Activation of adult human derived microglia by myelin phagocytosis in vitro

The role of foam cells in spinal cord injury: challenges and opportunities for intervention

Spinal cord injury (SCI) results in a large amount of tissue cell debris in the lesion site, which interacts with various cytokines, including inflammatory factors, and the intrinsic glial environment of the central nervous system (CNS) to form an inhibitory microenvironment that impedes nerve regeneration. The efficient clearance of tissue debris is crucial for the resolution of the inhibitory microenvironment after SCI. Macrophages are the main cells responsible for tissue debris removal after SCI. However, the high lipid content in tissue debris and the dysregulation of lipid metabolism within macrophages lead to their transformation into foamy macrophages during the phagocytic process. This phenotypic shift is associated with a further pro-inflammatory polarization that may aggravate neurological deterioration and hamper nerve repair. In this review, we summarize the phenotype and metabolism of macrophages under inflammatory conditions, as well as the mechanisms and consequences of foam cell formation after SCI. Moreover, we discuss two strategies for foam cell modulation and several potential therapeutic targets that may enhance the treatment of SCI.

The roles of microglia and astrocytes in myelin phagocytosis in the central nervous system

Myelination is an important process in the central nervous system (CNS). Oligodendrocytes (OLs) extend multiple layers to densely sheath on axons, composing the myelin to achieve efficient electrical signal conduction. The myelination during developmental stage maintains a balanced state. However, numerous CNS diseases including neurodegenerative and cerebrovascular diseases cause demyelination and disrupt the homeostasis, resulting in inflammation and white matter deficits. Effective clearance of myelin debris is needed in the region of demyelination, which is a key step for remyelination and tissue regeneration. Microglia and astrocytes are the major resident phagocytic cells in the brain, which may play different or collaborative roles in myelination. Microglia and astrocytes participate in developmental myelination through engulfing excessive unneeded myelin. They are also involved in the clearance of degenerated myelin debris for accelerating remyelination, or engulfing healthy myelin sheath for inhibiting remyelination. This review focuses on the roles of microglia and astrocytes in phagocytosing myelin in the developmental brain and diseased brain. In addition, the interaction between microglia and astrocytes to mediate myelin engulfment is also summarized.

Alzheimer's Disease-Associated Alternative Splicing of CD33 Is Regulated by the HNRNPA Family Proteins

Genetic variations of CD33 have been implicated as a susceptibility factor of Alzheimer's disease (AD). A polymorphism on exon 2 of CD33, rs12459419, affects the alternative splicing of this exon. The minor allele is associated with a reduced risk of AD and promotes the skipping of exon 2 to produce a shorter CD33 isoform lacking the extracellular ligand-binding domain, leading to decreased suppressive signaling on microglial activity. Therefore, factors that regulate the splicing of exon 2 may alter the disease-associated properties of CD33. Herein, we sought to identify the regulatory proteins of CD33 splicing. Using a panel of RNA-binding proteins and a human CD33 minigene, we found that exon 2 skipping of CD33 was promoted by HNRNPA1. Although the knockdown of HNRNPA1 alone did not reduce exon 2 skipping, simultaneous knockdown of HNRNPA1 together with that of HNRNPA2B1 and HNRNPA3 promoted exon 2 inclusion, suggesting functional redundancy among HNRNPA proteins. Similar redundant regulation by HNRNPA proteins was observed in endogenous CD33 of THP-1 and human microglia-like cells. Although mouse Cd33 showed a unique splicing pattern of exon 2, we confirmed that HNRNPA1 promoted the skipping of this exon. Collectively, our results revealed novel regulatory relationships between CD33 and HNRNPA proteins.

Intrathecal minocycline does not block the adverse effects of repeated, intravenous morphine administration on recovery of function after SCI

Opioids are among the most effective analgesics for the management of pain in the acute phase of a spinal cord injury (SCI), and approximately 80% of patients are treated with morphine in the first 24 h following SCI. We have found that morphine treatment in the first 7 days after SCI increases symptoms of pain at 42 days post-injury and undermines the recovery of locomotor function in a rodent model. Prior research has implicated microglia/macrophages in opioid-induced hyperalgesia and the development of neuropathic pain. We hypothesized that glial activation may also underlie the development of morphine-induced pain and cell death after SCI. Supporting this hypothesis, our previous studies found that intrathecal and intravenous morphine increase the number of activated microglia and macrophages present at the spinal lesion site, and that the adverse effects of intrathecal morphine can be blocked with intrathecal minocycline. Recognizing that the cellular expression of opioid receptors, and the intracellular signaling pathways engaged, can change with repeated administration of opioids, the current study tested whether minocycline was also protective with repeated intravenous morphine administration, more closely simulating clinical treatment. Using a rat model of SCI, we co-administered intravenous morphine and intrathecal minocycline for the first 7 days post injury and monitored sensory and locomotor recovery. Contrary to our hypothesis and previous findings with intrathecal morphine, we found that minocycline did not prevent the negative effects of morphine. Surprisingly, we also found that intrathecal minocycline alone is detrimental for locomotor recovery after SCI. Using ex vivo cell cultures, we investigated how minocycline and morphine altered microglia/macrophage function. Commensurate with published studies, we found that minocycline blocked the effects of morphine on the release of pro-inflammatory cytokines but, like morphine, it increased glial phagocytosis. While phagocytosis is critical for the removal of cellular and extracellular debris at the spinal injury site, increased phagocytosis after injury has been linked to the clearance of stressed but viable neurons and protracted inflammation. In sum, our data suggest that both morphine and minocycline alter the acute immune response, and reduce locomotor recovery after SCI.

CNS remyelination and inflammation: From basic mechanisms to therapeutic opportunities

Remyelination, the myelin regenerative response that follows demyelination, restores saltatory conduction and function and sustains axon health. Its declining efficiency with disease progression in the chronic autoimmune disease multiple sclerosis (MS) contributes to the currently untreatable progressive phase of the disease. Although some of the bona fide myelin regenerative medicine clinical trials have succeeded in demonstrating proof-of-principle, none of these compounds have yet proceeded toward approval. There therefore remains a need to increase our understanding of the fundamental biology of remyelination so that existing targets can be refined and new ones discovered. Here, we review the role of inflammation, in particular innate immunity, in remyelination, describing its many and complex facets and discussing how our evolving understanding can be harnessed to translational goals.

Citrullinated myelin induces microglial TNFα and inhibits endogenous repair in the cuprizone model of demyelination

BACKGROUND

Microglia are the primary phagocytes of the central nervous system and are responsible for removing damaged myelin following demyelination. Previous investigations exploring the consequences of myelin phagocytosis on microglial activation overlooked the biochemical modifications present on myelin debris. Such modifications, including citrullination, are increased within the inflammatory environment of multiple sclerosis lesions.

