Activation status of human microglia is dependent on lesion formation stage and remyelination in multiple sclerosis

Neuroadhesive L1 coating attenuates acute microglial attachment to neural electrodes as revealed by live two-photon microscopy

Implantable neural electrode technologies for chronic neural recordings can restore functional control to paralysis and limb loss victims through brain-machine interfaces. These probes, however, have high failure rates partly due to the biological responses to the probe which generate an inflammatory scar and subsequent neuronal cell death. L1 is a neuronal specific cell adhesion molecule and has been shown to minimize glial scar formation and promote electrode-neuron integration when covalently attached to the surface of neural probes. In this work, the acute microglial response to L1-coated neural probes was evaluated in vivo by implanting coated devices into the cortex of mice with fluorescently labeled microglia, and tracking microglial dynamics with multi-photon microscopy for the ensuing 6 h in order to understand L1's cellular mechanisms of action. Microglia became activated immediately after implantation, extending processes towards both L1-coated and uncoated control probes at similar velocities. After the processes made contact with the probes, microglial processes expanded to cover 47.7% of the control probes' surfaces. For L1-coated probes, however, there was a statistically significant 83% reduction in microglial surface coverage. This effect was sustained through the experiment. At 6 h post-implant, the radius of microglia activation was reduced for the L1 probes by 20%, shifting from 130.0 to 103.5 μm with the coating. Microglia as far as 270 μm from the implant site displayed significantly lower morphological characteristics of activation for the L1 group. These results suggest that the L1 surface treatment works in an acute setting by microglial mediated mechanisms.

Schizophrenia associated sensory gating deficits develop after adolescent microglia activation

Maternal infection during pregnancy is a well-established risk factor for schizophrenia in the adult offspring. Consistently, prenatal Poly(I:C) treatment in mice has been validated to model behavioral and neurodevelopmental abnormalities associated with schizophrenia. By using the Poly(I:C) BALB/c mouse model, we investigated the functional profile of microglia by flow cytometry in relation to progressive behavioral changes from adolescence to adulthood. Prenatal Poly(I:C) treatment induced the expected sensory gating deficits (pre-pulse inhibition (PPI) of the acoustic startle response) in 100day-old adult offspring, but only in female not in male descendants. No PPI-deficits were present in 30day-old adolescent mice. Sensory gating deficits in adult females were preceded by a strong M1-type microglia polarization pattern during puberty as determined by flow cytometric analysis of multiple pro- and anti-inflammatory surface markers. Microglia activation in females did not persist until adulthood and was absent in behaviorally unaffected male descendants. Further, the specific activation pattern of microglia was not mirrored by a similar activation of peripheral immune cells. We conclude that prenatal Poly(I:C) treatment induces post pubertal deficits in sensory gating which are specifically preceded by a pro-inflammatory activation pattern of microglia during puberty.

Inflammation after Ischemic Stroke: The Role of Leukocytes and Glial Cells

The immune response after stroke is known to play a major role in ischemic brain pathobiology. The inflammatory signals released by immune mediators activated by brain injury sets off a complex series of biochemical and molecular events which have been increasingly recognized as a key contributor to neuronal cell death. The primary immune mediators involved are glial cells and infiltrating leukocytes, including neutrophils, monocytes and lymphocyte. After ischemic stroke, activation of glial cells and subsequent release of pro- and anti-inflammatory signals are important for modulating both neuronal cell damage and wound healing. Infiltrated leukocytes release inflammatory mediators into the site of the lesion, thereby exacerbating brain injury. This review describes how the roles of glial cells and circulating leukocytes are a double-edged sword for neuroinflammation by focusing on their detrimental and protective effects in ischemic stroke. Here, we will focus on underlying characterize of glial cells and leukocytes under inflammation after ischemic stroke.

