Microglial M1/M2 polarization and metabolic states

Influence of extracellular zinc on M1 microglial activation

Extracellular zinc, which is released from hippocampal neurons in response to brain ischaemia, triggers morphological changes in microglia. Under ischaemic conditions, microglia exhibit two opposite activation states (M1 and M2 activation), which may be further regulated by the microenvironment. We examined the role of extracellular zinc on M1 activation of microglia. Pre-treatment of microglia with 30–60 μM ZnCl₂ resulted in dose-dependent increases in interleukin-1 beta (IL-1β), interleukin-6 (IL-6), and tumour necrosis factor-alpha (TNFα) secretion when M1 activation was induced by lipopolysaccharide administration. In contrast, the cell-permeable zinc chelator TPEN, the radical scavenger Trolox, and the P2X7 receptor antagonist A438079 suppressed the effects of zinc pre-treatment on microglia. Furthermore, endogenous zinc release was induced by cerebral ischaemia–reperfusion, resulting in increased expression of IL-1β, IL-6, TNFα, and the microglial M1 surface marker CD16/32, without hippocampal neuronal cell loss, in addition to impairments in object recognition memory. However, these effects were suppressed by the zinc chelator CaEDTA. These findings suggest that extracellular zinc may prime microglia to enhance production of pro-inflammatory cytokines via P2X7 receptor activation followed by reactive oxygen species generation in response to stimuli that trigger M1 activation, and that these inflammatory processes may result in deficits in object recognition memory.

Methylglyoxal-Derived Advanced Glycation Endproducts in Multiple Sclerosis

Multiple sclerosis (MS) is a demyelinating disease of the central nervous system (CNS). The activation of inflammatory cells is crucial for the development of MS and is shown to induce intracellular glycolytic metabolism in pro-inflammatory microglia and macrophages, as well as CNS-resident astrocytes. Advanced glycation endproducts (AGEs) are stable endproducts formed by a reaction of the dicarbonyl compounds methylglyoxal (MGO) and glyoxal (GO) with amino acids in proteins, during glycolysis. This suggests that, in MS, MGO-derived AGEs are formed in glycolysis-driven cells. MGO and MGO-derived AGEs can further activate inflammatory cells by binding to the receptor for advanced glycation endproducts (RAGE). Recent studies have revealed that AGEs are increased in the plasma and brain of MS patients. Therefore, AGEs might contribute to the inflammatory status in MS. Moreover, the main detoxification system of dicarbonyl compounds, the glyoxalase system, seems to be affected in MS patients, which may contribute to high MGO-derived AGE levels. Altogether, evidence is emerging for a contributing role of AGEs in the pathology of MS. In this review, we provide an overview of the current knowledge on the involvement of AGEs in MS.

Distinct Effects of miR-210 Reduction on Neurogenesis: Increased Neuronal Survival of Inflammation But Reduced Proliferation Associated with Mitochondrial Enhancement

Neurogenesis is essential to brain development and plays a central role in the response to brain injury. Stroke and head trauma stimulate proliferation of endogenous neural stem cells (NSCs); however, the survival of young neurons is sharply reduced by postinjury inflammation. Cellular mitochondria are critical to successful neurogenesis and are a major target of inflammatory injury. Mitochondrial protection was shown to improve survival of young neurons. This study tested whether reducing cellular microRNA-210 (miR-210) would enhance mitochondrial function and improve survival of young murine neurons under inflammatory conditions. Several studies have demonstrated the potential of miR-210 inhibition to enhance and protect mitochondrial function through upregulation of mitochondrial proteins. Here, miR-210 inhibition significantly increased neuronal survival and protected the activity of mitochondrial enzymes cytochrome c oxidase and aconitase in differentiating NSC cultures exposed to inflammatory mediators. Unexpectedly, we found that reducing miR-210 significantly attenuated NSC proliferation upon induction of differentiation. Further investigation revealed that increased mitochondrial function suppressed the shift to primarily glycolytic metabolism and reduced mitochondrial length characteristic of dividing cells. Activation of AMP-regulated protein kinase-retinoblastoma signaling is important in NSC proliferation and the reduction of this activation observed by miR-210 inhibition is one mechanism contributing to the reduced proliferation. Postinjury neurogenesis occurs as a burst of proliferation that peaks in days, followed by migration and differentiation over weeks. Our studies suggest that mitochondrial protective miR-210 inhibition should be delayed until after the initial burst of proliferation, but could be beneficial during the prolonged differentiation stage.SIGNIFICANCE STATEMENT Increasing the success of endogenous neurogenesis after brain injury holds therapeutic promise. Postinjury inflammation markedly reduces newborn neuron survival. This study found that enhancement of mitochondrial function by reducing microRNA-210 (miR-210) levels could improve survival of young neurons under inflammatory conditions. miR-210 inhibition protected the activity of mitochondrial enzymes cytochrome c oxidase and aconitase. Conversely, we observed decreased precursor cell proliferation likely due to suppression of the AMP-regulated protein kinase-retinoblastoma axis with miR-210 inhibition. Therefore, mitochondrial protection is a double-edged sword: early inhibition reduces proliferation, but inhibition later significantly increases neuroblast survival. This explains in part the contradictory published reports of the effects of miR-210 on neurogenesis.

Microglial Function during Glucose Deprivation: Inflammatory and Neuropsychiatric Implications

Inflammation is increasingly recognized as a contributor to the pathophysiology of neuropsychiatric disorders, including depression, anxiety disorders and autism, though the factors leading to contextually inappropriate or sustained inflammation in pathological conditions are yet to be elucidated. Microglia, as the key mediators of inflammation in the CNS, serve as likely candidates in initiating pathological inflammation and as an ideal point of therapeutic intervention. Glucose deprivation, as a component of the pathophysiology of ischemia or occurring transiently in diabetes, may serve to modify microglial function contributing to inflammatory injury. To this end, primary microglia were cultured from postnatal rat brain and subject to glucose deprivation in vitro. Microglia were characterized for their proliferation, phagocytic function and secretion of inflammatory factors, and tested for their capacity to respond to a potent inflammatory stimulus. In the absence of glucose, microglia remained capable of proliferation, phagocytosis and inflammatory activation and showed increased release of inflammatory factors after presentation of an inflammatory stimulus. Glucose-deprived microglia demonstrated increased phagocytic activity and decreased accumulation of lipids in lipid droplets over a 48-h timecourse, suggesting they may use scavenged lipids as a key alternate energy source during metabolic stress. In the present manuscript, we present novel findings that glucose deprivation may sensitize microglial release of inflammatory mediators and prime microglial functions for both survival and inflammatory roles, which may contribute to psychiatric comorbidities of ischemia, diabetes and/or metabolic disorder.