METHODS

Mouse cortical myelin isolated by ultracentrifugation was citrullinated ex vivo by incubation with the calcium-dependent peptidyl arginine deiminase PAD2. Demyelination was induced by 6 weeks of cuprizone (0.3%) treatment and spontaneous repair was initiated by reversion to normal chow. Citrullinated or unmodified myelin was injected into the primary motor cortex above the cingulum bundle at the time of reversion to normal chow and the consequent impact on remyelination was assessed by measuring the surface area of myelin basic protein-positive fibers in the cortex 3 weeks later. Microglial responses to myelin were characterized by measuring cytokine release, assessing flow cytometric markers of microglial activation, and RNAseq profiling of transcriptional changes.

RESULTS

Citrullinated myelin induced a unique microglial response marked by increased tumor necrosis factor α (TNFα) production both in vitro and in vivo. This response was not induced by unmodified myelin. Injection of citrullinated myelin but not unmodified myelin into the cortex of cuprizone-demyelinated mice significantly inhibited spontaneous remyelination. Antibody-mediated neutralization of TNFα blocked this effect and restored remyelination to normal levels.

CONCLUSIONS

These findings highlight the role of post-translation modifications such as citrullination in the determination of microglial activation in response to myelin during demyelination. The inhibition of endogenous repair induced by citrullinated myelin and the reversal of this effect by neutralization of TNFα may have implications for therapeutic approaches to patients with inflammatory demyelinating disorders.

Nano-structured myelin: new nanovesicles for targeted delivery to white matter and microglia, from brain-to-brain

Neurodegenerative diseases affect millions of people worldwide and the presence of various physiological barriers limits the accessibility to the brain and reduces the efficacy of various therapies. Moreover, new carriers having targeting properties to specific brain regions and cells are needed in order to improve therapies for the brain disorder treatment. In this study, for the first time, Myelin nanoVesicles (hereafter defined MyVes) from brain-extracted myelin were produced. The MyVes have an average diameter of 100–150 ​nm, negative zeta potential, spheroidal morphology, and contain lipids and the key proteins of the myelin sheath. Furthermore, they exhibit good cytocompatibility. The MyVes were able to target the white matter and interact mainly with the microglia cells. The preliminary results here presented allow us to suppose the employment of MyVes as potential carrier to target the white matter and microglia in order to counteract white matter microglia-related diseases.

•

Bio-fabrication of brain tissue derived nanovesicles: myelin nanovesicles.

•

Myelin nanovesicles contain the main proteins of the myelin sheath (myelin basic protein and myelin proteolipid protein).

•

Myelin nanovesicles can lade a drug/molecule and cross a blood–brain barrier model.

•

Myelin nanovesicles target white matter and microglia cells.

Macrophage phagocytosis after spinal cord injury: when friends become foes

After spinal cord injury (SCI), macrophages can exert either beneficial or detrimental effects depending on their phenotype. Aside from their critical role in inflammatory responses, macrophages are also specialized in the recognition, engulfment, and degradation of pathogens, apoptotic cells, and tissue debris. They promote remyelination and axonal regeneration by removing inhibitory myelin components and cellular debris. However, excessive intracellular presence of lipids and dysregulated intracellular lipid homeostasis result in the formation of foamy macrophages. These develop a pro-inflammatory phenotype that may contribute to further neurological decline. Additionally, myelin-activated macrophages play a crucial role in axonal dieback and retraction. Here, we review the opposing functional consequences of phagocytosis by macrophages in SCI, including remyelination and regeneration versus demyelination, degeneration, and axonal dieback. Furthermore, we discuss how targeting the phagocytic ability of macrophages may have therapeutic potential for the treatment of SCI.

Connecting Neuroinflammation and Neurodegeneration in Multiple Sclerosis: Are Oligodendrocyte Precursor Cells a Nexus of Disease?

The pathology in neurodegenerative diseases is often accompanied by inflammation. It is well-known that many cells within the central nervous system (CNS) also contribute to ongoing neuroinflammation, which can promote neurodegeneration. Multiple sclerosis (MS) is both an inflammatory and neurodegenerative disease in which there is a complex interplay between resident CNS cells to mediate myelin and axonal damage, and this communication network can vary depending on the subtype and chronicity of disease. Oligodendrocytes, the myelinating cell of the CNS, and their precursors, oligodendrocyte precursor cells (OPCs), are often thought of as the targets of autoimmune pathology during MS and in several animal models of MS; however, there is emerging evidence that OPCs actively contribute to inflammation that directly and indirectly contributes to neurodegeneration. Here we discuss several contributors to MS disease progression starting with lesion pathology and murine models amenable to studying particular aspects of disease. We then review how OPCs themselves can play an active role in promoting neuroinflammation and neurodegeneration, and how other resident CNS cells including microglia, astrocytes, and neurons can impact OPC function. Further, we outline the very complex and pleiotropic role(s) of several inflammatory cytokines and other secreted factors classically described as solely deleterious during MS and its animal models, but in fact, have many neuroprotective functions and promote a return to homeostasis, in part via modulation of OPC function. Finally, since MS affects patients from the onset of disease throughout their lifespan, we discuss the impact of aging on OPC function and CNS recovery. It is becoming clear that OPCs are not simply a bystander during MS progression and uncovering the active roles they play during different stages of disease will help uncover potential new avenues for therapeutic intervention.

The effects of myelin on macrophage activation are phenotypic specific via cPLA2 in the context of spinal cord injury inflammation

Spinal cord injury (SCI) produces chronic, pro-inflammatory macrophage activation that impairs recovery. The mechanisms driving this chronic inflammation are not well understood. Here, we detail the effects of myelin debris on macrophage physiology and demonstrate a novel, activation state-dependent role for cytosolic phospholipase-A2 (cPLA₂) in myelin-mediated potentiation of pro-inflammatory macrophage activation. We hypothesized that cPLA₂ and myelin debris are key mediators of persistent pro-inflammatory macrophage responses after SCI. To test this, we examined spinal cord tissue 28-days after thoracic contusion SCI in 3-month-old female mice and observed both cPLA₂ activation and intracellular accumulation of lipid-rich myelin debris in macrophages. In vitro, we utilized bone marrow-derived macrophages to determine myelin's effects across a spectrum of activation states. We observed phenotype-specific responses with myelin potentiating only pro-inflammatory (LPS + INF-γ; M1) macrophage activation, whereas myelin did not induce pro-inflammatory responses in unstimulated or anti-inflammatory (IL-4; M2) macrophages. Specifically, myelin increased levels of pro-inflammatory cytokines, reactive oxygen species, and nitric oxide production in M1 macrophages as well as M1-mediated neurotoxicity. PACOCF3 (cPLA₂ inhibitor) blocked myelin's detrimental effects. Collectively, we provide novel spatiotemporal evidence that myelin and cPLA₂ play an important role in the pathophysiology of SCI inflammation and the phenotype-specific response to myelin implicate diverse roles of myelin in neuroinflammatory conditions.