Complement C3 on microglial clusters in multiple sclerosis occur in chronic but not acute disease: Implication for disease pathogenesis

Microglial clusters with C3d deposits are observed in the periplaque of multiple sclerosis (MS) brains and were proposed as early stage of lesion formation. As such they should appear in the brain of MS donors with acute disease but thus far this has not been shown. Using postmortem brain tissue from acute (n = 10) and chronic (n = 15) MS cases we investigated whether C3d+ microglial clusters are part of an acute attack against myelinated axons, which could have implications for disease pathogenesis. The specificity of our findings to MS was tested in ischemic stroke cases (n = 8) with initial or advanced lesions and further analyzed in experimental traumatic brain injury (TBI, n = 26), as both conditions are primarily nondemyelinating but share essential features of neurodegeneration with MS lesions. C3d+ microglial clusters were found in chronic but not acute MS. They were not associated with antibody deposits or terminal complement activation. They were linked to slowly expanding lesions, localized on axons with impaired transport and associated with neuronal C3 production. C3d+ microglial clusters were not specific to MS as they were also found in stroke and experimental TBI. We conclude that C3d+ microglial clusters in MS are not part of an acute attack against myelinated axons. As such it is unlikely that they drive formation of new lesions but could represent a physiological mechanism to remove irreversibly damaged axons in chronic disease. GLIA 2017;65:264–277

Multiple Sclerosis and Obesity: Possible Roles of Adipokines

Multiple Sclerosis (MS) is an autoimmune disorder of the Central Nervous System that has been associated with several environmental factors, such as diet and obesity. The possible link between MS and obesity has become more interesting in recent years since the discovery of the remarkable properties of adipose tissue. Once MS is initiated, obesity can contribute to increased disease severity by negatively influencing disease progress and treatment response, but, also, obesity in early life is highly relevant as a susceptibility factor and causally related risk for late MS development. The aim of this review was to discuss recent evidence about the link between obesity, as a chronic inflammatory state, and the pathogenesis of MS as a chronic autoimmune and inflammatory disease. First, we describe the main cells involved in MS pathogenesis, both from neural tissue and from the immune system, and including a new participant, the adipocyte, focusing on their roles in MS. Second, we concentrate on the role of several adipokines that are able to participate in the mediation of the immune response in MS and on the possible cross talk between the latter. Finally, we explore recent therapy that involves the transplantation of adipocyte precursor cells for the treatment of MS.

Myeloid cells - targets of medication in multiple sclerosis

Discussions of multiple sclerosis (MS) pathophysiology tend to focus on T cells and B cells of the adaptive immune response. The innate immune system is less commonly considered in this context, although dendritic cells, monocytes, macrophages and microglia - collectively referred to as myeloid cells - have prominent roles in MS pathogenesis. These populations of myeloid cells function as antigen-presenting cells and effector cells in neuroinflammation. Furthermore, a vicious cycle of interactions between T cells and myeloid cells exacerbates pathology. Several disease-modifying therapies are now available to treat MS, and insights into their mechanisms of action have largely focused on the adaptive immune system, but these therapies also have important effects on myeloid cells. In this Review, we discuss the evidence for the roles of myeloid cells in MS and the experimental autoimmune encephalomyelitis model of MS, and consider how interactions between myeloid cells and T cells and/or B cells promote MS pathology. Finally, we discuss the direct and indirect effects of existing MS medications on myeloid cells.

Ageing and recurrent episodes of neuroinflammation promote progressive experimental autoimmune encephalomyelitis in Biozzi ABH mice

Current therapies for multiple sclerosis (MS) reduce the frequency of relapses by modulating adaptive immune responses but fail to limit the irreversible neurodegeneration driving progressive disability. Experimental autoimmune encephalomyelitis (EAE) in Biozzi ABH mice recapitulates clinical features of MS including relapsing-remitting episodes and secondary-progressive disability. To address the contribution of recurrent inflammatory events and ageing as factors that amplify progressive neurological disease, we examined EAE in 8- to 12-week-old and 12-month-old ABH mice. Compared with the relapsing-remitting (RREAE) and secondary progressive (SPEAE) EAE observed in young mice, old mice developed progressive disease from onset (PEAE) associated with pronounced axonal damage and increased numbers of CD3(+) T cells and microglia/macrophages, but not B cells. Whereas the clinical neurological features of PEAE and SPEAE were comparable, the pathology was distinct. SPEAE was associated with significantly reduced perivascular infiltrates and T-cell numbers in the central nervous system (CNS) compared with PEAE and the acute phase of RREAE. In contrast to perivascular infiltrates that declined during progression from RREAE into SPEAE, the numbers of microglia clusters remained constant. Similar to what is observed during MS, the microglia clusters emerging during EAE were associated with axonal damage and oligodendrocytes expressing heat-shock protein B5, but not lymphocytes. Taken together, our data reveal that the course of EAE is dependent on the age of the mice. Younger mice show a relapsing-remitting phase followed by progressive disease, whereas old mice immediately show progression. This indicates that recurrent episodes of inflammation in the CNS, as well as age, contribute to progressive neurological disease.

