASK1 during experimental autoimmune encephalomyelitis (EAE). We found that neuroinflammation was reduced in both the early and later stages of EAE in microglia/macrophage-specific ASK1 knockout mice, whereas only the later-stage neuroinflammation was ameliorated in astrocyte-specific ASK1 knockout mice. ASK1 deficiency in T cells and dendritic cells had no significant effects on EAE severity. Further, we found that ASK1 in microglia/macrophages induces a proinflammatory environment, which subsequently activates astrocytes to exacerbate neuroinflammation. Microglia-specific ASK1 deletion was achieved using a CX3CR1^CreER system, and we found that ASK1 signaling in microglia played a major role in generating and maintaining disease. Activated astrocytes produce key inflammatory mediators, including CCL2, that further activated and recruited microglia/macrophages, in an astrocytic ASK1-dependent manner. Astrocyte-specific analysis revealed CCL2 expression was higher in the later stage compared with the early stage, suggesting a greater proinflammatory role of astrocytes in the later stage. Our findings demonstrate cell-type–specific roles of ASK1 and suggest phase-specific ASK1-dependent glial cell interactions in EAE pathophysiology. We propose glial ASK1 as a promising therapeutic target for reducing neuroinflammation.

Microglia activate early anti-viral responses upon HSV-1 entry into the brain to counteract development of encephalitis-like disease in mice

Spread of herpes simplex virus 1 (HSV1) from the periphery to the central nervous system (CNS) can lead to extensive infection and pathological inflammation in the brain, causing herpes simplex encephalitis (HSE). It has been shown that microglia, the CNS-resident macrophages, are involved in early sensing of HSV1 and an induction of antiviral responses. In addition, infiltration of peripheral immune cells may contribute to control of viral infection. In this study, we tested the effect of microglia depletion in a mouse model of HSE. Increased viral titers and increased disease severity were observed in microglia-depleted mice. The effect of microglia depletion was more pronounced in wild-type than in cGas^-/- mice, revealing that this immune sensor contributes to the antiviral activity of microglia. Importantly, microglia depletion led to reduced production of type I interferon (IFN), pro-inflammatory cytokines and chemokines at early time points after viral entry into the CNS. In line with this, in vitro experiments on murine primary CNS cells demonstrated microglial presence to be essential for IFN RNA induction, and control of HSV1 replication. However, the effect of microglia depletion on expression of IFNs, and inflammatory cytokines was restricted to early time point of HSV1 entry into the CNS. There was no major alteration of infiltration of CD45-positive cells in microglia-depleted mice. Collectively, our data demonstrate a key role for microglia in controlling HSV1 replication early after viral entry into the CNS and highlight the importance of a prompt antiviral innate response to reduce the risk of HSE development. Importance One of the most devastating and acute neurological conditions is encephalitis, i.e. inflammation of brain tissue. Herpes simplex virus 1 (HSV1) is a highly prevalent pathogen in humans, and the most frequent cause of viral sporadic encephalitis, called herpes simplex encephalitis (HSE). HSV1 has the ability to infect peripheral neurons and reach the central nervous system (CNS) of humans, where it can be detected by brain resident cells and infiltrating immune cells, leading to protective and damaging immune responses. In this study, we investigated the effects of a depletion of microglia, the main brain-resident immune cell type. For this purpose, we used a mouse model of HSE. We found that viral levels increased and disease symptoms worsened in microglia-depleted mice. In addition, mice lacking a major sensor of viral DNA, cGAS, manifested more pronounced disease than wild-type mice, highlighting the importance of this immune sensor in the activity of microglia. Evidently, microglia depletion led to a reduced production of many known antiviral factors, most notably type I interferon (IFN). The importance of microglia in the early control of HSV1 spread and the generation of antiviral responses is further demonstrated by experiments on murine mixed glial cell cultures. Interestingly, mice with microglia depletion exhibited an unaltered activation of antiviral responses and recruitment of immune cells from the periphery at later time points of infection, but this did not prevent the development of the disease. Overall, the data highlight the importance of a rapid activation of the host defense, with microglia playing a critical role in controlling HSV1 infection, which eventually prevents damage to neurons and brain tissue.

SK-Channel Activation Alters Peripheral Metabolic Pathways in Mice, but Not Lipopolysaccharide-Induced Fever or Inflammation

Purpose

Previously, we have shown that CyPPA (cyclohexyl-[2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-pyrimidin-4-yl]-amine), a pharmacological small-conductance calcium-activated potassium (SK)–channel positive modulator, antagonizes lipopolysaccharide (LPS)-induced cytokine expression in microglial cells. Here, we aimed to test its therapeutic potential for brain-controlled sickness symptoms, brain inflammatory response during LPS-induced systemic inflammation, and peripheral metabolic pathways in mice.