Thyroid hormone and thyromimetics inhibit myelin and axonal degeneration and oligodendrocyte loss in EAE

We have previously demonstrated that thyromimetics stimulate oligodendrocyte precursor cell differentiation and promote remyelination in murine demyelination models. We investigated whether a thyroid receptor-beta selective thyromimetic, sobetirome (Sob), and its CNS-targeted prodrug, Sob-AM2, could prevent myelin and axonal degeneration in experimental autoimmune encephalomyelitis (EAE). Compared to controls, EAE mice receiving triiodothyronine (T3, 0.4 mg/kg), Sob (5 mg/kg) or Sob-AM2 (5 mg/kg) had reduced clinical disease and, within the spinal cord, less tissue damage, more normally myelinated axons, fewer degenerating axons and more oligodendrocytes. T3 and Sob also protected cultured oligodendrocytes against cell death. Thyromimetics thus might protect against oligodendrocyte death, demyelination and axonal degeneration as well as stimulate remyelination in multiple sclerosis.

Stearoyl-CoA desaturase-1 impairs the reparative properties of macrophages and microglia in the brain

Failure of remyelination underlies the progressive nature of demyelinating diseases such as multiple sclerosis. Macrophages and microglia are crucially involved in the formation and repair of demyelinated lesions. Here we show that myelin uptake temporarily skewed these phagocytes toward a disease-resolving phenotype, while sustained intracellular accumulation of myelin induced a lesion-promoting phenotype. This phenotypic shift was controlled by stearoyl-CoA desaturase-1 (SCD1), an enzyme responsible for the desaturation of saturated fatty acids. Monounsaturated fatty acids generated by SCD1 reduced the surface abundance of the cholesterol efflux transporter ABCA1, which in turn promoted lipid accumulation and induced an inflammatory phagocyte phenotype. Pharmacological inhibition or phagocyte-specific deficiency of Scd1 accelerated remyelination ex vivo and in vivo. These findings identify SCD1 as a novel therapeutic target to promote remyelination.

Docosahexaenoic acid reduces microglia phagocytic activity via miR-124 and induces neuroprotection in rodent models of spinal cord contusion injury

Microglia are activated after spinal cord injury (SCI), but their phagocytic mechanisms and link to neuroprotection remain incompletely characterized. Docosahexaenoic acid (DHA) has been shown to have significant neuroprotective effects after hemisection and compression SCI and can directly affect microglia in these injury models. In rodent contusion SCI, we demonstrate that DHA (500 nmol/kg) administered acutely post-injury confers neuroprotection and enhances locomotor recovery, and also exerts a complex modulation of the microglial response to injury. In rodents, at 7 days after SCI, the level of phagocytosed myelin within Iba1-positive or P2Y12-positive cells was significantly lower after DHA treatment, and this occurred in parallel with an increase in intracellular miR-124 expression. Furthermore, intraspinal administration of a miR-124 inhibitor significantly reduced the DHA-induced decrease in myelin phagocytosis in mice at 7 days post-SCI. In rat spinal primary microglia cultures, DHA reduced the phagocytic response to myelin, which was associated with an increase in miR-124, but not miR-155. A similar response was observed in a microglia cell line (BV2) treated with DHA, and the effect was blocked by a miR-124 inhibitor. Furthermore, the phagocytic response of BV2 cells to stressed neurones was also reduced in the presence of DHA. In peripheral monocyte-derived macrophages, the expression of the M1, but not the M0 or M2 phenotype, was reduced by DHA, but the phagocytic activation was not altered. These findings show that DHA induces neuroprotection in contusion injury. Furthermore, the improved outcome is via a miR-124-dependent reduction in the phagocytic response of microglia.

Label-free Imaging of Tissue Architecture during Axolotl Peripheral Nerve Regeneration in Comparison to Functional Recovery

Human peripheral nerves hold the potential to regenerate after injuries; however, whether a successful axonal regrowth was achieved can be elucidated only months after injury by assessing function. The axolotl salamander is a regenerative model where nerves always regenerate quickly and fully after all types of injury. Here, de- and regeneration of the axolotl sciatic nerve were investigated in a single and double injury model by label-free multiphoton imaging in comparison to functional recovery. We used coherent anti-Stokes Raman scattering to visualize myelin fragmentation and axonal regeneration. The presence of axons at the lesion site corresponded to onset of functional recovery in both lesion models. In addition, we detected axonal regrowth later in the double injury model in agreement with a higher severity of injury. Moreover, endogenous two-photon excited fluorescence visualized macrophages and revealed a similar timecourse of inflammation in both injury models, which did not correlate with functional recovery. Finally, using the same techniques, axonal structure and status of myelin were visualized in vivo after sciatic nerve injury. Label-free imaging is a new experimental approach that provides mechanistic insights in animal models, with the potential to be used in the future for investigation of regeneration after nerve injuries in humans.

Alzheimer's Disease Research Using Human Microglia

Experimental studies of neuroinflammation in Alzheimer's disease (AD) have mostly investigated microglia, the brain-resident macrophages. This review focused on human microglia obtained at rapid autopsies. Studies employing methods to isolate and culture human brain microglia in high purity for experimental studies were discussed. These methods were employed to isolate human microglia for investigation of a number of features of neuroinflammation, including activation phenotypes, neurotoxicity, responses to abnormal aggregated proteins such as amyloid beta, phagocytosis, and the effects of aging and disease on microglia cellular properties. In recent years, interest in human microglia and neuroinflammation has been renewed due to the identification of inflammation-related AD genetic risk factors, in particular the triggering receptor expressed on myeloid cells (TREM)-2. Because of the difficulties in developing effective treatments for AD, there has been a general need for greater understanding of the functions of microglia in normal and AD brains. While most experimental studies on neuroinflammation have employed rodent microglia, this review considered the role of human microglia in experimental studies. This review focused on the development of in vitro methodology for the culture of postmortem human microglia and the key findings obtained from experimental studies with these cells.

Phagocytosis in the Brain: Homeostasis and Disease

Microglia are resident macrophages of the central nervous system and significantly contribute to overall brain function by participating in phagocytosis during development, homeostasis, and diseased states. Phagocytosis is a highly complex process that is specialized for the uptake and removal of opsonized and non-opsonized targets, such as pathogens, apoptotic cells, and cellular debris. While the role of phagocytosis in mediating classical innate and adaptive immune responses has been known for decades, it is now appreciated that phagocytosis is also critical throughout early neural development, homeostasis, and initiating repair mechanisms. As such, modulating phagocytic processes has provided unexplored avenues with the intent of developing novel therapeutics that promote repair and regeneration in the CNS. Here, we review the functional consequences that phagocytosis plays in both the healthy and diseased CNS, and summarize how phagocytosis contributes to overall pathophysiological mechanisms involved in brain injury and repair.

