Microglial diversity by responses and responders

Microglia are continuously activated in the circumventricular organs of mouse brain

Microglia are the primary resident immune cells of the brain parenchyma and transform into the amoeboid form in the "activated state" under pathological conditions from the ramified form in the "resting state" under physiologically healthy conditions. In the present study, we found that microglia in the circumventricular organs (CVOs) of adult mice displayed the amoeboid form with fewer branched cellular processes even under normal conditions; however, those in other brain regions showed the ramified form, which is characterized by well-branched and dendritic cellular processes. Moreover, microglia in the CVOs showed the strong protein expression of the M1 markers CD16/32 and CD86 and M2 markers CD206 and Ym1 without any pathological stimulation. Thus, the present results indicate that microglia in the CVOs of adult mice are morphologically and functionally activated under normal conditions, possibly due to the specialized features of the CVOs, namely, the entry of blood-derived molecules into parenchyma through fenestrated capillaries and the presence of neural stem cells.

Transcriptional control of microglia phenotypes in health and disease

Microglia are the main resident macrophage population of the CNS and perform numerous functions required for CNS development, homeostasis, immunity, and repair. Many lines of evidence also indicate that dysregulation of microglia contributes to the pathogenesis of neurodegenerative and behavioral diseases. These observations provide a compelling argument to more clearly define the mechanisms that control microglia identity and function in health and disease. In this Review, we present a conceptual framework for how different classes of transcription factors interact to select and activate regulatory elements that control microglia development and their responses to internal and external signals. We then describe functions of specific transcription factors in normal and pathological contexts and conclude with a consideration of open questions to be addressed in the future.

Potassium 2-(l-hydroxypentyl)-benzoate attenuates neuroinflammatory responses and upregulates heme oxygenase-1 in systemic lipopolysaccharide-induced inflammation in mice

A neuroinflammatory response is commonly involved in the progression of many neurodegenerative diseases. Potassium 2-(1-hydroxypentyl)-benzoate (PHPB), a novel neuroprotective compound, has shown promising effects in the treatment of ischemic stroke and Alzheimer׳s disease (AD). In the present study, the anti-inflammatory effects of PHPB were investigated in the plasma and brain of C57BL/6 mice administered a single intraperitoneal (i.p.) injection of lipopolysaccharide (LPS). Levels of iNOS and the cytokines TNFα, IL-1β and IL-10 were elevated in plasma, cerebral cortex and hippocampus after LPS injection and the number of microglia and astrocytes in cortex and hippocampus were increased. LPS also upregulated the expression of heme oxygenase-1 (HO-1) in the cortex and hippocampus. PHPB reduced the levels of iNOS and cytokines in the plasma and brain, decreased the number of microglia and astrocytes and further enhanced the upregulation of HO-1. In addition, PHPB inhibited the LPS-induced phosphorylation of ERK, P38 and JNK. These results suggest that PHPB is a potential candidate in the treatment of neurodegenerative diseases through inhibiting neuroinflammation.

Interferon-β regulates the production of IL-10 by toll-like receptor-activated microglia

Pattern recognition receptors, such as toll‐like receptors (TLRs), perceive tissue alterations and initiate local innate immune responses. Microglia, the resident macrophages of the brain, encode TLRs which primary role is to protect the tissue integrity. However, deregulated activation of TLRs in microglia may lead to chronic neurodegeneration. This double role of microglial responses is often reported in immune‐driven neurologic diseases, as in multiple sclerosis (MS). Consequently, strategies to manipulate microglia inflammatory responses may help to ameliorate disease progression. In this context, the anti‐inflammatory cytokine interleukin (IL)‐10 appears as an attractive target. In this study, we investigated how activation of microglia by TLRs with distinct roles in MS impacts on IL‐10 production. We found that activation of TLR2, TLR4, and TLR9 induced the production of IL‐10 to a greater extent than activation of TLR3. This was surprising as both TLR3 and IL‐10 play protective roles in animal models of MS. Interestingly, combination of TLR3 triggering with the other TLRs, enhanced IL‐10 through the modulation of its transcription, via interferon (IFN)‐β, but independently of IL‐27. Thus, in addition to the modulation of inflammatory responses of the periphery described for the axis TLR3/IFN‐β, we now report a direct modulation of microglial responses. We further show that the presence of IFN‐γ in the microenvironment abrogated the modulation of IL‐10 by TLR3, whereas that of IL‐17 had no effect. Considering the therapeutic application of IFN‐β in MS, our study bears important implications for the understanding of the cytokine network regulating microglia responses in this setting.

Microglial Interferon Signaling and White Matter

Microglia, the resident immune cells of the CNS, are primary regulators of the neuroimmune response to injury. Type I interferons (IFNs), including the IFNαs and IFNβ, are key cytokines in the innate immune system. Their activity is implicated in the regulation of microglial function both during development and in response to neuroinflammation, ischemia, and neurodegeneration. Data from numerous studies in multiple sclerosis (MS) and stroke suggest that type I IFNs can modulate the microglial phenotype, influence the overall neuroimmune milieu, regulate phagocytosis, and affect blood-brain barrier integrity. All of these IFN-induced effects result in numerous downstream consequences on white matter pathology and microglial reactivity. Dysregulation of IFN signaling in mouse models with genetic deficiency in ubiquitin specific protease 18 (USP18) leads to a severe neurological phenotype and neuropathological changes that include white matter microgliosis and pro-inflammatory gene expression in dystrophic microglia. A class of genetic disorders in humans, referred to as pseudo-TORCH syndrome (PTS) for the clinical resemblance to infection-induced TORCH syndrome, also show dysregulation of IFN signaling, which leads to severe neurological developmental disease. In these disorders, the excessive activation of IFN signaling during CNS development results in a destructive interferonopathy with similar induction of microglial dysfunction as seen in USP18 deficient mice. Other recent studies implicate "microgliopathies" more broadly in neurological disorders including Alzheimer's disease (AD) and MS, suggesting that microglia are a potential therapeutic target for disease prevention and/or treatment, with interferon signaling playing a key role in regulating the microglial phenotype.