Modulation of Microglial Cell Fcγ Receptor Expression Following Viral Brain Infection

Fcγ receptors (FcγRs) for IgG couple innate and adaptive immunity through activation of effector cells by antigen-antibody complexes. We investigated relative levels of activating and inhibitory FcγRs on brain-resident microglia following murine cytomegalovirus (MCMV) infection. Flow cytometric analysis of microglial cells obtained from infected brain tissue demonstrated that activating FcγRs were expressed maximally at 5 d post-infection (dpi), while the inhibitory receptor (FcγRIIB) remained highly elevated during both acute and chronic phases of infection. The highly induced expression of activating FcγRIV during the acute phase of infection was also noteworthy. Furthermore, in vitro analysis using cultured primary microglia demonstrated the role of interferon (IFN)γ and interleukin (IL)-4 in polarizing these cells towards a M1 or M2 phenotype, respectively. Microglial cell-polarization correlated with maximal expression of either FcγRIV or FcγRIIB following stimulation with IFNγ or IL-4, respectively. Finally, we observed a significant delay in polarization of microglia towards an M2 phenotype in the absence of FcγRs in MCMV-infected Fcer1g and FcgR2b knockout mice. These studies demonstrate that neuro-inflammation following viral infection increases expression of activating FcγRs on M1-polarized microglia. In contrast, expression of the inhibitory FcγRIIB receptor promotes M2-polarization in order to shut-down deleterious immune responses and limit bystander brain damage.

Psychosocial stress on neuroinflammation and cognitive dysfunctions in Alzheimer's disease: the emerging role for microglia?

Chronic psychosocial stress is increasingly recognized as a risk factor for late-onset Alzheimer's disease (LOAD) and associated cognitive deficits. Chronic stress also primes microglia and induces inflammatory responses in the adult brain, thereby compromising synapse-supportive roles of microglia and deteriorating cognitive functions during aging. Substantial evidence demonstrates that failure of microglia to clear abnormally accumulating amyloid-beta (Aβ) peptide contributes to neuroinflammation and neurodegeneration in AD. Moreover, genome-wide association studies have linked variants in several immune genes, such as TREM2 and CD33, the expression of which in the brain is restricted to microglia, with cognitive dysfunctions in LOAD. Thus, inflammation-promoting chronic stress may create a vicious cycle of aggravated microglial dysfunction accompanied by increased Aβ accumulation, collectively exacerbating neurodegeneration. Surprisingly, however, little is known about whether and how chronic stress contributes to microglia-mediated neuroinflammation that may underlie cognitive impairments in AD. This review aims to summarize the currently available clinical and preclinical data and outline potential molecular mechanisms linking stress, microglia and neurodegeneration, to foster future research in this field.

The Polarization States of Microglia in TBI: A New Paradigm for Pharmacological Intervention

Traumatic brain injury (TBI) is a serious medical and social problem worldwide. Because of the complex pathophysiological mechanisms of TBI, effective pharmacotherapy is still lacking. The microglial cells are resident tissue macrophages located in the brain and have two major polarization states, M1 phenotype and M2 phenotype, when activated. The M1 phenotype is related to the release of proinflammatory cytokines and secondary brain injury, while the M2 phenotype has been proved to be responsible for the release of anti-inflammation cytokines and for central nervous system (CNS) repair. In animal models, pharmacological strategies inhibiting the M1 phenotype and promoting the M2 phenotype of microglial cells could alleviate cerebral damage and improve neurological function recovery after TBI. In this review, we aimed to summarize the current knowledge about the pathological significance of microglial M1/M2 polarization in the pathophysiology of TBI. In addition, we reviewed several drugs that have provided neuroprotective effects against brain injury following TBI by altering the polarization states of the microglia. We emphasized that future investigation of the regulation mechanisms of microglial M1/M2 polarization in TBI is anticipated, which could contribute to the development of new targets of pharmacological intervention in TBI.

Iso-α-acids, Bitter Components of Beer, Prevent Inflammation and Cognitive Decline Induced in a Mouse Model of Alzheimer's Disease

Alongside the rapid growth in aging populations worldwide, prevention and therapy for age-related memory decline and dementia are in great demand to maintain a long, healthy life. Here we found that iso-α-acids, hop-derived bitter compounds in beer, enhance microglial phagocytosis and suppress inflammation via activation of the peroxisome proliferator-activated receptor γ. In normal mice, oral administration of iso-α-acids led to a significant increase both in CD11b and CD206 double-positive anti-inflammatory type microglia (p < 0.05) and in microglial phagocytosis in the brain. In Alzheimer's model 5xFAD mice, oral administration of iso-α-acids resulted in a 21% reduction in amyloid β in the cerebral cortex as observed by immunohistochemical analysis, a significant reduction in inflammatory cytokines such as IL-1β and chemokines including macrophage inflammatory protein-1α in the cerebral cortex (p < 0.05) and a significant improvement in a novel object recognition test (p < 0.05), as compared with control-fed 5xFAD mice. The differences in iso-α-acid-fed mice were due to the induction of microglia to an anti-inflammatory phenotype. The present study is the first to report that amyloid β deposition and inflammation are suppressed in a mouse model of Alzheimer's disease by a single component, iso-α-acids, via the regulation of microglial activation. The suppression of neuroinflammation and improvement in cognitive function suggests that iso-α-acids contained in beer may be useful for the prevention of dementia.

Paliperidone reverts Toll-like receptor 3 signaling pathway activation and cognitive deficits in a maternal immune activation mouse model of schizophrenia

The pathophysiology of psychotic disorders is multifactorial, including alterations in the immune system caused by exogenous or endogenous factors. Epidemiological and experimental studies indicate that infections during the gestational period represent a risk factor to develop schizophrenia (SZ) along lifetime. Here, we tested the hypothesis that the antipsychotic paliperidone regulates immune-related brain effects in an experimental model of SZ. A well described prenatal immune activation model of SZ in mice by maternal injection of the viral mimetic poly(I:C) during pregnancy was used. Young-adult offspring animals (60PND) received paliperidone ip (0.05 mg/kg) for 21 consecutive days. One day after last injection, animals were submitted to a cognitive test and brain frontal cortex (FC) samples were obtained for biochemical determinations. The adults showed an activated innate immune receptor TLR-3 signaling pathway, oxidative/nitrosative stress and accumulation of pro-inflammatory mediators such as nuclear transcription factors (i.e., NFκB) and inducible enzymes (i.e., iNOS) in FC. Chronic paliperidone blocked this neuroinflammatory response possibly by the synergic activation and preservation of endogenous antioxidant/anti-inflammatory mechanisms such as NRF2 and PPARγ pathways, respectively. Paliperidone administration also stimulated the alternative polarization of microglia to the M2 anti-inflammatory profile. In addition, paliperidone treatment improved spatial working memory deficits of this SZ-like animal model. In conclusion, chronic administration of paliperidone to young-adult mice prenatally exposed to maternal immune (MIA) challenge elicits a general preventive anti-inflammatory/antioxidant effect at both intracellular and cellular polarization (M1/M2) level in FC, as well as ameliorates specific cognitive deficits.