Methods

Mice were pretreated with CyPPA (15 mg/kg IP) 24 hours before and simultaneously with LPS stimulation (2.5 mg/kg IP), and the sickness response was recorded by a telemetric system for 24 hours. A second cohort of mice were euthanized 2 hours after CyPPA or solvent treatment to assess underlying CyPPA-induced mechanisms. Brain, blood, and liver samples were analyzed for inflammatory mediators or nucleotide concentrations using immunohistochemistry, real-time PCR and Western blot, or HPLC. Moreover, we investigated CyPPA-induced changes of UCP1 expression in brown adipose tissue (BAT)–explant cultures.

Results

CyPPA treatment did not affect LPS-induced fever, anorexia, adipsia, or expression profiles of inflammatory mediators in the hypothalamus or plasma or microglial reactivity to LPS (CD11b staining and CD68 mRNA expression). However, CyPPA alone induced a rise in core body temperature linked to heat production via altered metabolic pathways like reduced levels of adenosine, increased protein content, and increased UCP1 expression in BAT-explant cultures, but no alteration in ATP/ADP concentrations in the liver. CyPPA treatment was accompanied by altered pathways, including NFκB signaling, in the hypothalamus and cortex, while circulating cytokines remained unaltered.

Conclusion

Overall, while CyPPA has promise as a treatment strategy, in particular according to results from in vitro experiments, we did not reveal anti-inflammatory effects during severe LPS-induced systemic inflammation. Interestingly, we found that CyPPA alters metabolic pathways inducing short hyperthermia, most likely due to increased energy turnover in the liver and heat production in BAT.

The Colony Stimulating Factor-1 Receptor (CSF-1R)-Mediated Regulation of Microglia/Macrophages as a Target for Neurological Disorders (Glioma, Stroke)

Immunomodulatory therapies have fueled interest in targeting microglial cells as part of the innate immune response after infection or injury. In this context, the colony-stimulating factor 1 (CSF-1) and its receptor (CSF-1R) have gained attention in various neurological conditions to deplete and reprogram the microglia/macrophages compartment. Published data in physiological conditions support the use of small-molecule inhibitors to study microglia/macrophages dynamics under inflammatory conditions and as a therapeutic strategy in pathologies where those cells support disease progression. However, preclinical and clinical data highlighted that the complexity of the spatiotemporal inflammatory response could limit their efficiency due to compensatory mechanisms, ultimately leading to therapy resistance. We review the current state-of-art in the field of CSF-1R inhibition in glioma and stroke and provide an overview of the fundamentals, ongoing research, potential developments of this promising therapeutic strategy and further application toward molecular imaging.

Microglial depletion abolishes ischemic preconditioning in white matter

Ischemic preconditioning (IPC) is a phenomenon whereby a brief, non-injurious ischemic exposure enhances tolerance to a subsequent ischemic challenge. The mechanism of IPC has mainly been studied in rodent stroke models where gray matter (GM) constitutes about 85% of the cerebrum. In humans, white matter (WM) is 50% of cerebral volume and is a critical component of stroke damage. We developed a novel CNS WM IPC model using the mouse optic nerve (MON) and identified the involved immune signaling pathways. Here we tested the hypothesis that microglia are necessary for WM IPC. Microglia were depleted by treatment with the colony stimulating factor 1 receptor (CSF1R) inhibitor PLX5622. MONs were exposed to transient ischemia in vivo, acutely isolated 72 h later, and subjected to oxygen-glucose deprivation (OGD) to simulate a severe ischemic injury (i.e., stroke). Functional and structural axonal recovery was assessed by recording compound action potentials (CAPs) and by microscopy using quantitative stereology. Microglia depletion eliminated IPC-mediated protection. In control mice, CAP recovery was improved in preconditioned MONs compared with non-preconditioned MONs, however, in PLX5622-treated mice, we observed no difference in CAP recovery between preconditioned and non-preconditioned MONs. Microgliadepletion also abolished IPC protective effects on axonal integrity and survival of mature (APC⁺ ) oligodendrocytes after OGD. IPC-mediated protection was independent of retinal injury suggesting it results from mechanistic processes intrinsic to ischemia-exposed WM. We conclude that preconditioned microglia are critical for IPC in WM. The "preconditioned microglia" phenotype might protect against other CNS pathologies and is a neurotherapeutic horizon worth exploring.