Sinomenine enhances microglia M2 polarization and attenuates inflammatory injury in intracerebral hemorrhage

Microglia polarization plays a vital role in brain inflammatory injury following intracerebral hemorrhage (ICH). Previous studies have shown that sinomenine possesses potential immunoregulatory capabilities. However, microglia polarization's exact mechanisms in ICH remain uncertain. Therefore, we examined the role of sinomenine on microglia polarization and brain inflammation following ICH. For the experiment, autologous blood models were constructed in C57/BL6 mice. Markers of classically activated (M1) and alternatively activated (M2) microglia were detected by real-time polymerase chain reaction, immunofluorescence, and flow cytometry. Microglial toxicity was assessed using MTT and FACS assays. In addition, the neurological deficit and cerebral water content of ICH mice were also observed. Sinomenine attenuated M1 markers while promoting M2 markers of microglia. Sinomenine also protected hippocampal neurons from indirect toxicity mediated by ICH-treated microglia. Additionally, administration of sinomenine inhibited matrix metalloproteinase (MMP) 3/9 expression, cerebral water content, and neurological deficit. Therefore, sinomenine protected brain function following ICH, perhaps via M2 microglia phenotype induction and MMP 3/9 inhibition. This result suggests that sinomenine is a promising therapeutical strategy in ICH.

Microglia activation is associated with IFN-α induced depressive-like behavior

Inflammatory immune activation has been frequently associated with the development of major depression. This association was confirmed in patients receiving long-term treatment with pro-inflammatory interferon-α (IFN-α). Microglia, the resident immune cells in the brain, might serve as an important interface in this immune system-to-brain communication. The aim of the present study was to investigate the role of microglia in an IFN-α mouse model of immune-mediated depression. Male BALB/c mice were treated with daily injections of IFN-α for two weeks. Depressive-like behavior was analyzed in the forced swim and tail suspension test. Activation of microglia was measured by flow cytometry. Pro-inflammatory M1 type (MHC-II, CD40, CD54, CD80, CD86, CCR7), anti-inflammatory M2 type (CD206, CD200R), and maturation markers (CD11c, CCR7) were tested, as well as the chemokine receptor CCR2. IFN-α led to a significant increase in depressive-like behavior and expression of the pro-inflammatory surface markers MHC-II, CD86, and CD54, indicating M1 polarization. Because IFN-α-treated mice showed great individual variance in the behavioral response to IFN-α, they were further divided into vulnerable and non-vulnerable subgroups. Only IFN-α vulnerable mice (characterized by their development of depressive-like behavior in response to IFN-α) showed an increased expression of MHC-II and CD86, while CD54 was similarly enhanced in both subgroups. Thus, IFN-α-induced activation of microglia was specifically associated with depressive-like behavior.

Cellular and Molecular Mechanisms Underpinning Macrophage Activation during Remyelination

Remyelination is an example of central nervous system (CNS) regeneration, whereby myelin is restored around demyelinated axons, re-establishing saltatory conduction and trophic/metabolic support. In progressive multiple sclerosis, remyelination is limited or fails altogether which is considered to contribute to axonal damage/loss and consequent disability. Macrophages have critical roles in both CNS damage and regeneration, such as remyelination. This diverse range in functions reflects the ability of macrophages to acquire tissue microenvironment-specific activation states. This activation is dynamically regulated during efficient regeneration, with a switch from pro-inflammatory to inflammation-resolution/pro-regenerative phenotypes. Although, some molecules and pathways have been implicated in the dynamic activation of macrophages, such as NFκB, the cellular and molecular mechanisms underpinning plasticity of macrophage activation are unclear. Identifying mechanisms regulating macrophage activation to pro-regenerative phenotypes may lead to novel therapeutic strategies to promote remyelination in multiple sclerosis.