Alterations in CD200-CD200R1 System during EAE Already Manifest at Presymptomatic Stages

In the brain of patients with multiple sclerosis, activated microglia/macrophages appear in active lesions and in normal appearing white matter. However, whether they play a beneficial or a detrimental role in the development of the pathology remains a controversial issue. The production of pro-inflammatory molecules by chronically activated microglial cells is suggested to contribute to the progression of neurodegenerative processes in neurological disease. In the healthy brain, neurons control glial activation through several inhibitory mechanisms, such as the CD200-CD200R1 interaction. Therefore, we studied whether alterations in the CD200-CD200R1 system might underlie the neuroinflammation in an experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. We determined the time course of CD200 and CD200R1 expression in the brain and spinal cord of an EAE mouse model from presymptomatic to late symptomatic stages. We also assessed the correlation with associated glial activation, inflammatory response and EAE severity. Alterations in CD200 and CD200R1 expression were mainly observed in spinal cord regions in the EAE model, mostly a decrease in CD200 and an increase in CD200R1 expression. A decrease in the expression of the mRNA encoding a full CD200 protein was detected before the onset of clinical signs, and remained thereafter. A decrease in CD200 protein expression was observed from the onset of clinical signs. By contrast, CD200R1 expression increased at EAE onset, when a glial reaction associated with the production of pro- and anti-inflammatory markers occurred, and continued to be elevated during the pathology. Moreover, the magnitude of the alterations correlated with severity of the EAE mainly in spinal cord. These results suggest that neuronal-microglial communication through CD200-CD200R1 interaction is compromised in EAE. The early decreases in CD200 expression in EAE suggest that this downregulation might also occur in the initial phases of multiple sclerosis, and that this early neuronal dysfunction might facilitate the development of neuroinflammation. The increased CD200R1 expression in the EAE model highlights the potential use of targeted agonist molecules as therapeutic tools to control neuroinflammation. In summary, the CD200-CD200R1 system is a potential therapeutic target in multiple sclerosis, and CD200R1 agonists are molecules that may be worth developing in this context.

Non-Neuronal Cells in the Hypothalamic Adaptation to Metabolic Signals

Although the brain is composed of numerous cell types, neurons have received the vast majority of attention in the attempt to understand how this organ functions. Neurons are indeed fundamental but, in order for them to function correctly, they rely on the surrounding "non-neuronal" cells. These different cell types, which include glia, epithelial cells, pericytes, and endothelia, supply essential substances to neurons, in addition to protecting them from dangerous substances and situations. Moreover, it is now clear that non-neuronal cells can also actively participate in determining neuronal signaling outcomes. Due to the increasing problem of obesity in industrialized countries, investigation of the central control of energy balance has greatly increased in attempts to identify new therapeutic targets. This has led to interesting advances in our understanding of how appetite and systemic metabolism are modulated by non-neuronal cells. For example, not only are nutrients and hormones transported into the brain by non-neuronal cells, but these cells can also metabolize these metabolic factors, thus modifying the signals reaching the neurons. The hypothalamus is the main integrating center of incoming metabolic and hormonal signals and interprets this information in order to control appetite and systemic metabolism. Hence, the factors transported and released from surrounding non-neuronal cells will undoubtedly influence metabolic homeostasis. This review focuses on what is known to date regarding the involvement of different cell types in the transport and metabolism of nutrients and hormones in the hypothalamus. The possible involvement of non-neuronal cells, in particular glial cells, in physiopathological outcomes of poor dietary habits and excess weight gain are also discussed.

Adiponectin Regulates the Polarization and Function of Microglia via PPAR-γ Signaling Under Amyloid β Toxicity

Alzheimer’s disease (AD), characterized by the abnormal accumulation of amyloid beta (Aβ), is gradually increasing globally. Given that AD is considered a neuroinflammatory disease, recent studies have focused on the cellular mechanisms in brain inflammatory conditions that underlie AD neuropathology. Microglia are macrophage cells in the central nervous system (CNS) that are activated in response to Aβ condition. The function of microglia contributes to the neuroinflammation in AD brain, suggesting that microglia regulate the production of inflammatory mediators and contribute to the regeneration of damaged tissues. Adiponectin, an adipokine derived from adipose tissue, has been known to regulate inflammation and control macrophages during oxidative stress conditions. In present study, we investigated whether adiponectin influences the polarization and function of microglia under Aβ toxicity by examining alterations of BV2 microglia function and polarization by Acrp30 (a globular form of adiponectin) treatment using reverse transcription PCR, western blotting and immunofluorescence staining. Acrp30 promoted the induction of the M2 phenotype, and regulated the inflammatory responses through peroxisome proliferator-activated receptor (PPAR)-γ signaling under Aβ toxicity. In addition, Acrp30 boosted the capacity of Aβ scavenging in microglia. Taken together, we suggest that adiponectin may control the function of microglia by promoting anti-inflammatory responses through PPAR- γ signaling. Hence, we conclude that adiponectin may act as a critical controller of microglia function in the AD brain.

Reactive astrogliosis in stroke: Contributions of astrocytes to recovery of neurological function

Alterations in neuronal connectivity, particularly in the "peri-infarct" tissue adjacent to the region of ischemic damage, are important contributors to the spontaneous recovery of function that commonly follows stroke. Peri-infarct astrocytes undergo reactive astrogliosis and play key roles in modulating the adaptive responses in neurons. This reactive astrogliosis shares many features with that induced by other forms of damage to the central nervous system but also differs in details that potentially influence neurological recovery. A subpopulation of astrocytes within a few hundred micrometers of the infarct proliferate and are centrally involved in the development of the glial scar that separates the damaged tissue in the infarct from surrounding normal brain. The intertwined processes of astrocytes adjacent to the infarct provide the core structural component of the mature scar. Interventions that cause early disruption of glial scar formation typically impede restoration of neurological function. Marked reactive astrogliosis also develops in cells more distant from the infarct but these cells largely remain in the spatial territories they occupied prior to stroke. These cells play important roles in controlling the extracellular environment and release proteins and other molecules that are able to promote neuronal plasticity and improve functional recovery. Treatments manipulating aspects of reactive astrogliosis can enhance neuronal plasticity following stroke. Optimising these treatments for use in human stroke would benefit from a more complete characterization of the specific responses of peri-infarct astrocytes to stroke as well as a better understanding of the influence of other factors including age, sex, comorbidities and reperfusion of the ischemic tissue.

Microglia in Physiology and Disease

As the immune-competent cells of the brain, microglia play an increasingly important role in maintaining normal brain function. They invade the brain early in development, transform into a highly ramified phenotype, and constantly screen their environment. Microglia are activated by any type of pathologic event or change in brain homeostasis. This activation process is highly diverse and depends on the context and type of the stressor or pathology. Microglia can strongly influence the pathologic outcome or response to a stressor due to the release of a plethora of substances, including cytokines, chemokines, and growth factors. They are the professional phagocytes of the brain and help orchestrate the immunological response by interacting with infiltrating immune cells. We describe here the diversity of microglia phenotypes and their responses in health, aging, and disease. We also review the current literature about the impact of lifestyle on microglia responses and discuss treatment options that modulate microglial phenotypes.