Colony-stimulating factor-1 receptor inhibition attenuates microgliosis and myelin loss but exacerbates neurodegeneration in the chronic cuprizone model

Multiple sclerosis (MS), especially in its progressive phase, involves early axonal and neuronal damage resulting from a combination of inflammatory mediators, demyelination, and loss of trophic support. During progressive disease stages, a microenvironment is created within the central nervous system (CNS) favoring the arrival and retention of inflammatory cells. Active demyelination and neurodegeneration have also been linked to microglia (MG) and astrocyte (AST)-activation in early lesions. While reactive MG can damage tissue, exacerbate deleterious effects, and contribute to neurodegeneration, it should be noted that activated MG possess neuroprotective functions as well, including debris phagocytosis and growth factor secretion. The progressive form of MS can be modelled by the prolonged administration to cuprizone (CPZ) in adult mice, as CPZ induces highly reproducible demyelination of different brain regions through oligodendrocyte (OLG) apoptosis, accompanied by MG and AST activation and axonal damage. Therefore, our goal was to evaluate the effects of a reduction in microglial activation through orally administered brain-penetrant colony-stimulating factor-1 receptor (CSF-1R) inhibitor BLZ945 (BLZ) on neurodegeneration and its correlation with demyelination, astroglial activation and behavior in a chronic CPZ-induced demyelination model. Our results show that BLZ treatment successfully reduced the microglial population and myelin loss. However, no correlation was found between myelin preservation and neurodegeneration, as axonal degeneration was more prominent upon BLZ treatment. Concomitantly, BLZ failed to significantly offset CPZ-induced astroglial activation and behavioral alterations. These results should be taken into account when proposing the modulation of microglial activation in the design of therapies relevant for demyelinating diseases.

NeuroD1 induces microglial apoptosis and cannot induce microglia-to-neuron cross-lineage reprogramming

The regenerative capacity of neurons is limited in the central nervous system (CNS), with irreversible neuronal loss upon insult. In contrast, microglia exhibit extraordinary capacity for repopulation. Matsuda et al. (2019) recently reported NeuroD1-induced microglia-to-neuron conversion, aiming to provide an "unlimited" source to regenerate neurons. However, the extent to which NeuroD1 can exert cross-lineage reprogramming of microglia (myeloid lineage) to neurons (neuroectodermal lineage) is unclear. In this study, we unexpectedly found that NeuroD1 cannot convert microglia to neurons in mice. Instead, NeuroD1 expression induces microglial cell death. Moreover, lineage tracing reveals non-specific leakage of similar lentiviruses as previously used for microglia-to-neuron conversion, which confounds the microglia-to-neuron observation. In summary, we demonstrated that NeuroD1 cannot induce microglia-to-neuron cross-lineage reprogramming. We here propose rigid principles for verifying glia-to-neuron conversion. This Matters Arising paper is in response to Matsuda et al. (2019), published in Neuron.

Treatment With the CSF1R Antagonist GW2580, Sensitizes Microglia to Reactive Oxygen Species

Microglia activation and proliferation are hallmarks of many neurodegenerative disorders and may contribute to disease pathogenesis. Neurons actively regulate microglia survival and function, in part by secreting the microglia mitogen interleukin (IL)-34. Both IL-34 and colony stimulating factor (CSF)-1 bind colony stimulating factor receptor (CSFR)1 expressed on microglia. Systemic treatment with central nervous system (CNS) penetrant, CSFR1 antagonists, results in microglia death in a dose dependent matter, while others, such as GW2580, suppress activation during disease states without altering viability. However, it is not known how treatment with non-penetrant CSF1R antagonists, such as GW2580, affect the normal physiology of microglia. To determine how GW2580 affects microglia function, C57BL/6J mice were orally gavaged with vehicle or GW2580 (80mg/kg/d) for 8 days. Body weights and burrowing behavior were measured throughout the experiment. The effects of GW2580 on circulating leukocyte populations, brain microglia morphology, and the transcriptome of magnetically isolated adult brain microglia were determined. Body weights, burrowing behavior, and circulating leukocytes were not affected by treatment. Analysis of Iba-1 stained brain microglia indicated that GW2580 treatment altered morphology, but not cell number. Analysis of RNA-sequencing data indicated that genes related to reactive oxygen species (ROS) regulation and survival were suppressed by treatment. Treatment of primary microglia cultures with GW2580 resulted in a dose-dependent reduction in viability only when the cells were concurrently treated with LPS, an inducer of ROS. Pre-treatment with the ROS inhibitor, YCG063, blocked treatment induced reductions in viability. Finally, GW2580 sensitized microglia to hydrogen peroxide induced cell death. Together, these data suggest that partial CSF1R antagonism may render microglia more susceptible to reactive oxygen and nitrogen species.