Inflammation: maladies, models, mechanisms and molecules

The continued focus of attention on the diversity of mechanisms underpinning inflammation has improved our understanding of the potential to target specific pathways in the inflammatory network to achieve meaningful therapeutic gains. In this themed issue of the British Journal of Pharmacology our scope was deliberately broad, ranging across both acute and chronic disease in various organs. Pro- and anti-inflammatory mechanisms receive attention as does the phenotype of macrophages. Whilst the manifestations of neuro-inflammation are less obvious than those in peripheral tissues, central innate and adaptive immunity in brain and the M1/M2 phenotypes of microglia are topics of special interest. The contributions to the inflammatory milieu of cytokines, chemokines and associated signalling cascades are considered. Overall, the coverage herein advances the basic science underpinning our understanding of inflammation and emphasizes its importance in different pathologies.

Microglia in Physiology and Disease

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

Protective Effects of α-Tocopherol, γ-Tocopherol and Oleic Acid, Three Compounds of Olive Oils, and No Effect of Trolox, on 7-Ketocholesterol-Induced Mitochondrial and Peroxisomal Dysfunction in Microglial BV-2 Cells

Lipid peroxidation products, such as 7-ketocholesterol (7KC), may be increased in the body fluids and tissues of patients with neurodegenerative diseases and trigger microglial dysfunction involved in neurodegeneration. It is therefore important to identify synthetic and natural molecules able to impair the toxic effects of 7KC. We determined the impact of 7KC on murine microglial BV-2 cells, especially its ability to trigger mitochondrial and peroxisomal dysfunction, and evaluated the protective effects of α- and γ-tocopherol, Trolox, and oleic acid (OA). Multiple complementary chemical assays, flow cytometric and biochemical methods were used to evaluate the antioxidant and cytoprotective properties of these molecules. According to various complementary assays to estimate antioxidant activity, only α-, and γ-tocopherol, and Trolox had antioxidant properties. However, only α-tocopherol, γ-tocopherol and OA were able to impair 7KC-induced loss of mitochondrial transmembrane potential, which is associated with increased permeability to propidium iodide, an indicator of cell death. In addition, α-and γ-tocopherol, and OA were able to prevent the decrease in Abcd3 protein levels, which allows the measurement of peroxisomal mass, and in mRNA levels of Abcd1 and Abcd2, which encode for two transporters involved in peroxisomal β-oxidation. Thus, 7KC-induced side effects are associated with mitochondrial and peroxisomal dysfunction which can be inversed by natural compounds, thus supporting the hypothesis that the composition of the diet can act on the function of organelles involved in neurodegenerative diseases.

Neuroinflammation Induces Neurodegeneration

Neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), and Multiple Sclerosis (MS) are characterized by neuronal degeneration and neuronal death in specific regions of the central nervous system (CNS). In AD, neurons of the hippocampus and entorhinal cortex are the first to degenerate, whereas in PD, dopaminergic neurons in the substantia nigra degenerate. MS patients show destruction of the myelin sheath. Once the CNS neurons are damaged, they are unable to regenerate unlike any other tissue in the body. Neurodegeneration is mediated by inflammatory and neurotoxic mediators such as interleukin-1beta (IL-1β), IL-6, IL-8, IL-33, tumor necrosis factor-alpha (TNF-α), chemokine (C-C motif) ligand 2 (CCL2), CCL5, matrix metalloproteinase (MMPs), granulocyte macrophage colony-stimulating factor (GM-CSF), glia maturation factor (GMF), substance P, reactive oxygen species (ROS), reactive nitrogen species (RNS), mast cells-mediated histamine and proteases, protease activated receptor-2 (PAR-2), CD40, CD40L, CD88, intracellular Ca⁺ elevation, and activation of mitogen-activated protein kinases (MAPKs) and nuclear factor kappa-B (NF-kB). Activated microglia, astrocytes, neurons, T-cells and mast cells release these inflammatory mediators and mediate neuroinflammation and neurodegeneration in a vicious manner. Further, immune and inflammatory cells and inflammatory mediators from the periphery cross the defective blood-brain-barrier (BBB) and augment neuroinflammation. Though inflammation is crucial in the onset and the progression of neurodegenerative diseases, anti-inflammatory drugs do not provide significant therapeutic effects in these patients till date, as the disease pathogenesis is not yet clearly understood. In this review, we discuss the possible factors involved in neuroinflammation-mediated neurodegeneration.

MFG-E8 Selectively Inhibited Aβ-Induced Microglial M1 Polarization via NF-κB and PI3K-Akt Pathways

Activated microglia are classified into two specific states: classically activated (M1) and alternatively activated (M2) subtypes. It is believed that the polarization of M1/M2 phenotype plays an important role in Alzheimer's disease (AD). However, the mechanisms regulating this process remain unclear. Thus, we addressed this question focusing on milk fat globule epidermal growth factor 8 (MFG-E8). MFG-E8 is a unique protein which can bind to microglia and regulate its inflammatory responses. It is speculated that it might play a role in the balance of microglial polarization. In the current study, we used fibril Aβ42 in vitro to stimulate mouse primary microglial cultures and found subsequent M1 marker expression, along with retained M2 marker production. Then, we discovered that MFG-E8 pretreatment reversed the increased trend of M1 markers and the decreased expression of M2 markers, which were induced by Aβ42. Moreover, MFG-E8 effects could be effectively blocked by an MFG-E8 antibody. Further analysis on the signaling pathways showed that NF-κB upregulation and Akt downregulation in microglial cultures were observed after Aβ42 incubation. And the alteration of these pathways could also be reversed by MFG-E8. We then assessed the effects of NF-κB and PI3K-Akt on M1/M2 alteration using their specific inhibitors. Pyrrolidine dithiocarbamate, a NF-κB inhibitor, inhibited M1 marker expression; moreover, LY294002, an Akt inhibitor, enhanced M1 marker expression. Our study indicated the regulatory role of MFG-E8 on microglia M1/M2 alteration for the first time, providing a basis for understanding the potential role of microglia activation in AD.

NADPH oxidase isoform expression is temporally regulated and may contribute to microglial/macrophage polarization after spinal cord injury

Spinal cord injury (SCI) results in both acute and chronic inflammation, as a result of activation of microglia, invasion of macrophages and activation of the NADPH oxidase (NOX) enzyme. The NOX enzyme is a primary source of reactive oxygen species (ROS) and is expressed by microglia and macrophages after SCI. These cells can assume either a pro- (M1) or anti-inflammatory (M2) polarization phenotype and contribute to tissue response to SCI. However, the contribution of NOX expression and ROS production to this polarization and vice versa is currently undefined. We therefore investigated the impact of SCI on NOX expression and microglial/macrophage polarization over time in a mouse model of contusion injury. Adult C57Bl/6 mice were exposed to a moderate T9 contusion SCI and tissue was assessed at acute, sub-acute and chronic time points for NOX isoform expression and co-expression with M1 and M2 microglia/macrophage polarization markers. Two NOX isoforms were increased after injury and were associated with both M1 and M2 markers, with an M1 preference for NOX2 acutely and NOX4 chronically. M2 cells were primarily found at acute time points only; the peak of NOX2 expression was associated with the decline in M2 polarization. In vitro, NOX2 inhibition shifted microglial polarization toward the M2 phenotype. These results now show that microglial/macrophage expression of NOX isoforms is independent of polarization state, but that NOX activity can influence subsequent polarization. These data can contribute to the therapeutic targeting of NOX as a therapy for SCI.