Inflammatory Response in the CNS: Friend or Foe?

Inflammatory reactions could be both beneficial and detrimental to the brain, depending on strengths of their activation in various stages of neurodegeneration. Mild activation of microglia and astrocytes usually reveals neuroprotective effects and ameliorates early symptoms of neurodegeneration; for instance, released cytokines help maintain synaptic plasticity and modulate neuronal excitability, and stimulated toll-like receptors (TLRs) promote neurogenesis and neurite outgrowth. However, strong activation of glial cells gives rise to cytokine overexpression/dysregulation, which accelerates neurodegeneration. Altered mutual regulation of p53 protein, a major tumor suppressor, and NF-κB, the major regulator of inflammation, seems to be crucial for the shift from beneficial to detrimental effects of neuroinflammatory reactions in neurodegeneration. Therapeutic intervention in the p53-NF-κB axis and modulation of TLR activity are future challenges to cope with neurodegeneration.

Targeting Kv1.3 channels to reduce white matter pathology after traumatic brain injury

Axonal injury is present in essentially all clinically significant cases of traumatic brain injury (TBI). While no effective treatment has been identified to date, experimental TBI models have shown promising axonal protection using immunosuppressants FK506 and Cyclosporine-A, with treatment benefits attributed to calcineurin inhibition or protection of mitochondrial function. However, growing evidence suggests neuroprotective efficacy of these compounds may also involve direct modulation of ion channels, and in particular Kv1.3. The present study tested whether blockade of Kv1.3 channels, using Clofazimine (CFZ), would alleviate TBI-induced white matter pathology in rodents. Postinjury CFZ administration prevented suppression of compound action potential (CAP) amplitude in the corpus callosum of adult rats following midline fluid percussion TBI, with injury and treatment effects primarily expressed in unmyelinated CAPs. Kv1.3 protein levels in callosal tissue extracts were significantly reduced postinjury, but this loss was prevented by CFZ treatment. In parallel, CFZ also attenuated the injury-induced elevation in pro-inflammatory cytokine IL1-β. The effects of CFZ on glial function were further studied using mixed microglia/astrocyte cell cultures derived from P3-5 mouse corpus callosum. Cultures of callosal glia challenged with lipopolysaccharide exhibited a dramatic increase in IL1-β levels, accompanied by reactive morphological changes in microglia, both of which were attenuated by CFZ treatment. These results support a cell specific role for Kv1.3 signaling in white matter pathology after TBI, and suggest a treatment approach based on the blockade of these channels. This therapeutic strategy may be especially efficacious for normalizing neuro-glial interactions affecting unmyelinated axons after TBI.

Copper disrupts S-nitrosothiol signaling in activated BV2 microglia

Microglia, the primary resident immune cells of the central nervous system (CNS), responds rapidly to pathogens and injury by secreting immune mediators including nitric oxide (NO). The reaction of NO with the anti-oxidant glutathione forms S-nitrosoglutathione (GSNO), the major pool of biologic NO in the body. GSNO is degraded by GSNO reductase (GSNOR). Recently, we have shown that copper (Cu(I)) inhibits the release of NO in lipopolysaccharide (LPS)-stimulated BV2 microglia and induces BV2 microglia to acquire a mixed a profile with both pro- and anti-inflammatory characteristics. Since GSNOR is the critical enzyme in GSNO metabolism, we sought to determine whether Cu(I) affects GSNOR activity and S-nitrosothiol (SNO) accumulation in activated BV2 microglia. Our results show that GSNOR protein expression is reduced by Cu(I) treatment in LPS-stimulated BV2 microglia. Our results also show a decrease in S-nitrosothiol content despite a reduced GSNOR expression. This effect is most likely due to Cu(I) reacting with the central thiol of the SNO bond resulting in the degradation of SNO. A dose of 1 μM Cu(I) did not affect SNO protein accumulation in LPS-stimulated BV2 microglia, however, a dose of 100 μM Cu(I) inhibited SNO protein in accordance with inhibition of S-nitrosothiols. These data provide direct evidence that Cu(I) disrupts S-nitrosothiol homeostasis and NO metabolism, and, thus, provide new insights into the mechanisms involved in microglia-mediated-CNS disorders.

A case of mistaken identity: CD11c-eYFP(+) cells in the normal mouse brain parenchyma and neural retina display the phenotype of microglia, not dendritic cells

Under steady-state conditions the central nervous system (CNS) is traditionally thought to be devoid of antigen presenting cells; however, putative dendritic cells (DCs) expressing enhanced yellow fluorescent protein (eYFP) are present in the retina and brain parenchyma of CD11c-eYFP mice. We previously showed that these mice carry the Crb1(rd8) mutation, which causes retinal dystrophic lesions; therefore we hypothesized that the presence of CD11c-eYFP(+) cells within the CNS may be due to pathology associated with the Crb1(rd8) mutation. We generated CD11c-eYFP Crb1(wt/wt) mice and compared the distribution and immunophenotype of CD11c-eYFP(+) cells in CD11c-eYFP mice with and without the Crb1(rd8) mutation. The number and distribution of CD11c-eYFP(+) cells in the CNS was similar between CD11c-eYFP Crb1(wt/wt) and CD11c-eYFP Crb1(rd8/rd8) mice. CD11c-eYFP(+) cells were distributed throughout the inner retina, and clustered in brain regions that receive input from the external environment or lack a blood-brain barrier. CD11c-eYFP(+) cells within the retina and cerebral cortex of CD11c-eYFP Crb1(wt/wt) mice expressed CD11b, F4/80, CD115 and Iba-1, but not DC or antigen presentation markers, whereas CD11c-eYFP(+) cells within the choroid plexus and pia mater expressed CD11c, I-A/I-E, CD80, CD86, CD103, DEC205, CD8α and CD135. The immunophenotype of CD11c-eYFP(+) cells and microglia within the CNS was similar between CD11c-eYFP Crb1(wt/wt) and CD11c-eYFP Crb1(rd8/rd8) mice; however, CD11c and I-A/I-E expression was significantly increased in CD11c-eYFP Crb1(rd8/rd8) mice. This study demonstrates that the overwhelming majority of CNS CD11c-eYFP(+) cells do not display the phenotype of DCs or their precursors and are most likely a subpopulation of microglia. GLIA 2016. GLIA 2016;64:1331-1349.