The physiology of foamy phagocytes in multiple sclerosis

Multiple sclerosis (MS) is a chronic disease of the central nervous system characterized by massive infiltration of immune cells, demyelination, and axonal loss. Active MS lesions mainly consist of macrophages and microglia containing abundant intracellular myelin remnants. Initial studies showed that these foamy phagocytes primarily promote MS disease progression by internalizing myelin debris, presenting brain-derived autoantigens, and adopting an inflammatory phenotype. However, more recent studies indicate that phagocytes can also adopt a beneficial phenotype upon myelin internalization. In this review, we summarize and discuss the current knowledge on the spatiotemporal physiology of foamy phagocytes in MS lesions, and elaborate on extrinsic and intrinsic factors regulating their behavior. In addition, we discuss and link the physiology of myelin-containing phagocytes to that of foamy macrophages in other disorders such atherosclerosis.

Decoding microglia responses to psychosocial stress reveals blood-brain barrier breakdown that may drive stress susceptibility

An animal's ability to cope with or succumb to deleterious effects of chronic psychological stress may be rooted in the brain's immune responses manifested in microglial activity. Mice subjected to chronic social defeat (CSD) were categorized as susceptible (CSD-S) or resilient (CSD-R) based on behavioral phenotyping, and their microglia were isolated and analyzed by microarray. Microglia transcriptomes from CSD-S mice were enriched for pathways associated with inflammation, phagocytosis, oxidative stress, and extracellular matrix remodeling. Histochemical experiments confirmed the array predictions: CSD-S microglia showed elevated phagocytosis and oxidative stress, and the brains of CSD-S but not CSD-R or non-stressed control mice showed vascular leakage of intravenously injected fluorescent tracers. The results suggest that the inflammatory profile of CSD-S microglia may be precipitated by extracellular matrix degradation, oxidative stress, microbleeds, and entry and phagocytosis of blood-borne substances into brain parenchyma. We hypothesize that these CNS-centric responses contribute to the stress-susceptible behavioral phenotype.

Microglial Phenotypes and Functions in Multiple Sclerosis

Microglia are the resident immune cells that constantly survey the central nervous system. They can adapt to their environment and respond to injury or insult by altering their morphology, phenotype, and functions. It has long been debated whether microglial activation is detrimental or beneficial in multiple sclerosis (MS). Recently, the two opposing yet connected roles of microglial activation have been described with the aid of novel microglial markers, RNA profiling, and in vivo models. In this review, microglial phenotypes and functions in the context of MS will be discussed with evidence from both human pathological studies, in vitro and in vivo models. Microglial functional diversity-phagocytosis, antigen presentation, immunomodulation, support, and repair-will also be examined in detail. In addition, this review discusses the emerging evidence for microglia-related targets as biomarkers and therapeutic targets for MS.

Low-Density Lipoprotein Receptor Deficiency Attenuates Neuroinflammation through the Induction of Apolipoprotein E

Objective

We aimed to determine the role of the low-density lipoprotein receptor (LDLr) in neuroinflammation by inducing experimental autoimmune encephalomyelitis (EAE) in ldlr knock out mice.

Methods

MOG_35-55 induced EAE in male and female ldlr^-/- mice was assessed clinically and histopathologically. Expression of inflammatory mediators and apolipoprotein E (apoE) was investigated by qPCR. Changes in protein levels of apoE and tumor necrosis factor alpha (TNFα) were validated by western blot and ELISA, respectively.

Results

Ldlr^-/--attenuated EAE disease severity in female, but not in male, EAE mice marked by a reduced proinflammatory cytokine production in the central nervous system of female ldlr^-/- mice. Macrophages from female ldlr^-/- mice showed a similar decrease in proinflammatory mediators, an impaired capacity to phagocytose myelin and enhanced secretion of the anti-inflammatory apoE. Interestingly, apoE/ldlr double knock out abrogated the beneficial effect of ldlr depletion in EAE.

Conclusion

Collectively, we show that ldlr^-/- reduces EAE disease severity in female but not in male EAE mice, and that this can be explained by increased levels of apoE in female ldlr^-/- mice. Although the reason for the observed sexual dimorphism remains unclear, our findings show that LDLr and associated apoE levels are involved in neuroinflammatory processes.

Krabbe's leukodystrophy: Approaches and models in vitro

This Review describes some in vitro approaches used to investigate the mechanisms involved in Krabbe's disease, with particular regard to the cellular systems employed to study processes of inflammation, apoptosis, and angiogenesis. The aim was to update the knowledge on the results obtained from in vitro models of this neurodegenerative disorder and provide stimuli for future research. For a long time, the nonavailability of established neural cells has limited the understanding of neuropathogenic mechanisms in Krabbe's leukodystrophy. More recently, the development of new Krabbe's disease cell models has allowed the identification of neurologically relevant pathogenic cascades, including the major role of elevated psychosine levels. Thus, direct and/or indirect roles of psychosine in the release of cytokines, reactive oxygen species, and nitric oxide and in the activation of kinases, caspases, and angiogenic factors results should be clearer. In parallel, it is now understood that the presence of globoid cells precedes oligodendrocyte apoptosis and demyelination. The information described here will help to continue the research on Krabbe's leukodystrophy and on potential new therapeutic approaches for this disease that even today, despite numerous attempts, is without cure. © 2016 Wiley Periodicals, Inc.

Nogo presence is inversely associated with shifts in cortical microglial morphology following experimental diffuse brain injury

Diffuse traumatic brain injury (TBI) initiates secondary pathology, including inflammation and reduced myelination. Considering these injury-related pathologies, the many states of activated microglia as demonstrated by differing morphologies would form, migrate, and function in and through fields of growth-inhibitory myelin byproduct, specifically Nogo. Here we evaluate the relationship between inflammation and reduced myelin antigenicity in the wake of diffuse TBI and present the hypothesis that the Nogo-66 receptor antagonist peptide NEP(1-40) would reverse the injury-induced shift in distribution of microglia morphologies by limiting myelin-based inhibition. Adult male rats were subjected to midline fluid percussion sham or brain injury. At 2h, 6h, 1d, 2d, 7d, and 21d post-injury, immunohistochemical staining was analyzed in sensory cortex (S1BF) for myelin antigens (myelin basic protein; MBP and CNPase), microglia morphology (ionized calcium-binding adapter protein; Iba1), Nogo receptor and Nogo. Pronounced reduction in myelin antigenicity was evident transiently at 1d post-injury, as evidenced by decreased MBP and CNPase staining, as well as loss of white matter organization, compared to sham and later injury time points. Concomitant with reduced myelin antigenicity, injury shifted microglia morphology from the predominantly ramified morphology observed in sham-injured cortex to hyper-ramified, activated, fully activated, or rod. Changes in microglial morphology were evident as early as 2h post-injury, and remained at least until day 21. Additional cohorts of uninjured and brain-injured animals received vehicle or drug (NEP(1-40), i.p., 15min and 19h post-injury) and brains were collected at 2h, 6h, 1d, 2d, or 7d post-injury. NEP(1-40) administration further shifted distributions of microglia away from an injury-induced activated morphology toward greater proportions of rod and macrophage-like morphologies compared to vehicle-treated. By 7d post-injury, no differences in the distributions of microglia were noted between vehicle and NEP(1-40). This study begins to link secondary pathologies of white matter damage and inflammation after diffuse TBI. In the injured brain, secondary pathologies co-occur and likely interact, with consequences for neuronal circuit disruption leading to neurological symptoms.