Interleukin-13 immune gene therapy prevents CNS inflammation and demyelination via alternative activation of microglia and macrophages

Detrimental inflammatory responses in the central nervous system are a hallmark of various brain injuries and diseases. With this study we provide evidence that lentiviral vector-mediated expression of the immune-modulating cytokine interleukin 13 (IL-13) induces an alternative activation program in both microglia and macrophages conferring protection against severe oligodendrocyte loss and demyelination in the cuprizone mouse model for multiple sclerosis (MS). First, IL-13 mediated modulation of cuprizone induced lesions was monitored using T₂ -weighted magnetic resonance imaging and magnetization transfer imaging, and further correlated with quantitative histological analyses for inflammatory cell influx, oligodendrocyte death, and demyelination. Second, following IL-13 immune gene therapy in cuprizone-treated eGFP⁺ bone marrow chimeric mice, we provide evidence that IL-13 directs the polarization of both brain-resident microglia and infiltrating macrophages towards an alternatively activated phenotype, thereby promoting the conversion of a pro-inflammatory environment toward an anti-inflammatory environment, as further evidenced by gene expression analyses. Finally, we show that IL-13 immune gene therapy is also able to limit lesion severity in a pre-existing inflammatory environment. In conclusion, these results highlight the potential of IL-13 to modulate microglia/macrophage responses and to improve disease outcome in a mouse model for MS. GLIA 2016;64:2181-2200.

Folic Acid Is Able to Polarize the Inflammatory Response in LPS Activated Microglia by Regulating Multiple Signaling Pathways

We investigated the ability of folic acid to modulate the inflammatory responses of LPS activated BV-2 microglia cells and the signal transduction pathways involved. To this aim, the BV-2 cell line was exposed to LPS as a proinflammatory response inducer, in presence or absence of various concentrations of folic acid. The production of nitric oxide (NO) was determined by the Griess test. The levels of tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), and IL-10 were determined by ELISA. Inducible NO synthase (iNOS), nuclear transcription factor-kappa B (NF-κB) p65, MAPKs protein, and suppressors of cytokine signaling (SOCS)1 and SOCS3 were analyzed by western blotting. TNF-α and IL-1β, as well as iNOS dependent NO production, resulted significantly inhibited by folic acid pretreatment in LPS-activated BV-2 cells. We also observed that folic acid dose-dependently upregulated both SOCS1 and SOCS3 expression in BV-2 cells, leading to an increased expression of the anti-inflammatory cytokine IL-10. Finally, p-IκBα, which indirectly reflects NF-κB complex activation, and JNK phosphorylation resulted dose-dependently downregulated by folic acid pretreatment of LPS-activated cells, whereas p38 MAPK phosphorylation resulted significantly upregulated by folic acid treatment. Overall, these results demonstrated that folic acid was able to modulate the inflammatory response in microglia cells, shifting proinflammatory versus anti-inflammatory responses through regulating multiple signaling pathways.

Everolimus (RAD001) ameliorates vascular cognitive impairment by regulating microglial function via the mTORC1 signaling pathway

Vascular dementia (VaD) is a widely prevalent and devastating disease. Despite the tremendous complexity that limits understanding of the pathophysiology of VaD, microglial dysfunction has been attributed, in part, to immune microenviroment disorder and finally leads to cognitive deficits. Considered the mammalian target of rapamycin complex 1 (mTORC1) is a key player in regulation of glial function, our work focused on whether the mTOR inhibitor everolimus (RAD001) could overcome the destructive microglial function, change the phenotype and ameliorate cognitive decline induced by chronic cerebral hypoperfusion. Strikingly, the results suggest that inhibition of the mTORC1 activity by RAD001 ameliorates VaD by restoring microglia's M1/M2 balance. Therefore, RAD001 may have promise as a therapy for VaD disease.

1-Oleyl-lysophosphatidic acid (LPA) promotes polarization of BV-2 and primary murine microglia towards an M1-like phenotype

Background

Microglia, the immunocompetent cells of the CNS, rapidly respond to brain injury and disease by altering their morphology and phenotype to adopt an activated state. Microglia can exist broadly between two different states, namely the classical (M1) and the alternative (M2) phenotype. The first is characterized by the production of pro-inflammatory cytokines/chemokines and reactive oxygen and/or nitrogen species. In contrast, alternatively activated microglia are typified by an anti-inflammatory phenotype supporting wound healing and debris clearance. The objective of the present study was to determine the outcome of lysophosphatidic acid (LPA)-mediated signaling events on microglia polarization.

Methods

LPA receptor expression and cyto-/chemokine mRNA levels in BV-2 and primary murine microglia (PMM) were determined by qPCR. M1/M2 marker expression was analyzed by Western blotting, immunofluorescence microscopy, or flow cytometry. Cyto-/chemokine secretion was quantitated by ELISA.

Results

BV-2 cells express LPA receptor 2 (LPA2), 3, 5, and 6, whereas PMM express LPA1, 2, 4, 5, and 6. We show that LPA treatment of BV-2 and PMM leads to a shift towards a pro-inflammatory M1-like phenotype. LPA treatment increased CD40 and CD86 (M1 markers) and reduced CD206 (M2 marker) expression. LPA increased inducible nitric oxide synthase (iNOS) and COX-2 levels (both M1), while the M2 marker Arginase-1 was suppressed in BV-2 cells. Immunofluorescence studies (iNOS, COX-2, Arginase-1, and RELMα) extended these findings to PMM. Upregulation of M1 markers in BV-2 and PMM was accompanied by increased cyto-/chemokine transcription and secretion (IL-1β, TNFα, IL-6, CCL5, and CXCL2). The pharmacological LPA5 antagonist TCLPA5 blunted most of these pro-inflammatory responses.

Conclusions

LPA drives BV-2 and PMM towards a pro-inflammatory M1-like phenotype. Suppression by TCLPA5 indicates that the LPA/LPA5 signaling axis could represent a potential pharmacological target to interfere with microglia polarization in disease.

Early applications of granulocyte colony-stimulating factor (G-CSF) can stabilize the blood-optic-nerve barrier and ameliorate inflammation in a rat model of anterior ischemic optic neuropathy (rAION)