PDE4B as a microglia target to reduce neuroinflammation

The importance of microglia in immune homeostasis within the brain is undisputed. Their role in a diversity of neurological and psychiatric diseases as well as CNS injury is the subject of much investigation. Cyclic adenosine monophosphate (AMP) is a critical regulator of microglia homeostasis; as the predominant negative modulator of cyclic AMP signaling within microglia, phosphodiesterase 4 (PDE4) represents a promising target for modulating immune function. PDE4 expression is regulated by inflammation, and in turn, PDE4 inhibition can alter microglia reactivity. As the prototypic PDE4 inhibitor, rolipram, was tested clinically in the 1980s, drug discovery and clinical development of PDE4 inhibitors have been severely hampered by tolerability issues involving nausea and emesis. The two PDE4 inhibitors approved for peripheral inflammatory disorders (roflumilast and apremilast) lack brain penetration and are dose-limited by side effects making them unsuitable for modulating microglial function. Subtype selective inhibitors targeting PDE4B are of high interest given the critical role PDE4B plays in immune function versus the association of PDE4D with nausea and emesis. The challenges and requirements for successful development of a novel brain-penetrant PDE4B inhibitor are discussed in the context of early clinical development strategies. Furthermore, the challenges of monitoring the state of microglia in vivo are highlighted, including a description of the currently available tools and their limitations. Continued drug discovery efforts to identify safe and well-tolerated, brain-penetrant PDE4 inhibitors are a reflection of the confidence in the rationale for modulation of this target to produce meaningful therapeutic benefit in a wide range of neurological conditions and injury. GLIA 2016;64:1698-1709.

L-3-n-Butylphthalide attenuates neuroinflammatory responses by downregulating JNK activation and upregulating Heme oxygenase-1 in lipopolysaccharide-treated mice

Microglia activation-induced neuroinflammation contributes to neuronal damage in neurodegenerative diseases. Inhibition of microglia activation and reduction of major neurotoxic cytokines have been becoming a therapeutic strategy for neurodegenerative diseases. L-3-n-Butylphthalide (L-NBP) has shown the potent neuroprotective effects in stroke and Alzheimer's disease animal models. The present study investigated the immune modulatory effects of L-NBP on pro-inflammatory cytokines and microglia activation in brain tissue induced by systemic lipopolysaccharide (LPS) treatment in C57BL/6 mice. Our results showed that systemic LPS treatment induced microglia activation in the brain. L-NBP treatment significantly suppressed the expression of proinflammatory cytokines, such as tumor necrosis factor (TNFα), interlukin-1β (IL-1β), interlukin-6 (IL-6), and interlukin-10 (IL-10) in LPS-treated mice. At the meantime, L-NBP treatment decreased the morphological activation of microglia. In addition, the phosphorylation level of JNK MAP kinase-signaling pathway was also inhibited by L-NBP in LPS-treated mice. Furthermore, L-NBP upregulated the expression of heme oxygenase (HO)-1, a key element in the anti-inflammation and anti-oxidative stress. These results suggested that L-NBP might be a promising candidate in delaying and reversing the progress of neurodegenerative diseases by inhibiting microglia activation.

The mTOR Inhibitor Rapamycin Mitigates Perforant Pathway Neurodegeneration and Synapse Loss in a Mouse Model of Early-Stage Alzheimer-Type Tauopathy

The perforant pathway projection from layer II of the entorhinal cortex to the hippocampal dentate gyrus is especially important for long-term memory formation, and is preferentially vulnerable to developing a degenerative tauopathy early in Alzheimer's disease (AD) that may spread over time trans-synaptically. Despite the importance of the perforant pathway to the clinical onset and progression of AD, a therapeutic has not been identified yet that protects it from tau-mediated toxicity. Here, we used an adeno-associated viral vector-based mouse model of early-stage AD-type tauopathy to investigate effects of the mTOR inhibitor and autophagy stimulator rapamycin on the tau-driven loss of perforant pathway neurons and synapses. Focal expression of human tau carrying a P301L mutation but not eGFP as a control in layer II of the lateral entorhinal cortex triggered rapid degeneration of these neurons, loss of lateral perforant pathway synapses in the dentate gyrus outer molecular layer, and activation of neuroinflammatory microglia and astroglia in the two locations. Chronic systemic rapamycin treatment partially inhibited phosphorylation of a mechanistic target of rapamycin substrate in brain and stimulated LC3 cleavage, a marker of autophagic flux. Compared with vehicle-treated controls, rapamycin protected against the tau-induced neuronal loss, synaptotoxicity, reactive microgliosis and astrogliosis, and activation of innate neuroimmunity. It did not alter human tau mRNA or total protein levels. Finally, rapamycin inhibited trans-synaptic transfer of human tau expression to the dentate granule neuron targets for the perforant pathway, likely by preventing the synaptic spread of the AAV vector in response to pathway degeneration. These results identify systemic rapamycin as a treatment that protects the entorhinal cortex and perforant pathway projection from tau-mediated neurodegeneration, axonal and synapse loss, and neuroinflammatory reactive gliosis. The findings support the potential for slowing the progression of AD by abrogating tau-mediated neurotoxicity at its earliest neuropathological stages.

Pathologic and Protective Roles for Microglial Subsets and Bone Marrow- and Blood-Derived Myeloid Cells in Central Nervous System Inflammation

Inflammation is a series of processes designed for eventual clearance of pathogens and repair of damaged tissue. In the context of autoimmune recognition, inflammatory processes are usually considered to be pathological. This is also true for inflammatory responses in the central nervous system (CNS). However, as in other tissues, neuroinflammation can have beneficial as well as pathological outcomes. The complex role of encephalitogenic T cells in multiple sclerosis and its animal model experimental autoimmune encephalomyelitis (EAE) may derive from heterogeneity of the myeloid cells with which these T cells interact within the CNS. Myeloid cells, including resident microglia and infiltrating bone marrow-derived cells, such as dendritic cells (DC) and monocytes/macrophages [bone marrow-derived macrophages (BMDM)], are highly heterogeneous populations that may be involved in neurotoxicity and also immunoregulation and regenerative processes. Better understanding and characterization of myeloid cell heterogeneity is essential for future development of treatments controlling inflammation and inducing neuroprotection and neuroregeneration in diseased CNS. Here, we describe and compare three populations of myeloid cells: CD11c⁺ microglia, CD11c⁻ microglia, and CD11c⁺ blood-derived cells in terms of their pathological versus protective functions in the CNS of mice with EAE. Our data show that CNS-resident microglia include functionally distinct subsets that can be distinguished by their expression of CD11c. These subsets differ in their expression of Arg-1, YM1, iNOS, IL-10, and IGF-1. Moreover, in contrast to BMDM/DC, both subsets of microglia express protective interferon-beta (IFNβ), high levels of colony-stimulating factor-1 receptor, and do not express the Th1-associated transcription factor T-bet. Taken together, our data suggest that CD11c⁺ microglia, CD11c⁻ microglia, and infiltrating BMDM/DC represent separate and distinct populations and illustrate the heterogeneity of the CNS inflammatory environment.