Myelin as an inflammatory mediator: Myelin interactions with complement, macrophages, and microglia in spinal cord injury

Spinal cord injury (SCI) triggers chronic intraspinal inflammation consisting of activated resident and infiltrating immune cells (especially microglia/macrophages). The environmental factors contributing to this protracted inflammation are not well understood; however, myelin lipid debris is a hallmark of SCI. Myelin is also a potent macrophage stimulus and target of complement-mediated clearance and inflammation. The downstream effects of these neuroimmune interactions have the potential to contribute to ongoing pathology or facilitate repair. This depends in large part on whether myelin drives pathological or reparative macrophage activation states, commonly referred to as M1 (proinflammatory) or M2 (alternatively) macrophages, respectively. Here we review the processes by which myelin debris may be cleared through macrophage surface receptors and the complement system, how this differentially influences macrophage and microglial activation states, and how the cellular functions of these myelin macrophages and complement proteins contribute to chronic inflammation and secondary injury after SCI.

Microglia activation and phagocytosis: relationship with aging and cognitive impairment in the rhesus monkey

While cognitive decline is observed in the normal aging monkey, neurons are not lost with age. Instead, frontal white matter is lost as myelin degenerates and both correlate with age-related cognitive decline. As age-related myelin damage increases, there should be an increase in clearance of damaged myelin by microglial phagocytosis. In this study, brains of behaviorally tested rhesus monkeys were assessed using unbiased stereology to quantify the density of activated microglia (LN3 antibody positive) and phagocytic microglia (galectin-3 (Gal-3) antibody positive) in three white matter regions: the corpus callosum, cingulum bundle (CGB), and frontal white matter (FWM). LN3 cell density was significantly increased in the CGB, whereas Gal-3 cell density was significantly increased in all regions. Increases in Gal-3 cell density in the FWM were associated with cognitive impairment. In the FWM of old animals, Gal-3-positive microglia were classified by morphological subtype as ramified, hypertrophic, or amoeboid. The densities of hypertrophic and amoeboid microglia significantly correlated with cognitive impairment. Finally, microglia were double-labeled with LN3 and Gal-3 showing that 91% of Gal-3 cells were also LN3 positive, thus expressing an "activated" phenotype. Furthermore, 15% of all double-labeled cells formed phagocytic cups. Overall, these results suggest that microglia become activated in white matter with age where the majority express a phagocytic phenotype. We hypothesize that age-related phagocytic activation of microglia is a response to accumulating myelin pathology. The association of Gal-3 in the FWM with cognitive impairment may reflect regional differences in damage or dysfunction of normal clearance mechanisms.

Complex molecular and functional outcomes of single versus sequential cytokine stimulation of rat microglia

BACKGROUND

Microglia are the "professional" phagocytes of the CNS. Phagocytosis is crucial for normal CNS development and maintenance, but it can be either beneficial or detrimental after injury or disease. For instance, white matter damage releases myelin debris that must be cleared by microglia in order for re-myelination to occur. However, phagocytosis can also produce damaging reactive oxygen species (ROS). Furthermore, microglia can acquire pro-inflammatory (M1) or anti-inflammatory (M2) activation states that affect cell functions. Although microglia are exposed to a changing cytokine environment after injury or disease, little is known about the molecular and functional consequences. Therefore, we applied several microglial activation paradigms, with or without myelin debris. We assessed (i) gene expression changes reflecting microglial activation and inflammatory states, and receptors and enzymes related to phagocytosis and ROS production, (ii) myelin phagocytosis and production of ROS, and (iii) expression and contributions of several ion channels that are considered potential targets for regulating microglial behavior.

METHODS

Primary rat microglia were exposed to cytokines, individually or sequentially. First, responses to individual M1 or M2 stimuli were compared: IFN-γ plus TNF-α ("I + T"; M1 activation), interleukin-4 (M2a/alternative activation), and interleukin-10 (M2c/acquired deactivation). Second, sequential cytokine addition was used to assess microglia repolarization and cell functions. The paradigms were M2a→M1, M2c→M1, M1→M2a, and M1→M2c.

RESULTS

M1 stimulation increased pro-inflammatory genes, phagocytosis, and ROS, as well as expression of Kv1.3, KCa3.1, and Kir2.1 channels. M2a stimulation increased anti-inflammatory genes, ROS production, and Kv1.3 and KCa3.1 expression. Myelin phagocytosis enhanced the M1 profile and dampened the M2a profile, and both phagocytosis and ROS production were dependent on NOX enzymes and Kir2.1 and CRAC channels. Importantly, microglia showed some capacity for re-polarization between M1 and M2a states, based on gene expression changes, myelin phagocytosis, and ROS production.

CONCLUSIONS

In response to polarizing and re-polarizing cytokine treatments, microglia display complex changes in gene transcription profiles, phagocytic capacity, NOX-mediated ROS production, and in ion channels involved in microglial activation. Because these changes might affect microglia-mediated CNS inflammation, they should be considered in future experimental, pre-clinical studies.

New Directions in Multiple Sclerosis Therapy: Matching Therapy with Pathogenesis

All currently approved therapies for multiple sclerosis (MS) modulate systemic immune components prior to their entry into the central nervous system (CNS). Available data indicate they lack impact on the progressive phases of disease; the more potent systemic immune-directed agents predispose to development of infectious or neoplastic disorders. Development of new agents that enhance disease stage related efficacy and limit systemic toxicity will need to consider the underlying mechanisms related to each phase of the clinical disorder, namely relapses, remission, and progression. This report focuses on disease related mechanisms ongoing within the CNS that contribute to the different phases of MS and how these may serve as potential therapeutic targets. Such mechanisms include CNS compartment specific immunologic properties especially as related to the innate immune system and neural cell-related properties that are determinants of the extent of actual tissue injury and repair (or lack thereof).

A parasitic helminth-derived peptide that targets the macrophage lysosome is a novel therapeutic option for autoimmune disease

Parasitic worms (helminths) reside in their mammalian hosts for many years. This is attributable, in part, to their ability to skew the host's immune system away from pro-inflammatory responses and towards anti-inflammatory or regulatory responses. This immune modulatory ability ensures helminth longevity within the host, while simultaneously minimises tissue destruction for the host. The molecules that the parasite releases clearly exert potent immune-modulatory actions, which could be exploited clinically, for example in the prophylactic and therapeutic treatment of pro-inflammatory and autoimmune diseases. We have identified a novel family of immune-modulatory proteins, termed helminth defence molecules (HDMs), which are secreted by several medically important helminth parasites. These HDMs share biochemical and structural characteristics with mammalian cathelicidin-like host defence peptides (HDPs), which are significant components of the innate immune system. Like their mammalian counterparts, parasite HDMs block the activation of macrophages via toll like receptor (TLR) 4 signalling, however HDMs are significantly less cytotoxic than HDPs. HDMs can traverse the cell membrane of macrophages and enter the endolysosomal system where they reduce the acidification of lysosomal compartments by inhibiting vacuolar (v)-ATPase activity. In doing this, HDMs can modulate critical cellular functions, such as cytokine secretion and antigen processing/presentation. Here, we review the role of macrophages, specifically their lysosomal mediated activities, in the initiation and perpetuation of pro-inflammatory immune responses. We also discuss the potential of helminth defence molecules (HDMs) as therapeutics to counteract the pro-inflammatory responses underlying autoimmune disease. Given the current lack of effective, non-cytotoxic treatment options to limit the progression of autoimmune pathologies, HDMs open novel treatment avenues.