Granulocyte colony-stimulating factor (G-CSF) was reported to have a neuroprotective effect in a rat model of anterior ischemic optic neuropathy (rAION model). However, the therapeutic window and anti-inflammatory effects of G-CSF in a rAION model have yet to be elucidated. Thus, this study aimed to determine the therapeutic window of G-CSF and investigate the mechanisms of G-CSF via regulation of optic nerve (ON) inflammation in a rAION model. Rats were treated with G-CSF on day 0, 1, 2 or 7 post-rAION induction for 5 consecutive days, and a control group were treated with phosphate-buffered saline (PBS). Visual function was assessed by flash visual evoked potentials at 4 weeks post-rAION induction. The survival rate and apoptosis of retinal ganglion cells were determined by FluoroGold labeling and TUNEL assay, respectively. ON inflammation was evaluated by staining of ED1 and Iba1, and ON vascular permeability was determined by Evans Blue extravasation. The type of macrophage polarization was evaluated using quantitative real-time PCR (qRT-PCR). The protein levels of TNF-α and IL-1β were analyzed by western blotting. A therapeutic window during which G-CSF could rescue visual function and retinal ganglion cell survival was demonstrated at day 0 and day 1 post-infarct. Macrophage infiltration was reduced by 3.1- and 1.6-fold by G-CSF treatment starting on day 0 and 1 post-rAION induction, respectively, compared with the PBS-treated group (P<0.05). This was compatible with 3.3- and 1.7-fold reductions in ON vascular permeability after G-CSF treatment compared with PBS treatment (P<0.05). Microglial activation was increased by 3.8- and 3.2-fold in the early (beginning treatment at day 0 or 1) G-CSF-treated group compared with the PBS-treated group (P<0.05). Immediate (within 30 mins of infarct) treatment with G-CSF also induced M2 microglia/macrophage activation. The cytokine levels were lower in the group that received immediate G-CSF treatment compared to those in the later G-CSF treatment group (P<0.05). Early treatment with G-CSF stabilized the blood-ON barrier to reduce macrophage infiltration and induced M2 microglia/macrophage polarization to decrease the expressions of pro-inflammatory cytokines in this rAION model.

Ethanol-Induced Neurodegeneration and Glial Activation in the Developing Brain

Ethanol induces neurodegeneration in the developing brain, which may partially explain the long-lasting adverse effects of prenatal ethanol exposure in fetal alcohol spectrum disorders (FASD). While animal models of FASD show that ethanol-induced neurodegeneration is associated with glial activation, the relationship between glial activation and neurodegeneration has not been clarified. This review focuses on the roles of activated microglia and astrocytes in neurodegeneration triggered by ethanol in rodents during the early postnatal period (equivalent to the third trimester of human pregnancy). Previous literature indicates that acute binge-like ethanol exposure in postnatal day 7 (P7) mice induces apoptotic neurodegeneration, transient activation of microglia resulting in phagocytosis of degenerating neurons, and a prolonged increase in glial fibrillary acidic protein-positive astrocytes. In our present study, systemic administration of a moderate dose of lipopolysaccharides, which causes glial activation, attenuates ethanol-induced neurodegeneration. These studies suggest that activation of microglia and astrocytes by acute ethanol in the neonatal brain may provide neuroprotection. However, repeated or chronic ethanol can induce significant proinflammatory glial reaction and neurotoxicity. Further studies are necessary to elucidate whether acute or sustained glial activation caused by ethanol exposure in the developing brain can affect long-lasting cellular and behavioral abnormalities observed in the adult brain.

Impact of macrophages on tumor growth characteristics in a murine ocular tumor model

Tumor associated macrophages (TAM), mean vascular density (MVD), PAS positive extravascular matrix patterns, and advanced patients' age are associated with a poor prognosis in uveal melanoma. These correlations may be influenced by M2 macrophages and their cytokine expression pattern. Thus, the effect of TAM and their characteristic cytokines on histologic tumor growth characteristics were studied under the influence of age. Ninety five CX3CR1(+/GFP) mice (young 8-12weeks, old 10-12months) received an intravitreal injection of 1 × 10(5) HCmel12 melanoma cells. Subgroups were either systemically macrophage-depleted by Clodronate liposomes (n = 23) or received melanoma cells, which were pre-incubated with the supernatant of M1- or M2-polarized macrophages (n = 26). Eyes were processed histologically/immunohistochemically (n = 75), or for flow cytometry (n = 20) to analyze tumor size, mean vascular density (MVD), extravascular matrix patterns, extracellular matrix (ECM) and the presence/polarization of TAM. Prognostically significant extravascular matrix patterns (parallels with cross-linkings, loops, networks) were found more frequently in tumors of untreated old compared to tumors of untreated young mice (p = 0.024); as well as in tumors of untreated mice compared to tumors of macrophage-depleted mice (p = 0.014). Independent from age, M2-conditioned tumors showed more TAM (p = 0.001), increased collagen IV levels (p = 0.024) and a higher MVD (p = 0.02) than M1-conditioned tumors. Flow cytometry revealed a larger proportion of M2-macrophages in old than in young mice. The results indicate that TAM and their cytokines appear to be responsible for a more aggressive tumor phenotype. Tumor favoring and pro-angiogenic effects can be directly attributed to a M2-dominated tumor microenvironment rather than to age-dependent factors alone. However, an aged immunoprofile with an increased number of M2-macrophages may provide a tumor-favoring basis. Further, old mice represent a more suitable tumor model instead of young mice since their histologic tumor pattern better resembles human tumors.

Transcranial Direct Current Stimulation Modulates Neurogenesis and Microglia Activation in the Mouse Brain

Transcranial direct current stimulation (tDCS) has been suggested as an adjuvant tool to promote recovery of function after stroke, but the mechanisms of its action to date remain poorly understood. Moreover, studies aimed at unraveling those mechanisms have essentially been limited to the rat, where tDCS activates resident microglia as well as endogenous neural stem cells. Here we studied the effects of tDCS on microglia activation and neurogenesis in the mouse brain. Male wild-type mice were subjected to multisession tDCS of either anodal or cathodal polarity; sham-stimulated mice served as control. Activated microglia in the cerebral cortex and neuroblasts generated in the subventricular zone as the major neural stem cell niche were assessed immunohistochemically. Multisession tDCS at a sublesional charge density led to a polarity-dependent downregulation of the constitutive expression of Iba1 by microglia in the mouse cortex. In contrast, both anodal and, to an even greater extent, cathodal tDCS induced neurogenesis from the subventricular zone. Data suggest that tDCS elicits its action through multifacetted mechanisms, including immunomodulation and neurogenesis, and thus support the idea of using tDCS to induce regeneration and to promote recovery of function. Furthermore, data suggest that the effects of tDCS may be animal- and polarity-specific.

Linking Activation of Microglia and Peripheral Monocytic Cells to the Pathophysiology of Psychiatric Disorders

A wide variety of studies have identified microglial activation in psychiatric disorders, such as schizophrenia, bipolar disorder, and major depressive disorder. Relatively fewer, but robust, studies have detected activation of peripheral monocytic cells in psychiatric disorders. Considering the origin of microglia, as well as neuropsychoimmune interactions in the context of the pathophysiology of psychiatric disorders, it is reasonable to speculate that microglia interact with peripheral monocytic cells in relevance with the pathogenesis of psychiatric disorders; however, these interactions have drawn little attention. In this review, we summarize findings relevant to activation of microglia and monocytic cells in psychiatric disorders, discuss the potential association between these cell types and disease pathogenesis, and propose perspectives for future research on these processes.

Brain aerobic glycolysis and motor adaptation learning

Ten percent to 15% of glucose used by the brain is metabolized nonoxidatively despite adequate tissue oxygenation, a process termed aerobic glycolysis (AG). Because of the known role of glycolysis in biosynthesis, we tested whether learning-induced synaptic plasticity would lead to regionally appropriate, learning-dependent changes in AG. Functional MRI (fMRI) before, during, and after performance of a visual-motor adaptation task demonstrated that left Brodmann area 44 (BA44) played a key role in adaptation, with learning-related changes to activity during the task and altered resting-state, functional connectivity after the task. PET scans before and after task performance indicated a sustained increase in AG in left BA 44 accompanied by decreased oxygen consumption. Intersubject variability in behavioral adaptation rate correlated strongly with changes in AG in this region, as well as functional connectivity, which is consistent with a role for AG in synaptic plasticity.