Benfotiamine upregulates antioxidative system in activated BV-2 microglia cells

Chronic microglial activation and resulting sustained neuroinflammatory reaction are generally associated with neurodegeneration. Activated microglia acquires proinflammatory cellular profile that generates oxidative burst. Their persistent activation exacerbates inflammation, which damages healthy neurons via cytotoxic mediators, such as superoxide radical anion and nitric oxide. In our recent study, we have shown that benfotiamine (S-benzoylthiamine O-monophosphate) possesses anti-inflammatory effects. Here, the effects of benfotiamine on the pro-oxidative component of activity of LPS-stimulated BV-2 cells were investigated. The activation of microglia was accompanied by upregulation of intracellular antioxidative defense, which was further promoted in the presence of benfotiamine. Namely, activated microglia exposed to non-cytotoxic doses of benfotiamine showed increased levels and activities of hydrogen peroxide- and superoxide-removing enzymes-catalase and glutathione system, and superoxide dismutase. In addition, benfotiamine showed the capacity to directly scavenge superoxide radical anion. As a consequence, benfotiamine suppressed the activation of microglia and provoked a decrease in NO and (·)O(-) 2 production and lipid peroxidation. In conclusion, benfotiamine might silence pro-oxidative activity of microglia to alleviate/prevent oxidative damage of neighboring CNS cells.

Low-dose ribavirin treatments attenuate neuroinflammatory activation of BV-2 Cells by interfering with inducible nitric oxide synthase

Microglia play a key role in defending central nervous system from various internal and external threats. However, their excessive and/or chronic activation is associated with deleterious effects in a variety of neurodegenerative diseases. Previously, we have shown that ribavirin when applied in clinically relevant dosage (10 μM) modulates activated microglia in complex fashion inducing both anti- and proinflammatory effects, simultaneously causing cytotoxicity. Here, we examined potential of low-dose ribavirin (0.1 and 1 μM) to modulate activated BV-2 microglia. Morphological and functional activation of BV-2 cells was achieved with lipopolysaccharide (LPS) stimulation. Our results demonstrated that low-dose ribavirin did not induce cell death, while 10 μM ribavirin promoted LPS induced apoptosis. We determined that 1 μM ribavirin was equally efficient in deactivation of LPS induced morphological changes as 10 μM ribavirin treatment. Ribavirin showed halfway success in reducing markers of functional activation of microglia. Namely, none of the doses had effect on LPS triggered production of proinflammatory cytokine tumor necrosis factor alpha. On the other hand, low-dose ribavirin proved its effectiveness in reduction of another inflammatory mediator, nitric oxide, by inhibiting inducible form of nitric oxide synthase. Our results imply that low-dose ribavirin may alleviate nitrosative stress during neuroinflammation.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

Do not judge a cell by its cover--diversity of CNS resident, adjoining and infiltrating myeloid cells in inflammation

Specialized populations of tissue-resident myeloid cells inhabit every organ of the body. While many of these populations appear similar morphologically and phenotypically, they exhibit great functional diversity. The central nervous system (CNS), as an immune privileged organ, possesses a unique tissue-resident macrophage population, the microglia, as well as numerous myeloid cell subsets at its boarders and barriers in CNS-adjoining tissues, namely the meninges, the perivascular space, and the choroid plexus. Recent research has added much to our knowledge about microglia, whereas the populations of CNS-surrounding phagocytes are just starting to be appreciated. As guardians of CNS homeostasis, these myeloid cells perform immune surveillance and immune modulatory tasks in health and disease. As such, microglia and CNS-surrounding antigen-presenting cells have been shown to be crucially involved not only in the initiation and progression but also resolution of multiple sclerosis (MS). MS and its rodent model, experimental autoimmune encephalomyelitis, are autoimmune inflammatory demyelinating CNS pathologies. While some crucial aspects of the disease pathogenesis have been solved, much of the complex involvement and interplay of the innate immune compartment remains yet to be clarified. Here, we will discuss the current understanding of the scope of phenotypes and functions of myeloid cells involved in CNS neuroinflammation.

(+)-Naltrexone is neuroprotective and promotes alternative activation in the mouse hippocampus after cardiac arrest/cardiopulmonary resuscitation

Despite dramatic improvement in cardiopulmonary resuscitation (CPR) and other techniques for cardiac arrest (CA), the majority of survivors continue to show signs of decreased memory or executive cognitive function. Such memory impairment may be due to hippocampal CA1 neuronal death, which is delayed by several days after CA/CPR. Classical microgliosis in the CA1 region may contribute to neuronal death, yet the role of a key activation receptor Toll Like Receptor 4 (TLR4) has not been previously investigated for such neuronal death after CA/CPR. We show that (+)-naltrexone was neuroprotective after CA/CPR. TLR4 blockade was associated with decreased expression of markers for microglial/macrophage activation and T cell and B cell infiltration, as well as decreased pro-inflammatory cytokine levels. Notably, IL-10 expression was elevated in response to CA/CPR, but was not attenuated by (+)-naltrexone, suggesting that the local monocyte/microglial phenotype had shifted towards alternative activation. This was confirmed by elevated expression of Arginase-1, and decreased expression of NFκB p65 subunit. Thus, (+)-naltrexone and other TLR4 antagonists may represent a novel therapeutic strategy to alleviate the substantial burden of memory or executive cognitive function impairment after CA/CPR.