Expression of IL-1β in rhesus EAE and MS lesions is mainly induced in the CNS itself

Background

Interleukin (IL)-1β is a pro-inflammatory cytokine that plays a role in the pathogenesis of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), the animal model for MS. Yet, detailed studies on IL-1β expression in different stages of MS lesion development and a comparison of IL-1β expression in MS and EAE are lacking.

Methods

Here, we performed an extensive characterization of IL-1β expression in brain tissue of MS patients, which included different MS lesion types, and in brain tissue of rhesus macaques with EAE.

Results

In rhesus EAE brain tissue, we observed prominent IL-1β staining in MHC class II⁺ cells within perivascular infiltrates and at the edges of large demyelinating lesions. Surprisingly, staining was localized to resident microglia or differentiated macrophages rather than to infiltrating monocytes, suggesting that IL-1β expression is induced within the central nervous system (CNS). By contrast, IL-1β staining in MS brain tissue was much less pronounced. Staining was found in the parenchyma of active and chronic active MS lesions and in nodules of MHC class II⁺ microglia in otherwise normal appearing white matter. IL-1β expression was detected in a minority of the nodules only, which could not be distinguished by the expression of pro- and anti-inflammatory markers. These nodules were exclusively found in MS, and it remains to be determined whether IL-1β⁺ nodules are destined to progress into active lesions or whether they merely reflect a transient response to cellular stress.

Conclusions

Although the exact localization and relative intensity of IL-1β expression in EAE and MS is different, the staining pattern in both neuroinflammatory disorders is most consistent with the idea that the expression of IL-1β during lesion development is induced in the tissue rather than in the periphery.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0605-8) contains supplementary material, which is available to authorized users.

Persistent 7-tesla phase rim predicts poor outcome in new multiple sclerosis patient lesions

BACKGROUND

In some active multiple sclerosis (MS) lesions, a strong immune reaction at the lesion edge may contain growth and thereby isolate the lesion from the surrounding parenchyma. Our previous studies suggest that this process involves opening of the blood-brain barrier in capillaries at the lesion edge, seen on MRI as centripetal contrast enhancement and a colocalized phase rim. We hypothesized that using these features to characterize early lesion evolution will allow in vivo tracking of tissue degeneration and/or repair, thus improving the evaluation of potential therapies for chronic active lesions.

METHODS

Centripetally and centrifugally enhancing lesions were studied in 17 patients with MS using 7-tesla MRI. High-resolution, susceptibility-weighted, T1-weighted (before/after gadolinium), and dynamic contrast-enhanced scans were acquired at baseline and months 1, 3, 6, and 12. For each lesion, time evolution of the phase rim, lesion volume, and T1 hypointensity were assessed. In autopsies of 3 progressive MS cases, the histopathology of the phase rim was determined.

RESULTS

In centripetal lesions, a phase rim colocalized with initial contrast enhancement. In 12 of 22, this phase rim persisted after enhancement resolved. Compared with centripetal lesions with transient rim, those with persistent rim had less volume shrinkage and became more T1 hypointense between months 3 and 12. No centrifugal lesions developed phase rims at any time point. Pathologically, persistent rims corresponded to an iron-laden inflammatory myeloid cell population at the edge of chronic demyelinated lesions.

CONCLUSION

In early lesion evolution, a persistent phase rim in lesions that shrink least and become more T1 hypointense over time suggests that the rim might mark failure of early lesion repair and/or irreversible tissue damage. In later stages of MS, phase rim lesions continue to smolder, exerting detrimental effects on affected brain tissue.

TRIAL REGISTRATION

NCT00001248.

FUNDING

The Intramural Research Program of NINDS supported this study.

Microglia-Neuron Communication in Epilepsy

Epilepsy has remained a significant social concern and financial burden globally. Current therapeutic strategies are based primarily on neurocentric mechanisms that have not proven successful in at least a third of patients, raising the need for novel alternative and complementary approaches. Recent evidence implicates glial cells and neuroinflammation in the pathogenesis of epilepsy with the promise of targeting these cells to complement existing strategies. Specifically, microglial involvement, as a major inflammatory cell in the epileptic brain, has been poorly studied. In this review, we highlight microglial reaction to experimental seizures, discuss microglial control of neuronal activities, and propose the functions of microglia during acute epileptic phenotypes, delayed neurodegeneration, and aberrant neurogenesis. Future research that would help fill in the current gaps in our knowledge includes epilepsy-induced alterations in basic microglial functions, neuro-microglial interactions during chronic epilepsy, and microglial contribution to developmental seizures. Studying the role of microglia in epilepsy could inform therapies to better alleviate the disease. GLIA 2016;65:5-18.