Metabolic reprogramming by the pyruvate dehydrogenase kinase-lactic acid axis: Linking metabolism and diverse neuropathophysiologies

Emerging evidence indicates that there is a complex interplay between metabolism and chronic disorders in the nervous system. In particular, the pyruvate dehydrogenase (PDH) kinase (PDK)-lactic acid axis is a critical link that connects metabolic reprogramming and the pathophysiology of neurological disorders. PDKs, via regulation of PDH complex activity, orchestrate the conversion of pyruvate either aerobically to acetyl-CoA, or anaerobically to lactate. The kinases are also involved in neurometabolic dysregulation under pathological conditions. Lactate, an energy substrate for neurons, is also a recently acknowledged signaling molecule involved in neuronal plasticity, neuron-glia interactions, neuroimmune communication, and nociception. More recently, the PDK-lactic acid axis has been recognized to modulate neuronal and glial phenotypes and activities, contributing to the pathophysiologies of diverse neurological disorders. This review covers the recent advances that implicate the PDK-lactic acid axis as a novel linker of metabolism and diverse neuropathophysiologies. We finally explore the possibilities of employing the PDK-lactic acid axis and its downstream mediators as putative future therapeutic strategies aimed at prevention or treatment of neurological disorders.

Role of neuro-immunological factors in the pathophysiology of mood disorders

Mood disorders, despite the widespread availability of monoamine-based antidepressant treatments, are associated with persistently high rates of disability, together with elevated rates of mortality due to suicide, cardiovascular disease, and other causes. The development of more effective treatments has been hindered by the lack of knowledge about the etiology and pathogenesis of mood disorders. An emerging area of science that promises novel pathways to antidepressant and mood stabilizing therapies surrounds evidence that immune cells and their signaling play a major role in the pathophysiology of major depressive disorder (MDD) and bipolar disorder (BD). Here, we review evidence that the release of neuroactive cytokines, particularly interleukins such as IL-1β, IL-6, and TNF-α, is altered in these disorders and discuss mechanisms such as the ATP-gated ion channel P2X7, through which cytokine signaling can influence neuro-glial interactions. Brain P2X7, an emerging target and antagonism of P2X7 holds promise as a novel mechanism for targeting treatment-resistant depression. We further discuss the role of microglia and astroglia in central neuroinflammation and their interaction with the peripheral immune system We present extant clinical evidence that bolsters the role of neuroinflammation and neuroactive cytokines in mood disorders. To that end, the role of clinical imaging by probing neuroinflammatory markers is also discussed briefly. Finally, we present data using preclinical neuroinflammation models that produce depression-like behaviors in experimental animals to identify neuroinflammatory mechanisms which may aid in novel neuroimmune target identification for the development of exciting pharmacological interventions in mood disorders.

Label-free identification of macrophage phenotype by fluorescence lifetime imaging microscopy

Macrophages adopt a variety of phenotypes that are a reflection of the many functions they perform as part of the immune system. In particular, metabolism is a phenotypic trait that differs between classically activated, proinflammatory macrophages, and alternatively activated, prohealing macrophages. Inflammatory macrophages have a metabolism based on glycolysis while alternatively activated macrophages generally rely on oxidative phosphorylation to generate chemical energy. We employ this shift in metabolism as an endogenous marker to identify the phenotype of individual macrophages via live-cell fluorescence lifetime imaging microscopy (FLIM). We demonstrate that polarized macrophages can be readily discriminated with the aid of a phasor approach to FLIM, which provides a fast and model-free method for analyzing fluorescence lifetime images.

Intermittent hypoxia-induced cardiorespiratory long-term facilitation: A new role for microglia

Intermittent hypoxia induces plasticity in neural networks controlling breathing and cardiovascular function. Studies demonstrate that mechanisms causing cardiorespiratory plasticity rely on intracellular signalling pathways that are activated by specific neurotransmitters. Peptides such as serotonin, PACAP and orexin are well-known for their physiological significance in regulating the cardiorespiratory system. Their receptor counterparts are present in cardiorespiratory centres of the brainstem medulla and spinal cord. Microglial cells are also important players in inducing plasticity. The phenotype and function of microglial cells can change based on the physiological state of the central nervous system. Here, we propose that in the autonomic nuclei of the ventral brainstem the relationship between neurotransmitters and neurokines, neurons and microglia determines the overall neural function of the central cardiorespiratory system.

Interleukin-4 Ameliorates the Functional Recovery of Intracerebral Hemorrhage Through the Alternative Activation of Microglia/Macrophage

Neuro-inflammation plays an important role in the recovery of brain injury after stroke. Microglia/macrophage is the major executor in the neuro-inflammation, which can be polarized into two distinct phenotypes: injurious/toxic classical activation (M1 phenotype) and protective alternative activation (M2 phenotype). Here, we investigated whether intracerebral administration of interleukin-4 (IL-4) at an early stage could affect the activation of microglia/macrophage and the corresponding outcome after intracerebral hemorrhage (ICH). The neuro-behavior was recorded between different groups in the rat ICH model. The M1 and M2 markers were then determined by qRT-PCR, western blotting, ELISA, and immunofluorescence, respectively. We observed aberrant activation of microglia/macrophage after ICH. After intracerebral injection of IL-4, M1 activation was greatly inhibited while M2 activation was enhanced, along with improving neurobehavioral recovery from deficits after ICH. Our study showed that early intracerebral injection of IL-4 potentially promotes neuro-functional recovery, probably through enhancing the alternative activation of microglia/macrophage.

Microglial Polarization and Inflammatory Mediators After Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is a subtype of stroke with high mortality and morbidity. When a diseased artery within the brain bursts, expansion and absorption of the resulting hematoma trigger a series of reactions that cause primary and secondary brain injury. Microglia are extremely important for removing the hematoma and clearing debris, but they are also a source of ongoing inflammation. This article discusses the role of microglial activation/polarization and related inflammatory mediators, such as Toll-like receptor 4, matrix metalloproteinases, high-mobility group protein box-1, nuclear factor erythroid 2-related factor 2, heme oxygenase, and iron, in secondary injury after ICH and highlights the potential targets for ICH treatment.

Combining Growth Factor and Bone Marrow Cell Therapy Induces Bleeding and Alters Immune Response After Stroke in Mice

BACKGROUND AND PURPOSE

Bone marrow cell (BMC)-based therapies, either the transplantation of exogenous cells or stimulation of endogenous cells by growth factors like the granulocyte colony-stimulating factor (G-CSF), are considered a promising means of treating stroke. In contrast to large preclinical evidence, however, a recent clinical stroke trial on G-CSF was neutral. We, therefore, aimed to investigate possible synergistic effects of co-administration of G-CSF and BMCs after experimental stroke in mice to enhance the efficacy compared with single treatments.