Glia-neuron interactions in the mammalian retina

The mammalian retina provides an excellent opportunity to study glia-neuron interactions and the interactions of glia with blood vessels. Three main types of glial cells are found in the mammalian retina that serve to maintain retinal homeostasis: astrocytes, Müller cells and resident microglia. Müller cells, astrocytes and microglia not only provide structural support but they are also involved in metabolism, the phagocytosis of neuronal debris, the release of certain transmitters and trophic factors and K(+) uptake. Astrocytes are mostly located in the nerve fibre layer and they accompany the blood vessels in the inner nuclear layer. Indeed, like Müller cells, astrocytic processes cover the blood vessels forming the retinal blood barrier and they fulfil a significant role in ion homeostasis. Among other activities, microglia can be stimulated to fulfil a macrophage function, as well as to interact with other glial cells and neurons by secreting growth factors. This review summarizes the main functional relationships between retinal glial cells and neurons, presenting a general picture of the retina recently modified based on experimental observations. The preferential involvement of the distinct glia cells in terms of the activity in the retina is discussed, for example, while Müller cells may serve as progenitors of retinal neurons, astrocytes and microglia are responsible for synaptic pruning. Since different types of glia participate together in certain activities in the retina, it is imperative to explore the order of redundancy and to explore the heterogeneity among these cells. Recent studies revealed the association of glia cell heterogeneity with specific functions. Finally, the neuroprotective effects of glia on photoreceptors and ganglion cells under normal and adverse conditions will also be explored.

Robust increase of microglia proliferation in the fornix of hippocampal axonal pathway after a single LPS stimulation

Microglia are resident immunocompetent cells having important roles in innate immunity in the brains. In the present study, we found that a single lipopolysaccharide (LPS) administration significantly increased microglial proliferation in the fornix and dentate gyrus (DG) but not the cerebral cortex and corpus callosum of adult mice. LPS-induced microglial proliferation was especially robust at the white matter of the fornix. The density of microglia increased in the fornix and DG for roughly one week and returned to basal levels at least 20days after a single LPS administration. Consecutive LPS administration did not induce such dramatic increase of microglial proliferation in the fornix. The inhibition of vascular endothelial growth factor signaling by AZD2171 largely suppressed LPS-induced increase of microglial proliferation in the fornix. In conclusion, the present study indicates that the hippocampal neuronal system has a higher proliferative microglial capability against LPS-induced inflammatory administration compared with other brain regions.

The ischemic environment drives microglia and macrophage function

Cells of myeloid origin, such as microglia and macrophages, act at the crossroads of several inflammatory mechanisms during pathophysiology. Besides pro-inflammatory activity (M1 polarization), myeloid cells acquire protective functions (M2) and participate in the neuroprotective innate mechanisms after brain injury. Experimental research is making considerable efforts to understand the rules that regulate the balance between toxic and protective brain innate immunity. Environmental changes affect microglia/macrophage functions. Hypoxia can affect myeloid cell distribution, activity, and phenotype. With their intrinsic differences, microglia and macrophages respond differently to hypoxia, the former depending on ATP to activate and the latter switching to anaerobic metabolism and adapting to hypoxia. Myeloid cell functions include homeostasis control, damage-sensing activity, chemotaxis, and phagocytosis, all distinctive features of these cells. Specific markers and morphologies enable to recognize each functional state. To ensure homeostasis and activate when needed, microglia/macrophage physiology is finely tuned. Microglia are controlled by several neuron-derived components, including contact-dependent inhibitory signals and soluble molecules. Changes in this control can cause chronic activation or priming with specific functional consequences. Strategies, such as stem cell treatment, may enhance microglia protective polarization. This review presents data from the literature that has greatly advanced our understanding of myeloid cell action in brain injury. We discuss the selective responses of microglia and macrophages to hypoxia after stroke and review relevant markers with the aim of defining the different subpopulations of myeloid cells that are recruited to the injured site. We also cover the functional consequences of chronically active microglia and review pivotal works on microglia regulation that offer new therapeutic possibilities for acute brain injury.

Vascular and inflammatory factors in the pathophysiology of blast-induced brain injury

Blast-related traumatic brain injury (TBI) has received much recent attention because of its frequency in the conflicts in Iraq and Afghanistan. This renewed interest has led to a rapid expansion of clinical and animal studies related to blast. In humans, high-level blast exposure is associated with a prominent hemorrhagic component. In animal models, blast exerts a variety of effects on the nervous system including vascular and inflammatory effects that can be seen with even low-level blast exposures which produce minimal or no neuronal pathology. Acutely, blast exposure in animals causes prominent vasospasm and decreased cerebral blood flow along with blood-brain barrier breakdown and increased vascular permeability. Besides direct effects on the central nervous system, evidence supports a role for a thoracically mediated effect of blast; whereby, pressure waves transmitted through the systemic circulation damage the brain. Chronically, a vascular pathology has been observed that is associated with alterations of the vascular extracellular matrix. Sustained microglial and astroglial reactions occur after blast exposure. Markers of a central and peripheral inflammatory response are found for sustained periods after blast injury and include elevation of inflammatory cytokines and other inflammatory mediators. At low levels of blast exposure, a microvascular pathology has been observed in the presence of an otherwise normal brain parenchyma, suggesting that the vasculature may be selectively vulnerable to blast injury. Chronic immune activation in brain following vascular injury may lead to neurobehavioral changes in the absence of direct neuronal pathology. Strategies aimed at preventing or reversing vascular damage or modulating the immune response may improve the chronic neuropsychiatric symptoms associated with blast-related TBI.

Benfotiamine attenuates inflammatory response in LPS stimulated BV-2 microglia

Microglial cells are resident immune cells of the central nervous system (CNS), recognized as key elements in the regulation of neural homeostasis and the response to injury and repair. As excessive activation of microglia may lead to neurodegeneration, therapeutic strategies targeting its inhibition were shown to improve treatment of most neurodegenerative diseases. Benfotiamine is a synthetic vitamin B1 (thiamine) derivate exerting potentially anti-inflammatory effects. Despite the encouraging results regarding benfotiamine potential to alleviate diabetic microangiopathy, neuropathy and other oxidative stress-induced pathological conditions, its activities and cellular mechanisms during microglial activation have yet to be elucidated. In the present study, the anti-inflammatory effects of benfotiamine were investigated in lipopolysaccharide (LPS)-stimulated murine BV-2 microglia. We determined that benfotiamine remodels activated microglia to acquire the shape that is characteristic of non-stimulated BV-2 cells. In addition, benfotiamine significantly decreased production of pro-inflammatory mediators such as inducible form of nitric oxide synthase (iNOS) and NO; cyclooxygenase-2 (COX-2), heat-shock protein 70 (Hsp70), tumor necrosis factor alpha α (TNF-α), interleukin-6 (IL-6), whereas it increased anti-inflammatory interleukin-10 (IL-10) production in LPS stimulated BV-2 microglia. Moreover, benfotiamine suppressed the phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2), c-Jun N-terminal kinases (JNK) and protein kinase B Akt/PKB. Treatment with specific inhibitors revealed that benfotiamine-mediated suppression of NO production was via JNK1/2 and Akt pathway, while the cytokine suppression includes ERK1/2, JNK1/2 and Akt pathways. Finally, the potentially protective effect is mediated by the suppression of translocation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) in the nucleus. Therefore, benfotiamine may have therapeutic potential for neurodegenerative diseases by inhibiting inflammatory mediators and enhancing anti-inflammatory factor production in activated microglia.