Macrophage subsets and microglia in multiple sclerosis

Along with microglia and monocyte-derived macrophages, macrophages in the perivascular space, choroid plexus, and meninges are the principal effector cells in neuroinflammatory and neurodegenerative disorders. These phagocytes are highly heterogeneous cells displaying spatial- and temporal-dependent identities in the healthy, injured, and inflamed CNS. In the last decade, researchers have debated on whether phagocytes subtypes and phenotypes are pathogenic or protective in CNS pathologies. In the context of this dichotomy, we summarize and discuss the current knowledge on the spatiotemporal physiology of macrophage subsets and microglia in the healthy and diseased CNS, and elaborate on factors regulating their behavior. In addition, the impact of macrophages present in lymphoid organs on CNS pathologies is defined. The prime focus of this review is on multiple sclerosis (MS), which is characterized by inflammation, demyelination, neurodegeneration, and CNS repair, and in which microglia and macrophages have been extensively scrutinized. On one hand, microglia and macrophages promote neuroinflammatory and neurodegenerative events in MS by releasing inflammatory mediators and stimulating leukocyte activity and infiltration into the CNS. On the other hand, microglia and macrophages assist in CNS repair through the production of neurotrophic factors and clearance of inhibitory myelin debris. Finally, we define how microglia and macrophage physiology can be harnessed for new therapeutics aimed at suppressing neuroinflammatory and cytodegenerative events, as well as promoting CNS repair. We conclude that microglia and macrophages are highly dynamic cells displaying disease stage and location-specific fates in neurological disorders. Changing the physiology of divergent phagocyte subsets at particular disease stages holds promise for future therapeutics for CNS pathologies.

The kinetics of myelin antigen uptake by myeloid cells in the central nervous system during experimental autoimmune encephalomyelitis

Induction of experimental autoimmune encephalomyelitis (EAE) in susceptible animals requires reactivation of encephalitogenic CD4(+) T cells by APCs in the CNS. However, it has remained unresolved from where APCs in the CNS acquire myelin Ag for T cell activation and under which conditions, that is, whether only during EAE or also in the naive CNS. In this study, we investigated the kinetics of myelin Ag uptake by CNS APCs during EAE and in the naive CNS. Our results show that during EAE CX3CR1(+)CD11b(+) microglia were the first APCs in the CNS to contain myelin Ag upon induction of disease, albeit in very small numbers. Dendritic cells (DCs) arrived in the CNS in sizable numbers significantly later (day 5 postimmunization), without detectable myelin Ag, but acquired it by day 7 postimmunization. Furthermore, a sharp increase in neuroantigen-containing DCs coincided with the onset of EAE symptoms. Importantly, in naive mice a low but consistent number of microglia contained myelin Ag, suggesting release by oligodendrocytes under steady state conditions. Although microglia isolated from naive brain and spinal cord did not elicit a strong CD4(+) T cell response in vitro, myelin Ag-containing microglia may still play a local role in modulating encephalitogenic CD4(+) T cell responses in early EAE prior to the arrival of other professional APCs, such as DCs. Finally, newly arriving DCs in the CNS not yet loaded with myelin Ag before the onset of EAE may be a potential therapeutic target.

Myelin alters the inflammatory phenotype of macrophages by activating PPARs

Background

Foamy macrophages, containing myelin degradation products, are abundantly found in active multiple sclerosis (MS) lesions. Recent studies have described an altered phenotype of macrophages after myelin internalization. However, mechanisms by which myelin affects the phenotype of macrophages and how this phenotype influences lesion progression remain unclear.

Results

We demonstrate that myelin as well as phosphatidylserine (PS), a phospholipid found in myelin, reduce nitric oxide production by macrophages through activation of peroxisome proliferator-activated receptor β/δ (PPARβ/δ). Furthermore, uptake of PS by macrophages, after intravenous injection of PS-containing liposomes (PSLs), suppresses the production of inflammatory mediators and ameliorates experimental autoimmune encephalomyelitis (EAE), an animal model of MS. The protective effect of PSLs in EAE animals is associated with a reduced immune cell infiltration into the central nervous system and decreased splenic cognate antigen specific proliferation. Interestingly, PPARβ/δ is activated in foamy macrophages in active MS lesions, indicating that myelin also activates PPARβ/δ in macrophages in the human brain.

Conclusion

Our data show that myelin modulates the phenotype of macrophages by PPAR activation, which may subsequently dampen MS lesion progression. Moreover, our results suggest that myelin-derived PS mediates PPARβ/δ activation in macrophages after myelin uptake. The immunoregulatory impact of naturally-occurring myelin lipids may hold promise for future MS therapeutics.

Polarization of macrophages and microglia in inflammatory demyelination

Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system, and microglia and macrophages play important roles in its pathogenesis. The activation of microglia and macrophages accompanies disease development, whereas depletion of these cells significantly decreases disease severity. Microglia and macrophages usually have diverse and plastic phenotypes. Both pro-inflammatory and antiinflammatory microglia and macrophages exist in MS and its animal model, experimental autoimmune encephalomyelitis. The polarization of microglia and macrophages may underlie the differing functional properties that have been reported. in this review, we discuss the responses and polarization of microglia and macrophages in MS, and their effects on its pathogenesis and repair. Harnessing their beneficial effects by modulating their polarization states holds great promise for the treatment of inflammatory demyelinating diseases.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Myelin-derived lipids modulate macrophage activity by liver X receptor activation

Multiple sclerosis is a chronic, inflammatory, demyelinating disease of the central nervous system in which macrophages and microglia play a central role. Foamy macrophages and microglia, containing degenerated myelin, are abundantly found in active multiple sclerosis lesions. Recent studies have described an altered macrophage phenotype after myelin internalization. However, it is unclear by which mechanisms myelin affects the phenotype of macrophages and how this phenotype can influence lesion progression. Here we demonstrate, by using genome wide gene expression analysis, that myelin-phagocytosing macrophages have an enhanced expression of genes involved in migration, phagocytosis and inflammation. Interestingly, myelin internalization also induced the expression of genes involved in liver-X-receptor signaling and cholesterol efflux. In vitro validation shows that myelin-phagocytosing macrophages indeed have an increased capacity to dispose intracellular cholesterol. In addition, myelin suppresses the secretion of the pro-inflammatory mediator IL-6 by macrophages, which was mediated by activation of liver-X-receptor β. Our data show that myelin modulates the phenotype of macrophages by nuclear receptor activation, which may subsequently affect lesion progression in demyelinating diseases such as multiple sclerosis.