Advanced MRI and staging of multiple sclerosis lesions

Over the past few decades, MRI-based visualization of demyelinated CNS lesions has become pivotal to the diagnosis and monitoring of multiple sclerosis (MS). In this Review, we outline current efforts to correlate imaging findings with the pathology of lesion development in MS, and the pitfalls that are being encountered in this research. Multimodal imaging at high and ultra-high magnetic field strengths is yielding biologically relevant insights into the pathophysiology of blood-brain barrier dynamics and both active and chronic inflammation, as well as mechanisms of lesion healing and remyelination. Here, we parallel the results in humans with advances in imaging of a primate model of MS - experimental autoimmune encephalomyelitis (EAE) in the common marmoset - in which demyelinated lesions resemble their human counterparts far more closely than do EAE lesions in the rodent. This approach holds promise for the identification of innovative biological markers, and for next-generation clinical trials that will focus more on tissue protection and repair.

Parallel Aspects of the Microenvironment in Cancer and Autoimmune Disease

Cancer and autoimmune diseases are fundamentally different pathological conditions. In cancer, the immune response is suppressed and unable to eradicate the transformed self-cells, while in autoimmune diseases it is hyperactivated against a self-antigen, leading to tissue injury. Yet, mechanistically, similarities in the triggering of the immune responses can be observed. In this review, we highlight some parallel aspects of the microenvironment in cancer and autoimmune diseases, especially hypoxia, and the role of macrophages, neutrophils, and their interaction. Macrophages, owing to their plastic mode of activation, can generate a pro- or antitumoral microenvironment. Similarly, in autoimmune diseases, macrophages tip the Th1/Th2 balance via various effector cytokines. The contribution of neutrophils, an additional plastic innate immune cell population, to the microenvironment and disease progression is recently gaining more prominence in both cancer and autoimmune diseases, as they can secrete cytokines, chemokines, and reactive oxygen species (ROS), as well as acquire an enhanced ability to produce neutrophil extracellular traps (NETs) that are now considered important initiators of autoimmune diseases. Understanding the contribution of macrophages and neutrophils to the cancerous or autoimmune microenvironment, as well as the role their interaction and cooperation play, may help identify new targets and improve therapeutic strategies.

Pentraxin-3 is upregulated in the central nervous system during MS and EAE, but does not modulate experimental neurological disease

Pentraxin-3 (PTX3), an acute-phase protein released during inflammation, aids phagocytic clearance of pathogens and apoptotic cells, and plays diverse immunoregulatory roles in tissue injury. In neuroinflammatory diseases, like MS, resident microglia could become activated by endogenous agonists for Toll like receptors (TLRs). Previously we showed a strong TLR2-mediated induction of PTX3 in cultured human microglia and macrophages by HspB5, which accumulates in glia during MS. Given the anti-inflammatory effects of HspB5, we examined the contribution of PTX3 to these effects in MS and its animal model EAE. Our data indicate that TLR engagement effectively induces PTX3 expression in human microglia, and that such expression is readily detectable in MS lesions. Enhanced PTX3 expression is prominently expressed in microglia in preactive MS lesions, and in microglia/macrophages engaged in myelin phagocytosis in actively demyelinating lesions. Yet, we did not detect PTX3 in cerebrospinal fluid of MS patients. PTX3 expression is also elevated in spinal cords during chronic relapsing EAE in Biozzi ABH mice, but the EAE severity and time course in PTX3-deficient mice did not differ from WT mice. Moreover, systemic PTX3 administration did not alter the disease onset or severity. Our findings reveal local functions of PTX3 during neuroinflammation in facilitating myelin phagocytosis, but do not point to a role for PTX3 in controlling the development of autoimmune neuroinflammation.