METHODS

We used an animal model for experimental stroke as paradigm to study possible synergistic effects of co-administration of G-CSF and BMCs on the functional outcome and the pathophysiological mechanism.

RESULTS

G-CSF treatment alone led to an improved functional outcome, a reduced infarct volume, increased blood vessel stabilization, and decreased overall inflammation. Surprisingly, the combination of G-CSF and BMCs abrogated G-CSFs' beneficial effects and resulted in increased hemorrhagic infarct transformation, altered blood-brain barrier, excessive astrogliosis, and altered immune cell polarization. These increased rates of infarct bleeding were mainly mediated by elevated matrix metalloproteinase-9-mediated blood-brain barrier breakdown in G-CSF- and BMCs-treated animals combined with an increased number of dilated and thus likely more fragile vessels in the subacute phase after cerebral ischemia.

CONCLUSIONS

Our results provide new insights into both BMC-based therapies and immune cell biology and help to understand potential adverse and unexpected side effects.

Microglial M1/M2 polarization and metabolic states

Microglia are critical nervous system-specific immune cells serving as tissue-resident macrophages influencing brain development, maintenance of the neural environment, response to injury and repair. As influenced by their environment, microglia assume a diversity of phenotypes and retain the capability to shift functions to maintain tissue homeostasis. In comparison with peripheral macrophages, microglia demonstrate similar and unique features with regards to phenotype polarization, allowing for innate immunological functions. Microglia can be stimulated by LPS or IFN-γ to an M1 phenotype for expression of pro-inflammatory cytokines or by IL-4/IL-13 to an M2 phenotype for resolution of inflammation and tissue repair. Increasing evidence suggests a role of metabolic reprogramming in the regulation of the innate inflammatory response. Studies using peripheral immune cells demonstrate that polarization to an M1 phenotype is often accompanied by a shift in cells from oxidative phosphorylation to aerobic glycolysis for energy production. More recently, the link between polarization and mitochondrial energy metabolism has been considered in microglia. Under these conditions, energy demands would be associated with functional activities and cell survival and thus, may serve to influence the contribution of microglia activation to various neurodegenerative conditions. This review examines the polarization states of microglia and their relationship to mitochondrial metabolism. Additional supporting experimental data are provided to demonstrate mitochondrial metabolic shifts in primary microglia and the BV-2 microglia cell line induced under LPS (M1) and IL-4/IL-13 (M2) polarization.

Metabolic Connection of Inflammatory Pain: Pivotal Role of a Pyruvate Dehydrogenase Kinase-Pyruvate Dehydrogenase-Lactic Acid Axis

Pyruvate dehydrogenase kinases (PDK1-4) are mitochondrial metabolic regulators that serve as decision makers via modulation of pyruvate dehydrogenase (PDH) activity to convert pyruvate either aerobically to acetyl-CoA or anaerobically to lactate. Metabolic dysregulation and inflammatory processes are two sides of the same coin in several pathophysiological conditions. The lactic acid surge associated with the metabolic shift has been implicated in diverse painful states. In this study, we investigated the role of PDK-PDH-lactic acid axis in the pathogenesis of chronic inflammatory pain. Deficiency of Pdk2 and/or Pdk4 in mice attenuated complete Freund's adjuvant (CFA)-induced pain hypersensitivities. Likewise, Pdk2/4 deficiency attenuated the localized lactic acid surge along with hallmarks of peripheral and central inflammation following intraplantar administration of CFA. In vitro studies supported the role of PDK2/4 as promoters of classical proinflammatory activation of macrophages. Moreover, the pharmacological inhibition of PDKs or lactic acid production diminished CFA-induced inflammation and pain hypersensitivities. Thus, a PDK-PDH-lactic acid axis seems to mediate inflammation-driven chronic pain, establishing a connection between metabolism and inflammatory pain.

SIGNIFICANCE STATEMENT

The mitochondrial pyruvate dehydrogenase (PDH) kinases (PDKs) and their substrate PDH orchestrate the conversion of pyruvate either aerobically to acetyl-CoA or anaerobically to lactate. Lactate, the predominant end product of glycolysis, has recently been identified as a signaling molecule for neuron-glia interactions and neuronal plasticity. Pathological metabolic shift and subsequent lactic acid production are thought to play an important role in diverse painful states; however, their contribution to inflammation-driven pain is still to be comprehended. Here, we report that the PDK-PDH-lactic acid axis constitutes a key component of inflammatory pain pathogenesis. Our findings establish an unanticipated link between metabolism and inflammatory pain. This study unlocks a previously ill-explored research avenue for the metabolic control of inflammatory pain pathogenesis.

Role of Neuro-Immunological Factors in the Pathophysiology of Mood Disorders: Implications for Novel Therapeutics for Treatment Resistant Depression

Mood disorders are associated with persistently high rates of morbidity and mortality, despite the widespread availability of antidepressant treatments. One limitation to extant therapeutic options has been that nearly all approved antidepressant pharmacotherapies exert a similar primary action of blocking monoamine transporters, and few options exist for transitioning treatment resistant patients to alternatives with distinct mechanisms. An emerging area of science that promises novel pathways to antidepressant and mood-stabilizing therapies has followed from evidence that immunological factors play major roles in the pathophysiology of at least some mood disorder subtypes. Here we review evidence that the compounds that reduce the release or signaling of neuroactive cytokines, particularly IL-1β, IL-6, and TNF-α, can exert antidepressant effects in subgroups of depressed patients who are identified by blood-based biomarkers associated with inflammation. Within this context we discuss the role of microglia in central neuroinflammation, and the interaction between the peripheral immune system and the central synaptic microenvironment during and after neuroinflammation. Finally we review data using preclinical neuroinflammation models that produce depression-like behaviors in experimental animals to guide the discovery of novel neuro-immune drug targets.

Targeting the Microglial Signaling Pathways: New Insights in the Modulation of Neuropathic Pain

The microglia, once thought only to be supporting cells of the central nervous system (CNS), are now recognized to play essential roles in many pathologies. Many studies within the last decades indicated that the neuro-immune interaction underlies the generation and maintenance of neuropathic pain. Through a large number of receptors and signaling pathways, the microglial cells communicate with neurons, astrocytes and other cells, including those of the immune system. A disturbance or loss of CNS homeostasis causes rapid responses of the microglia, which undergo a multistage activation process. The activated microglia change their cell shapes and gene expression profiles, which induce proliferation, migration, and the production of pro- or antinociceptive factors. The cells release a large number of mediators that can act in a manner detrimental or beneficial to the surrounding cells and can indirectly alter the nociceptive signals. This review discusses the most important microglial intracellular signaling cascades (MAPKs, NF-kB, JAK/STAT, PI3K/Akt) that are essential for neuropathic pain development and maintenance. Our objective was to identify new molecular targets that may result in the development of powerful tools to control the signaling associated with neuropathic pain.