BLBP-expression in astrocytes during experimental demyelination and in human multiple sclerosis lesions

Several lines of evidence indicate that remyelination represents one of the most effective mechanisms to achieve axonal protection. For reasons that are not yet understood, this process is often incomplete or fails in multiple sclerosis (MS). Activated astrocytes appear to be able to boost or inhibit endogenous repair processes. A better understanding of remyelination in MS and possible reasons for its failure is needed. Using the well-established toxic demyelination cuprizone model, we created lesions with either robust or impaired endogenous remyelination capacity. Lesions were analyzed for mRNA expression levels by Affymetrix GeneChip® arrays. One finding was the predominance of immune and stress response factors in the group of genes which were classified as remyelination-supporting factors. We further demonstrate that lesions with impaired remyelination capacity show weak expression of the radial-glia cell marker brain lipid binding protein (BLBP, also called B-FABP or FABP7). The expression of BLBP in activated astrocytes correlates with the presence of oligodendrocyte progenitor cells. BLBP-expressing astrocytes are also detected in experimental autoimmune encephalomyelitis during the remission phase. Furthermore, highest numbers of BLBP-expressing astrocytes were evident in lesions of early MS, whereas significantly less are present at the rim of (chronic)-active lesions from patients with long disease duration. Transfection experiments show that BLBP regulates growth factor expression in U87 astrocytoma cells. In conclusion, we provide evidence that expression of BLBP in activated astrocytes negatively correlates with disease duration and in parallel with remyelination failure.

Myelin-phagocytosing macrophages modulate autoreactive T cell proliferation

Introduction

Multiple sclerosis (MS) is a chronic, inflammatory, demyelinating disease of the central nervous system (CNS) in which macrophages play a central role. Initially, macrophages where thought to be merely detrimental in MS, however, recent evidence suggests that their functional phenotype is altered following myelin phagocytosis. Macrophages that have phagocytosed myelin may be less inflammatory and may exert beneficial effects. The presence of myelin-containing macrophages in CNS-draining lymph nodes and perivascular spaces of MS patients suggests that these cells are ideally positioned to exert an immune regulatory role. Therefore we evaluated in this study the effect of myelin-phagocytosing macrophages on lymphocyte reactivity.

Methods

Thioglycolate-elicited rat peritoneal macrophages were loaded with myelin and cocultured with myelin-basic protein (MBP) or ovalbumin (OVA) reactive lymphocytes. Lymphocyte proliferation was determined by CFSE-labeling. The role of nitric oxide in regulating lymphocyte proliferation was assessed by addition of an inhibitor of inducible nitric oxide synthase to the coculture. In vivo immune regulation was investigated by treating MBP- and OVA-immunized animals subcutaneously with myelin. Cognate antigen specific lymphocyte proliferation and nitric oxide production were determined 9d post-immunization.

Results

In this study we demonstrate that myelin-phagocytosing macrophages inhibit TCR-triggered lymphocyte proliferation in an antigen-independent manner. The observed immune suppression is mediated by an increase in NO production by myelin-phagocytosing macrophages upon contact with lymphocytes. Additionally, myelin delivery to primarily CD169⁺ macrophages in popliteal lymph nodes of OVA-immunized animals results in a reduced cognate antigen specific proliferation. In contrast to OVA-immunized animals, lymphocytes from MBP-immunized animals displayed an increased proliferation after stimulation with their cognate antigen, indicating that myelin-phagocytosing macrophages have dual effects depending on the specificity of surrounding lymphocytes.

Conclusions

Collectively our data show that myelin phagocytosis leads to an altered macrophage function that inhibits lymphocyte proliferation. Additionally, results from this study indicate that myelin-phagocytosing macrophages fulfill a dual role in vivo. On one hand they aggravate autoimmunity by activating myelin-reactive lymphocytes and on the other hand they suppress lymphocyte reactivity by producing NO.

The role of antigen presenting cells in multiple sclerosis

Multiple sclerosis (MS) is a debilitating T cell mediated autoimmune disease of the central nervous system (CNS). Animal models of MS, such as experimental autoimmune encephalomyelitis (EAE) and Theiler's murine encephalomyelitis virus-induced demyelinating disease (TMEV-IDD) have given light to cellular mechanisms involved in the initiation and progression of this organ-specific autoimmune disease. Within the CNS, antigen presenting cells (APC) such as microglia and astrocytes participate as first line defenders against infections or inflammation. However, during chronic inflammation they can participate in perpetuating the self-destructive environment by secretion of inflammatory factors and/or presentation of myelin epitopes to autoreactive T cells. Dendritic cells (DC) are also participants in the presentation of antigen to T cells, even within the CNS. While the APCs alone are not solely responsible for mediating the destruction to the myelin sheath, they are critical players in perpetuating the inflammatory milieu. This review will highlight relevant studies which have provided insight to the roles played by microglia, DCs and astrocytes in the context of CNS autoimmunity.

Dimethylfumarate inhibits microglial and astrocytic inflammation by suppressing the synthesis of nitric oxide, IL-1beta, TNF-alpha and IL-6 in an in-vitro model of brain inflammation

BACKGROUND

Brain inflammation plays a central role in multiple sclerosis (MS). Dimethylfumarate (DMF), the main ingredient of an oral formulation of fumaric acid esters with proven therapeutic efficacy in psoriasis, has recently been found to ameliorate the course of relapsing-remitting MS. Glial cells are the effector cells of neuroinflammation; however, little is known of the effect of DMF on microglia and astrocytes. The purpose of this study was to use an established in vitro model of brain inflammation to determine if DMF modulates the release of neurotoxic molecules from microglia and astrocytes, thus inhibiting glial inflammation.

METHODS

Primary microglial and astrocytic cell cultures were prepared from cerebral cortices of neonatal rats. The control cells were treated with LPS, an accepted inducer of pro-inflammatory properties in glial cells, and the experimental groups with LPS and DMF in different concentrations. After stimulation/incubation, the generation of nitric oxide (NO) in the cell culture supernatants was determined by measuring nitrite accumulation in the medium using Griess reagent. After 6 hours of treatment RT-PCR was used to determine transcription levels of iNOS, IL-1beta, IL-6 and TNF-alpha mRNA in microglial and astrocytic cell cultures initially treated with DMF, followed after 30 min by LPS treatment. Moreover, we investigated possible involvement of the ERK and Nrf-2 transduction pathway in microglia using western blot analysis.

RESULTS

Pretreatment with DMF decreased synthesis of the proinflammatory mediators iNOS, TNF-alpha, IL-1beta and IL-6 at the RNA level in activated microglia and astrocytes in vitro, associated with a decrease in ERK phosphorylation in microglia.