Production of IL-27 in multiple sclerosis lesions by astrocytes and myeloid cells: Modulation of local immune responses

The mechanisms whereby human glial cells modulate local immune responses are not fully understood. Interleukin-27 (IL-27), a pleiotropic cytokine, has been shown to dampen the severity of experimental autoimmune encephalomyelitis, but it is still unresolved whether IL-27 plays a role in the human disease multiple sclerosis (MS). IL-27 contribution to local modulation of immune responses in the brain of MS patients was investigated. The expression of IL-27 subunits (EBI3 and p28) and its cognate receptor IL-27R (the gp130 and TCCR chains) was elevated within post-mortem MS brain lesions compared with normal control brains. Moreover, astrocytes (GFAP(+) cells) as well as microglia and macrophages (Iba1(+) cells) were important sources of IL-27. Brain-infiltrating CD4 and CD8 T lymphocytes expressed the IL-27R specific chain (TCCR) implying that these cells could respond to local IL-27 sources. In primary cultures of human astrocytes inflammatory cytokines increased IL-27 production, whereas myeloid cell inflammatory M1 polarization and inflammatory cytokines enhanced IL-27 expression in microglia and macrophages. Astrocytes in postmortem tissues and in vitro expressed IL-27R. Moreover, IL-27 triggered the phosphorylation of the transcription regulator STAT1, but not STAT3 in human astrocytes; indeed IL-27 up-regulated MHC class I expression on astrocytes in a STAT1-dependent manner. These findings demonstrated that IL-27 and its receptor were elevated in MS lesions and that local IL-27 can modulate immune properties of astrocytes and infiltrating immune cells. Thus, therapeutic strategies targeting IL-27 may influence not only peripheral but also local inflammatory responses within the brain of MS patients.

Pioglitazone regulates myelin phagocytosis and multiple sclerosis monocytes

OBJECTIVE

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS). Myeloid phagocytes, including blood monocytes recruited to demyelinating lesions, may play a dual role in MS: on one hand, they might enhance CNS damage after differentiating toward a proinflammatory phenotype; on the other, they promote remyelination and repair through effective phagocytosis of myelin debris. We have previously determined that the retinoid X receptor (RXR) plays an important role in monocyte phagocytosis of myelin. Peroxisome proliferator-activated receptor γ is an RXR binding partner that plays a key role in myeloid cell biology and is targeted by the thiazolidinedione group of antidiabetics such as pioglitazone. Consequently, the purpose of this study was to determine if monocyte functions and differentiation profiles differ in MS patients compared to healthy volunteers (HV) and whether pioglitazone can reverse these differences to promote CNS recovery.

METHODS

Monocytes were isolated from MS patients and HV (n ≥ 36/group), and their ability to phagocytose myelin and modulate inflammation in the presence/absence of 1 μmol/L pioglitazone (the in vivo achievable concentration) was quantified by flow cytometry, transcriptional profiling, and proteomic assays.

RESULTS

MS monocytes display impaired phagocytosis of myelin debris and enhanced proinflammatory differentiation. Pioglitazone treatment causes partial normalization of identified monocyte abnormalities in MS and fully reverses the deficit in myelin phagocytosis.

INTERPRETATION

These findings suggest that by inhibiting proinflammatory differentiation of monocytes and enhancing their phagocytosis of myelin, pioglitazone may be a useful adjunct therapy to immunomodulatory agents that target dysregulated adaptive immunity in MS.

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography

Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

Translational utility of experimental autoimmune encephalomyelitis: recent developments

Multiple sclerosis (MS) is a complex autoimmune condition with firmly established genetic and environmental components. Genome-wide association studies (GWAS) have revealed a large number of genetic polymorphisms in the vicinity of, and within, genes that associate to disease. However, the significance of these single-nucleotide polymorphisms in disease and possible mechanisms of action remain, with a few exceptions, to be established. While the animal model for MS, experimental autoimmune encephalomyelitis (EAE), has been instrumental in understanding immunity in general and mechanisms of MS disease in particular, much of the translational information gathered from the model in terms of treatment development (glatiramer acetate and natalizumab) has been extensively summarized. In this review, we would thus like to cover the work done in EAE from a GWAS perspective, highlighting the research that has addressed the role of different GWAS genes and their pathways in EAE pathogenesis. Understanding the contribution of these pathways to disease might allow for the stratification of disease subphenotypes in patients and in turn open the possibility for new and individualized treatment approaches in the future.