Regulation of Macrophage, Dendritic Cell, and Microglial Phenotype and Function by the SOCS Proteins

Macrophages are innate immune cells of dynamic phenotype that rapidly respond to external stimuli in the microenvironment by altering their phenotype to respond to and to direct the immune response. The ability to dynamically change phenotype must be carefully regulated to prevent uncontrolled inflammatory responses and subsequently to promote resolution of inflammation. The suppressor of cytokine signaling (SOCS) proteins play a key role in regulating macrophage phenotype. In this review, we summarize research to date from mouse and human studies on the role of the SOCS proteins in determining the phenotype and function of macrophages. We will also touch on the influence of the SOCS on dendritic cell (DC) and microglial phenotype and function. The molecular mechanisms of SOCS function in macrophages and DCs are discussed, along with how dysregulation of SOCS expression or function can lead to alterations in macrophage/DC/microglial phenotype and function and to disease. Regulation of SOCS expression by microRNA is discussed. Novel therapies and unanswered questions with regard to SOCS regulation of monocyte-macrophage phenotype and function are highlighted.

Imaging robust microglial activation after lipopolysaccharide administration in humans with PET

Neuroinflammation is associated with a broad spectrum of neurodegenerative and psychiatric diseases. The core process in neuroinflammation is activation of microglia, the innate immune cells of the brain. We measured the neuroinflammatory response produced by a systemic administration of the Escherichia coli lipopolysaccharide (LPS; also called endotoxin) in humans with the positron emission tomography (PET) radiotracer [11C]PBR28, which binds to translocator protein, a molecular marker that is up-regulated by microglial activation. In addition, inflammatory cytokines in serum and sickness behavior profiles were measured before and after LPS administration to relate brain microglial activation with systemic inflammation and behavior. Eight healthy male subjects each had two 120-min [11C]PBR28 PET scans in 1 d, before and after an LPS challenge. LPS (1.0 ng/kg, i.v.) was administered 180 min before the second [11C]PBR28 scan. LPS administration significantly increased [11C]PBR28 binding 30-60%, demonstrating microglial activation throughout the brain. This increase was accompanied by an increase in blood levels of inflammatory cytokines, vital sign changes, and sickness symptoms, well-established consequences of LPS administration. To our knowledge, this is the first demonstration in humans that a systemic LPS challenge induces robust increases in microglial activation in the brain. This imaging paradigm to measure brain microglial activation with [11C]PBR28 PET provides an approach to test new medications in humans for their putative antiinflammatory effects.

Role of orexin A signaling in dietary palmitic acid-activated microglial cells

Excess dietary saturated fatty acids such as palmitic acid (PA) induce peripheral and hypothalamic inflammation. Hypothalamic inflammation, mediated in part by microglial activation, contributes to metabolic dysregulation. In rodents, high fat diet-induced microglial activation results in nuclear translocation of nuclear factor-kappa B (NFκB), and increased central pro-inflammatory cytokines tumor necrosis factor alpha (TNF-α) and interleukin-6 (IL-6). The hypothalamic neuropeptide orexin A (OXA, hypocretin 1) is neuroprotective in brain. In cortex, OXA can also reduce inflammation and neurodegeneration through a microglial-mediated pathway. Whether hypothalamic orexin neuroprotection mechanisms depend upon microglia is unknown. To address this issue, we evaluated effects of OXA and PA on inflammatory response in immortalized murine microglial and hypothalamic neuronal cell lines. We demonstrate for the first time in microglial cells that exposure to PA increases gene expression of orexin-1 receptor but not orexin-2 receptor. Pro-inflammatory markers IL-6, TNF-α, and inducible nitric oxide synthase in microglial cells are increased following PA exposure, but are reduced by pretreatment with OXA. The anti-inflammatory marker arginase-1 is increased by OXA. Finally, we show hypothalamic neurons exposed to conditioned media from PA-challenged microglia have increased cell survival only when microglia were pretreated with OXA. These data support the concept that OXA may act as an immunomodulatory regulator of microglia, reducing pro-inflammatory cytokines and increasing anti-inflammatory factors to promote a favorable neuronal microenvironment.

Parthenolide Relieves Pain and Promotes M2 Microglia/Macrophage Polarization in Rat Model of Neuropathy

Neuropathic pain treatment remains a challenge because pathomechanism is not fully understood. It is believed that glial activation and increased spinal nociceptive factors are crucial for neuropathy. We investigated the effect of parthenolide (PTL) on the chronic constriction injury to the sciatic nerve (CCI)-induced neuropathy in rat. We analyzed spinal changes in glial markers and M1 and M2 polarization factors, as well as intracellular signaling pathways. PTL (5 µg; i.t.) was preemptively and then daily administered for 7 days after CCI. PTL attenuated the allodynia and hyperalgesia and increased the protein level of IBA1 (a microglial/macrophage marker) but did not change GFAP (an astrocyte marker) on day 7 after CCI. PTL reduced the protein level of M1 (IL-1β, IL-18, and iNOS) and enhanced M2 (IL-10, TIMP1) factors. In addition, it downregulated the phosphorylated form of NF-κB, p38MAPK, and ERK1/2 protein level and upregulated STAT3. In primary microglial cell culture we have shown that IL-1β, IL-18, iNOS, IL-6, IL-10, and TIMP1 are of microglial origin. Summing up, PTL directly or indirectly attenuates neuropathy symptoms and promotes M2 microglia/macrophages polarization. We suggest that neuropathic pain therapies should be shifted from blanketed microglia/macrophage suppression toward maintenance of the balance between neuroprotective and neurotoxic microglia/macrophage phenotypes.

Activation and Regulation of NLRP3 Inflammasome by Intrathecal Application of SDF-1a in a Spinal Cord Injury Model

Stromal cell-derived factor-1 alpha (SDF-1a) or CXCL12 is an important cytokine with multiple functions in the brain during development and in adulthood. The inflammatory response initiated by spinal cord injury (SCI) involves the processing of interleukin-1beta (IL-1ß) and IL-18 mediated by caspase-1 which is under the control of an intracellular multiprotein complex termed inflammasome. Using an SCI rat model, we found improved functional long-term recovery which is paralleled by a reduction of apoptosis after intrathecal treatment with SDF-1a. An intriguing aspect is that SDF-1a changed the number of neuroinflammatory cells in the damaged area. We further examined the cellular localization and sequential expression of several inflammasomes during SCI at 6 h, 24 h, 3 days, and 7 days as well as the role of SDF-1a as a regulatory factor for inflammasomes. Using 14-week old male Wistar rats, spinal cord contusion was applied at the thoracic segment 9, and animals were subsequently treated with SDF-1a via intrathecal application through an osmotic pump. SCI temporally increased the expression of the inflammasomes NLRP3, ASC, the inflammatory marker tumor necrosis factor-a (TNF-a), interleukin-1ß (IL-1β) and IL-18. SDF-1a significantly reduced the levels of IL-18, IL-1b, TNF-a, NLRP3, ASC, and caspase-1. Immunofluorescence double-labeling demonstrated that microglia and neurons are major sources of the ASC and NLRP3 respectivley. Our data provide clear evidence that SCI stimulates a complex scenario of inflammasome activation at the injured site and that SDF-1a-mediated neuroprotection presumably depends on the attenuation of the inflammasome complex.