Axon initial segment-associated microglia

Microglia are the brain's resident immune cells and function as the main defense against pathogens or injury. However, in the absence of disease, microglia have other functions in the normal brain. For example, previous studies showed that microglia contribute to circuit refinement and synaptic plasticity in the developing and adult brain, respectively. Thus, microglia actively participate in regulating neuronal excitability and function. Here, we report that in the cortex, but not other brain regions, a subset of microglia extend a single process that specifically associates and overlaps with the axon initial segment (AIS), the site where action potentials are generated. Similar associations were not observed with dendrites or distal axons. Microglia-AIS interactions appear early in development, persist throughout adulthood, and are conserved across species including mice, rats, and primates. However, these interactions are lost after microglial activation following brain injury, suggesting that such interactions may be part of healthy brain function. Loss of microglial CX3CR1 receptors, or the specialized extracellular matrix surrounding the AIS, did not disrupt the interaction. However, loss of AIS proteins by the neuron-specific deletion of the master AIS scaffold AnkyrinG disrupted microglia-AIS interactions. These results reveal a unique population of microglia that specifically interact with the AIS in the adult cortex.

Innate immune response is differentially dysregulated between bipolar disease and schizophrenia

Schizophrenia (SZ) and bipolar disorder (BD) are severe psychiatric conditions with a neurodevelopmental component. Genetic findings indicate the existence of an overlap in genetic susceptibility across the disorders. Also, image studies provide evidence for a shared neurobiological basis, contributing to a dimensional diagnostic approach. This study aimed to identify the molecular mechanisms that differentiate SZ and BD patients from health controls but also that distinguish both from health individuals. Comparison of gene expression profiling in post-mortem brains of both disorders and health controls (30 cases), followed by a further comparison between 29 BD and 29 SZ revealed 28 differentially expressed genes. These genes were used in co-expression analysesthat revealed the pairs CCR1/SERPINA1, CCR5/HCST, C1QA/CD68, CCR5/S100A11 and SERPINA1/TLR1 as presenting the most significant difference in co-expression between SZ and BD. Next, a protein-protein interaction (PPI) network using the 28 differentially expressed genes as seeds revealed CASP4, TYROBP, CCR1, SERPINA1, CCR5 and C1QA as having a central role in the diseases manifestation. Both co-expression and network topological analyses pointed to genes related to microglia functions. Based on this data, we suggest that differences between SZ and BP are due to genes involved with response to stimulus, defense response, immune system process and response to stress biological processes, all having a role in the communication of environmental factors to the cells and associated to microglia.

Extracellular acidosis impairs P2Y receptor-mediated Ca(2+) signalling and migration of microglia

Microglia are the resident macrophage and immune cell of the brain and are critically involved in combating disease and assaults on the brain. Virtually all brain pathologies are accompanied by acidosis of the interstitial fluid, meaning that microglia are exposed to an acidic environment. However, little is known about how extracellular acidosis impacts on microglial function. The activity of microglia is tightly controlled by 'on' and 'off' signals, the presence or absence of which results in generation of distinct phenotypes in microglia. Activation of G protein coupled purinergic (P2Y) receptors triggers a number of distinct behaviours in microglia, including activation, migration, and phagocytosis. Using pharmacological tools and fluorescence imaging of the murine cerebellar microglia cell line C8B4, we show that extracellular acidosis interferes with P2Y receptor-mediated Ca(2+) signalling in these cells. Distinct P2Y receptors give rise to signature intracellular Ca(2+) signals, and Ca(2+) release from stores and Ca(2+) influx are differentially affected by acidotic conditions: Ca(2+) release is virtually unaffected, whereas Ca(2+) influx, mediated at least in part by store-operated Ca(2+) channels, is profoundly inhibited. Furthermore, P2Y1 and P2Y6-mediated stimulation of migration is inhibited under conditions of extracellular acidosis, whereas basal migration independent of P2Y receptor activation is not. Taken together, our results demonstrate that an acidic microenvironment impacts on P2Y receptor-mediated Ca(2+) signalling, thereby influencing microglial responses and responsiveness to extracellular signals. This may result in altered behaviour of microglia under pathological conditions compared with microglial responses in healthy tissue.

The evolving biology of microglia in Alzheimer's disease

Alzheimer's disease (AD) is typified by a robust microglial-mediated inflammatory response within the brain. Indeed, microglial accumulation around plaques in AD is one of the classical hallmarks of the disease pathology. Although microglia have the capacity to remove β-amyloid deposits and alleviate disease pathology, they fail to do so. Instead, they become chronically activated and promote inflammation-mediated impairment of cognition and cytotoxicity. However, if microglial function could be altered to engage their phagocytic response, promote their tissue maintenance functions, and prevent release of factors that promote tissue damage, this could provide therapeutic benefit. This review is focused on the current knowledge of microglial homeostatic mechanisms in AD, and mechanisms involved in the regulation of microglial phenotype in this context.

Role of non-neuronal cells in body weight and appetite control

The brain is composed of neurons and non-neuronal cells, with the latter encompassing glial, ependymal and endothelial cells, as well as pericytes and progenitor cells. Studies aimed at understanding how the brain operates have traditionally focused on neurons, but the importance of non-neuronal cells has become increasingly evident. Once relegated to supporting roles, it is now indubitable that these diverse cell types are fundamental for brain development and function, including that of metabolic circuits, and they may play a significant role in obesity onset and complications. They participate in processes of neurogenesis, synaptogenesis, and synaptic plasticity of metabolic circuits both during development and in adulthood. Some glial cells, such as tanycytes and astrocytes, transport circulating nutrients and metabolic factors that are fundamental for neuronal viability and activity into and within the hypothalamus. All of these cell types express receptors for a variety of metabolic factors and hormones, suggesting that they participate in metabolic function. They are the first line of defense against any assault to neurons. Indeed, microglia and astrocytes participate in the hypothalamic inflammatory response to high fat diet (HFD)-induced obesity, with this process contributing to inflammatory-related insulin and leptin resistance. Moreover, HFD-induced obesity and hyperleptinemia modify hypothalamic astroglial morphology, which is associated with changes in the synaptic inputs to neuronal metabolic circuits. Astrocytic contact with the microvasculature is increased by HFD intake and this could modify nutrient/hormonal uptake into the brain. In addition, progenitor cells in the hypothalamus are now known to have the capacity to renew metabolic circuits, and this can be affected by HFD intake and obesity. Here, we discuss our current understanding of how non-neuronal cells participate in physiological and physiopathological metabolic control.