Immune phenotypes of microglia in human neurodegenerative disease: challenges to detecting microglial polarization in human brains

Inflammatory responses in the brain, which can be demonstrated by changes in properties of microglia, the brain-resident macrophages, are a common feature of human neurodegenerative diseases. Different monocyte/macrophage phenotypes have been defined by changes in expression of cytokines, receptors and other markers as a response to different classes of stimuli. Monocytes, macrophages and microglia can have a range of phenotypes with associated properties depending on their microenvironment. Macrophage/microglia polarization states have been defined as classical activation (M1), alternative activation (M2a), type II alternative activation (M2b) or acquired deactivation (M2c). Available markers for identifying microglial phenotypes in human brains are still limited; those available provide incomplete information on the functions or polarization states of microglia observed in tissues from diseases such as Alzheimer's disease, Parkinson's disease and multiple sclerosis. The most widely used marker to describe activated microglia in human brains, particularly diseased brains, has been HLA-DR, the major histocompatibility complex II protein. HLA-DR-positive microglia can have a wide range of activation morphologies that are affected not only by disease pathology, but also by their differentiation states and brain regions. Two other widely used markers to identify microglia in human brains are ionized calcium binding adaptor molecule-1 and CD68. Although their expression changes in diseased brains, these markers do not show specificity for different phenotypes. Over the years there have been studies with additional markers that attempt to further define microglial properties, particularly in Alzheimer's disease brains. Most studies have employed immunohistochemical techniques to identify microglia in tissue sections, but recent advances in this field have allowed gene expression profiling of microglia upon immediate isolation from brains. We will review which markers might better define different activation phenotypes of microglia in human brains and whether they fit into current microglial polarization schemes.

Thyroid Hormone Potentially Benefits Multiple Sclerosis via Facilitating Remyelination

Myelin destruction due to inflammatory damage of oligodendrocytes (OLs) in conjunction with axonal degeneration is one of the major histopathological hallmarks of multiple sclerosis (MS), a common autoimmune disorder affecting the central nervous system (CNS). Therapies over the last 20 years mainly focus on the immune system and, more specifically, on the modulation of immune cell behavior. It seems to be effective in MS with relapse, while it is of little benefit to progressive MS in which neurodegeneration following demyelination outweighs inflammation. Otherwise, remyelination, as a result of oligodendrocyte production from oligodendrocyte precursor cells (OPCs), is considered to be a potential target for the treatment of progressive MS. In this review, positive effects of remyelination on MS will be discussed in view of the critical role played by thyroid hormone (TH), focusing on the following points: (1) promising treatment of TH on MS that potentially targets to remyelination; (2) the active role of TH that is able to promote remyelination; (3) the regulative role of TH that works on endogenous stem and precursor cells; (4) the effect of TH on gene transcription; and (5) a working hypothesis which is developed that TH can alleviate MS by promoting remyelination, and the mechanism of which is its regulative role in gene transcription of OPCs.

Myelin damage and repair in pathologic CNS: challenges and prospects

Injury to the central nervous system (CNS) results in oligodendrocyte cell death and progressive demyelination. Demyelinated axons undergo considerable physiological changes and molecular reorganizations that collectively result in axonal dysfunction, degeneration and loss of sensory and motor functions. Endogenous adult oligodendrocyte precursor cells and neural stem/progenitor cells contribute to the replacement of oligodendrocytes, however, the extent and quality of endogenous remyelination is suboptimal. Emerging evidence indicates that optimal remyelination is restricted by multiple factors including (i) low levels of factors that promote oligodendrogenesis; (ii) cell death among newly generated oligodendrocytes, (iii) inhibitory factors in the post-injury milieu that impede remyelination, and (iv) deficient expression of key growth factors essential for proper re-construction of a highly organized myelin sheath. Considering these challenges, over the past several years, a number of cell-based strategies have been developed to optimize remyelination therapeutically. Outcomes of these basic and preclinical discoveries are promising and signify the importance of remyelination as a mechanism for improving functions in CNS injuries. In this review, we provide an overview on: (1) the precise organization of myelinated axons and the reciprocal axo-myelin interactions that warrant properly balanced physiological activities within the CNS; (2) underlying cause of demyelination and the structural and functional consequences of demyelination in axons following injury and disease; (3) the endogenous mechanisms of oligodendrocyte replacement; (4) the modulatory role of reactive astrocytes and inflammatory cells in remyelination; and (5) the current status of cell-based therapies for promoting remyelination. Careful elucidation of the cellular and molecular mechanisms of demyelination in the pathologic CNS is a key to better understanding the impact of remyelination for CNS repair.