CONCLUSIONS

Collectively, these results suggest that the neuroprotective effects of DMF may be in part functionally attributable to the compound's ability to inhibit expression of multiple neuroinflammatory mediators in brain of MS patients.

Immune-mediated CNS damage

Multiple sclerosis (MS) is a demyelinating autoimmune disease. However, the persisting neurological deficits in MS patients result from acute axonal injury and chronic neurodegeneration, which are both triggered by the autoreactive immune response. Innate immunity, mainly mediated by activated microglial cells and invading macrophages, appears to contribute to chronic neurodegeneration. Activated microglia produce several reactive oxygen species and proinflammatory cytokines which affect neuronal function, integrity and survival. Adaptive immunity, particularly in cytotoxic CD8+ T cells, participates in acute demyelination and axonal injury by directly attacking oligodendrocytes and possibly neurons as well. Understanding the mechanisms of immune-mediated neuronal damage might help to design novel therapy strategies for MS.

Myelin-induced microglial neurotoxicity can be controlled by microglial metabotropic glutamate receptors

Microglia are present in an activated state in multiple sclerosis lesions. Incubation of primary cultured rat microglia with rat-brain derived myelin (0.1-1 microg/mL) for 24 h induced microglial activation; cells displayed enhanced ED1 staining, expression of inducible nitric oxide synthase, production and release of the cytokine tumour necrosis factor-alpha and glutamate release. Exposure of microglia to myelin induced the expression of neuronal caspases and ultimately neuronal death in cultured cerebellar granule cell neurons; neurotoxicity was directly because of microglial-derived soluble toxins. Co-incubation of microglia with agonists or antagonists of different metabotropic glutamate receptor (mGluR) subtypes ameliorated microglial neurotoxicity by inhibiting soluble neurotoxin production. Activation of microglial mGluR2 exacerbated myelin-evoked neurotoxicity whilst activation of mGluR3 was protective as was activation of group III mGluRs. These data show that myelin-induced microglial neurotoxicity can be prevented by regulation of mGluRs and suggest these receptors on microglia may be promising targets for therapeutic intervention in multiple sclerosis.

Psychosine-induced alterations in peroxisomes of twitcher mouse liver

Krabbe disease is a neuroinflammatory disorder in which galactosylsphingosine (psychosine) accumulates in nervous tissue. To gain insight into whether the psychosine-induced effects in nervous tissue extend to peripheral organs, we investigated the expression of cytokines and their effects on peroxisomal structure/functions in twitcher mouse liver (animal model of Krabbe disease). Immunofluorescence analysis demonstrated TNF-alpha and IL-6 expression, which was confirmed by mRNAs quantitation. Despite the presence of TNF-alpha, lipidomic analysis did not indicate a significant decrease in sphingomyelin or an increase in ceramide fractions. Ultrastructural analysis of catalase-dependent staining of liver sections showed reduced reactivity without significant changes in peroxisomal contents. This observation was confirmed by assaying catalase activity and quantitation of its mRNA, both of which were found significantly decreased in twitcher mouse liver. Western blot analysis demonstrated a generalized reduction of peroxisomal matrix and membrane proteins. These observations indicate that twitcher mouse pathobiology extends to the liver, where psychosine-induced TNF-alpha and IL-6 compromise peroxisomal structure and functions.

Clinical and immunopathologic alterations in rhesus macaques affected with globoid cell leukodystrophy

Globoid cell leukodystrophy, or Krabbe's disease, is a severe disorder of the central and peripheral nervous system caused by the absence of galactocerebrosidase (GALC) activity. Herein, we describe the clinical, neuropathological, histochemical, and immunohistological features observed in rhesus macaques affected with Krabbe's disease. Clinical signs included pronounced muscle tremors of head and limbs, difficulty ambulating, ataxia, hypermetria, proprioceptive deficits, and respiratory abnormalities. Histopathologically, all animals presented with evidence of demyelination in the peripheral and central nervous systems and accumulation of mononuclear and multinuclear globoid cells in the cerebral and cerebellar white matter associated with severe gliosis. Using immunohistochemistry and multi-label confocal microscopy, it was determined that globoid cells were CD68+, HAM56+, LN5+, CD163+, IBA-1+, and Glut-5+, suggesting that both peripheral blood-derived monocytes/macrophages and resident parenchymal microglia gave rise to globoid cells. Interestingly, many of the globoid cells and parenchymal microglia with a more ameboid morphology expressed HLA-DR, indicating immune activation. Increased expression of iNOS, TNF-alpha, and IL-1 beta were observed in the affected white matter, colocalizing with globoid cells, activated microglia, and astrocytes. Cytokine mRNA levels revealed markedly increased gene expression of CCL2 in the brain of affected macaques. CCL2-expressing cells were detected throughout the affected white matter, colocalizing with GFAP+ cells and astrocytes. Collectively, these data suggest that dysregulation of monocyte/macrophage/microglia and up-regulation of certain cytokines may contribute to the pathogenesis of Krabbe's disease.

Elevated activity and microglial expression of myeloperoxidase in demyelinated cerebral cortex in multiple sclerosis

Recent studies have revealed extensive cortical demyelination in patients with progressive multiple sclerosis (MS). Demyelination in gray matter lesions is associated with activation of microglia. Macrophages and microglia are known to express myeloperoxidase (MPO) and generate reactive oxygen species during myelin phagocytosis in the white matter. In the present study we examined the extent of microglial activation in the cerebral cortex and the relationship of microglial activation and MPO activity to cortical demyelination. Twenty-one cases of neuropathologically confirmed multiple sclerosis, with 34 cortical lesions, were used to assess microglial activation. HLA-DR immunolabeling of activated microglia was significantly higher in demyelinated MS cortex than control cortex and, within the MS cohort, was significantly greater within cortical lesions than in matched non-demyelinated areas of cortex. In homogenates of MS cortex, cortical demyelination was associated with significantly elevated MPO activity. Immunohistochemistry revealed MPO in CD68-positive microglia within cortical plaques, particularly toward the edge of the plaques, but not in microglia in adjacent non-demyelinated cortex. Cortical demyelination in MS is associated with increased activity of MPO, which is expressed by a CD68-positive subset of activated microglia, suggesting that microglial production of reactive oxygen species is likely to be involved in cortical demyelination.

Cannabinoid CB2 receptors in human brain inflammation

The presence of functional cannabinoid CB2 receptors in the CNS has provoked considerable controversy over the past few years. Formerly considered as an exclusively peripheral receptor, it is now accepted that it is also present in limited amounts and distinct locations in the brain of several animal species, including humans. Furthermore, the inducible nature of these receptors under neuroinflammatory conditions, in contrast to CB1, makes them attractive targets for the development of novel therapeutic approaches. In fact, the undesired psychoactive effects caused by CB1 activation have largely limited the clinical use of cannabinoid-related compounds that act on these receptors. In this review some recent findings on the antiinflammatory properties of CB2 receptors are presented, as well as new perspectives that have been obtained based on studies of human postmortem brain samples. In addition, various working hypotheses are also proposed and discussed.