Atypical "seizure-like" activity in cortical reverberating networks in vitro can be caused by LPS-induced inflammation: a multi-electrode array study from a hundred neurons

We show here that a mild sterile inflammation induced by the endotoxin lipopolysaccharide (LPS), in a neuron/astrocyte/microglial cortical network, modulates neuronal excitability and can initiate long-duration burst events resembling epileptiform seizures, a recognized feature of various central nervous neurodegenerative, neurological and acute systemic diseases associated with neuroinflammation. To study this action, we simultaneously analyzed the reverberating bursting activity of a hundred neurons by using in vitro multi-electrode array methods. ∼5 h after LPS application, we observed a net increase in the average number of spikes elicited in engaged cells and within each burst, but no changes neither in spike waveforms nor in burst rate. This effect was characterized by a slow, twofold exponential increase of the burst duration and the appearance of rarely occurring long burst events that were never seen during control recordings. These changes and the time-course of microglia-released proinflammatory cytokine, tumor necrosis factor-alpha (TNF-α), were blocked by pre-treatment with 50 nM minocycline, an established anti-inflammatory agent which was inactive when applied alone. Assay experiments also revealed that application of 60 pM exogenous TNF-α after 12–15 h, produced non-washable changes of neuronal excitability, completely different from those induced by LPS, suggesting that TNF-α release alone was not responsible for our observed findings. Our results indicate that the link between neuroinflammation and hyperexcitability can be unveiled by studying the long-term activity of in vitro neuronal/astrocyte/microglial networks.

Peripherally triggered and GSK-3β-driven brain inflammation differentially skew adult hippocampal neurogenesis, behavioral pattern separation and microglial activation in response to ibuprofen

Both familial and sporadic forms of Alzheimer disease (AD) present memory impairments. It has been proposed that these impairments are related to inflammation in relevant brain areas such as the hippocampus. Whether peripherally triggered and neuron-driven brain inflammation produce similar and equally reversible alterations is a matter of discussion. Here we studied the effects of ibuprofen administration on a familial AD mouse model overexpressing GSK-3β that presents severe brain inflammation. We compared these effects with those observed in a peripherally triggered brain inflammation model based on chronic lipopolysaccharide (LPS) administration. Both proinflammatory stimuli produced equivalent reversible morphological alterations in granule neurons; however, GSK-3β had a much more prominent role in newborn neuron connectivity, causing alterations that were not reversed by ibuprofen. Although both insults triggered similar behavioral impairments, ibuprofen rescued this defect in LPS-treated mice but did not produce any improvement in GSK-3β-overexpressing animals. This observation could be attributable to the different microglial phenotype induced by ibuprofen treatment. These data may be clinically relevant for AD therapies, as GSK-3β appears to determine the efficacy of ibuprofen treatment.

Microglial pathology

This paper summarizes pathological changes that affect microglial cells in the human brain during aging and in aging-related neurodegenerative diseases, primarily Alzheimer’s disease (AD). It also provides examples of microglial changes that have been observed in laboratory animals during aging and in some experimentally induced lesions and disease models. Dissimilarities and similarities between humans and rodents are discussed in an attempt to generate a current understanding of microglial pathology and its significance during aging and in the pathogenesis of Alzheimer dementia (AD). The identification of dystrophic (senescent) microglia has created an ostensible conflict with prior work claiming a role for activated microglia and neuroinflammation during normal aging and in AD, and this has raised a basic question: does the brain’s immune system become hyperactive (inflamed) or does it become weakened (senescent) in elderly and demented people, and what is the impact on neuronal function and cognition? Here we strive to reconcile these seemingly contradictory notions by arguing that both low-grade neuroinflammation and microglial senescence are the result of aging-associated free radical injury. Both processes are damaging for microglia as they synergistically exhaust this essential cell population to the point where the brain’s immune system is effete and unable to support neuronal function.

RNA sequencing of microglia and monocyte-derived macrophages from mice with experimental autoimmune encephalomyelitis illustrates a changing phenotype with disease course

The role of microglia and monocyte-derived macrophages in experimental autoimmune encephalomyelitis pathogenesis has been controversial. To gain insight into their respective roles, we developed a method for differentiating between microglia and monocyte-derived macrophages in the CNS by flow cytometry utilizing anti-CD44 antibodies. We used this system to monitor changes in cell number, activation status, and gene expression by RNA sequencing over the course of disease. This in vivo characterization and RNA-Seq dataset improves our understanding of macrophage biology in the brain under inflammatory conditions and may lead to strategies to identify therapies for neuroinflammatory diseases.

Microglia change from a reactive to an age-like phenotype with the time in culture

Age-related neurodegenerative diseases have been associated with chronic neuroinflammation and microglia activation. However, cumulative evidence supports that inflammation only occurs at an early stage once microglia change the endogenous characteristics with aging and switch to irresponsive/senescent and dystrophic phenotypes with disease progression. Thus, it will be important to have the means to assess the role of reactive and aged microglia when studying advanced brain neurodegeneration processes and age-associated related disorders. Yet, most studies are done with microglia from neonates since there are no adequate means to isolate degenerating microglia for experimentation. Indeed, only a few studies report microglia isolation from aged animals, using either short-term cultures or high concentrations of mitogens in the medium, which trigger microglia reactivity. The purpose of this study was to develop an experimental process to naturally age microglia after isolation from neonatal mice and to characterize the cultured cells at 2 days in vitro (DIV), 10 DIV, and 16 DIV. We found that 2 DIV (young) microglia had predominant amoeboid morphology and markers of stressed/reactive phenotype. In contrast, 16 DIV (aged) microglia evidenced ramified morphology and increased matrix metalloproteinase (MMP)-2 activation, as well as reduced MMP-9, glutamate release and nuclear factor kappa-B activation, in parallel with decreased expression of Toll-like receptor (TLR)-2 and TLR-4, capacity to migrate and phagocytose. These findings together with the reduced expression of microRNA (miR)-124, and miR-155, decreased autophagy, enhanced senescence associated beta-galactosidase activity and elevated miR-146a expression, are suggestive that 16 DIV cells mainly correspond to irresponsive/senescent microglia. Data indicate that the model represent an opportunity to understand and control microglial aging, as well as to explore strategies to recover microglia surveillance function.