Pathological differences between white and grey matter multiple sclerosis lesions

Multiple sclerosis (MS) is a debilitating disease characterized by demyelination of the central nervous system (CNS), resulting in widespread formation of white matter lesions (WMLs) and grey matter lesions (GMLs). WMLs are pathologically characterized by the presence of immune cells that infiltrate the CNS, whereas these immune cells are barely present in GMLs. This striking pathological difference between WMLs and GMLs raises questions about the underlying mechanism. It is known that infiltrating leukocytes contribute to the generation of WMLs; however, since GMLs show a paucity of infiltrating immune cells, their importance in GML formation remains to be determined. Here, we review pathological characteristics of WMLs and GMLs, and suggest some possible explanations for the observed pathological differences. In our view, cellular and molecular characteristics of WM and GM, and local differences within WMLs and GMLs (in particular, in glial cell populations and the molecules they express), determine the pathway to demyelination. Further understanding of GML pathogenesis, considered to contribute to chronic MS, may have a direct impact on the development of novel therapeutic targets to counteract this progressive neurological disorder.

N-Palmitoylethanolamine and Neuroinflammation: a Novel Therapeutic Strategy of Resolution

Inflammation is fundamentally a protective cellular response aimed at removing injurious stimuli and initiating the healing process. However, when prolonged, it can override the bounds of physiological control and becomes destructive. Inflammation is a key element in the pathobiology of chronic pain, neurodegenerative diseases, stroke, spinal cord injury, and neuropsychiatric disorders. Glia, key players in such nervous system disorders, are not only capable of expressing a pro-inflammatory phenotype but respond also to inflammatory signals released from cells of immune origin such as mast cells. Chronic inflammatory processes may be counteracted by a program of resolution that includes the production of lipid mediators endowed with the capacity to switch off inflammation. These naturally occurring lipid signaling molecules include the N-acylethanolamines, N-arachidonoylethanolamine (an endocannabinoid), and its congener N-palmitoylethanolamine (palmitoylethanolamide or PEA). PEA may play a role in maintaining cellular homeostasis when faced with external stressors provoking, for example, inflammation. PEA is efficacious in mast cell-mediated models of neurogenic inflammation and neuropathic pain and is neuroprotective in models of stroke, spinal cord injury, traumatic brain injury, and Parkinson disease. PEA in micronized/ultramicronized form shows superior oral efficacy in inflammatory pain models when compared to naïve PEA. Intriguingly, while PEA has no antioxidant effects per se, its co-ultramicronization with the flavonoid luteolin is more efficacious than either molecule alone. Inhibiting or modulating the enzymatic breakdown of PEA represents a complementary therapeutic approach to treat neuroinflammation. This review is intended to discuss the role of mast cells and glia in neuroinflammation and strategies to modulate their activation based on leveraging natural mechanisms with the capacity for self-defense against inflammation.

Microglia in action: how aging and injury can change the brain's guardians

Neuroinflammation, the inflammatory response in the central nervous system (CNS), is a major determinant of neuronal function and survival during aging and disease progression. Microglia, as the resident tissue-macrophages of the brain, provide constant support to surrounding neurons in healthy brain. Upon any stress signal (such as trauma, ischemia, inflammation) they are one of the first cells to react. Local and/or peripheral signals determine microglia stress response, which can vary within a continuum of states from beneficial to detrimental for neuronal survival, and can be shaped by aging and previous insults. In this review, we discuss the roles of microglia upon an ischemic or traumatic injury, and give our perspective how aging may contribute to microglia behavior in the injured brain. We speculate that a deeper understanding of specific microglia identities will pave the way to develop more potent therapeutics to treat the diseases of aging brain.