Transforming growth factor-β1 produced by hippocampal cells modulates microglial reactivity in culture

Whole Transcriptome RNA-Seq Reveals Drivers of Pathological Dysfunction in a Transgenic Model of Alzheimer's Disease

Alzheimer's disease (AD) affects more than 55 million people worldwide, yet current theories cannot fully explain its aetiology. Accordingly, gene expression profiling has been used to provide a holistic view of the biology underpinning AD. Focusing primarily on protein-coding genes, such approaches have highlighted a critical involvement of microglia-related inflammatory processes. Simultaneous investigation of transcriptional regulators and noncoding RNA (ncRNA) can offer further insight into AD biology and inform the development of disease-modifying therapies. We previously described a method for whole transcriptome sampling to simultaneously investigate protein-coding genes and ncRNA. Here, we use this technique to explore transcriptional changes in a murine model of AD (15-month-old APP/PS1 mice). We confirmed the extensive involvement of microglia-associated genes and gene networks, consistent with literature. We also report a wealth of differentially-expressed non-coding RNA - including microRNA, long non-coding RNA, small nuclear and small nucleolar RNA, and pseudogenes - many of which have been overlooked previously. Transcription factor analysis determined that six transcription factors likely regulate gene expression changes in this model (Irf8, Junb, c-Fos, Lmo2, Runx1, and Nfe2l2). We then utilised validated miRNA-target interactions, finding 60 interactions between 15 miRNA and 42 mRNA (messenger RNA) with largely consistent directionality. Furthermore, we found that eight transcription factors (Clock, Lmo2, Runx1, Nfe2l2, Egr2, c-Fos, Junb, and Nr4a1) are likely responsible for the regulation of miRNA expression. Taken together, these data indicate a complex interplay of coding and non-coding RNA, driven by a small number of specific transcription factors, contributing to transcriptional changes in 15-month-old APP/PS1 mice.

Neuron-glia crosstalk and inflammatory mediators in migraine pathophysiology

Migraine is a complex neurological disorder with neuroinflammation playing a crucial role in its pathogenesis. This review provides an overview of the neuroinflammation mechanisms in migraine, focusing on both cellular and molecular aspects. At the cellular level, we examine the role of glial cells, including astrocytes, microglia, oligodendrocytes in the central nervous system, and Schwann cells and satellite glial cells in the peripheral nervous system. On the molecular level, we explore the signaling pathways, including IL-1β, TNF-α, IL-6, and non-coding RNAs, that mediate cell interactions or independent actions. Some of the molecular signaling pathways mentioned, such as TNF-α and IL-1β, have been investigated as druggable targets. Recent advancements, such as [11C] PBR28-targeted imaging for visualizing astrocyte activation and single-cell sequencing for exploring cellular heterogeneity, represent breakthroughs in understanding the mechanisms of neuroinflammation in migraine. By considering factors for personalized treatments, estrogen and TRPM8 emerge as promising therapeutic targets regarding sexual dimorphism. These advancements may help bridge the gap between preclinical findings and clinical applications, ultimately leading to more precise and personalized options for migraine patients.

To re-examine the intersection of microglial activation and neuroinflammation in neurodegenerative diseases from the perspective of pyroptosis

Neurodegenerative diseases (NDs), such as Alzheimer's disease, Parkinson's disease, Huntington's disease, and motor neuron disease, are diseases characterized by neuronal damage and dysfunction. NDs are considered to be a multifactorial disease with diverse etiologies (immune, inflammatory, aging, genetic, etc.) and complex pathophysiological processes. Previous studies have found that neuroinflammation and typical microglial activation are important mechanisms of NDs, leading to neurological dysfunction and disease progression. Pyroptosis is a new mode involved in this process. As a form of programmed cell death, pyroptosis is characterized by the expansion of cells until the cell membrane bursts, resulting in the release of cell contents that activates a strong inflammatory response that promotes NDs by accelerating neuronal dysfunction and abnormal microglial activation. In this case, abnormally activated microglia release various pro-inflammatory factors, leading to the occurrence of neuroinflammation and exacerbating both microglial and neuronal pyroptosis, thus forming a vicious cycle. The recognition of the association between pyroptosis and microglia activation, as well as neuroinflammation, is of significant importance in understanding the pathogenesis of NDs and providing new targets and strategies for their prevention and treatment.

Molecular insights into the pathogenic impact of vitamin D deficiency in neurological disorders

Neurological disorders are the major cause of disability, leading to a decrease in quality of life by impairing cognitive, sensorimotor, and motor functioning. Several factors have been proposed in the pathogenesis of neurobehavioral changes, including nutritional, environmental, and genetic predisposition. Vitamin D (VD) is an environmental and nutritional factor that is widely distributed in the central nervous system's subcortical grey matter, neurons of the substantia nigra, hippocampus, thalamus, and hypothalamus. It is implicated in the regulation of several brain functions by preserving neuronal structures. It is a hormone rather than a nutritional vitamin that exerts a regulatory role in the pathophysiology of several neurological disorders, including Alzheimer's disease, Parkinson's disease, epilepsy, and multiple sclerosis. A growing body of epidemiological evidence suggests that VD is critical in neuronal development and shows neuroprotective effects by influencing the production and release of neurotrophins, antioxidants, immunomodulatory, regulation of intracellular calcium balance, and direct effect on the growth and differentiation of nerve cells. This review provides up-to-date and comprehensive information on vitamin D deficiency, risk factors, and clinical and preclinical evidence on its relationship with neurological disorders. Furthermore, this review provides mechanistic insight into the implications of vitamin D and its deficiency on the pathogenesis of neurological disorders. Thus, an understanding of the crucial role of vitamin D in the neurobiology of neurodegenerative disorders can assist in the better management of vitamin D-deficient individuals.

Ca2+ signaling in postsynaptic neurons: Neuroplastin-65 regulates the interplay between plasma membrane Ca2+ ATPases and ionotropic glutamate receptors

Upon postsynaptic glutamate receptor activation, the cytosolic Ca²⁺ concentration rises and initiates signaling and plasticity in spines. The plasma membrane Ca²⁺ ATPase (PMCA) is a major player to limit the duration of cytosolic Ca²⁺ signals. It forms complexes with the glycoprotein neuroplastin (Np) isoforms Np55 and Np65 and functionally interplays with N-methyl-D-aspartate (NMDA)-type ionotropic glutamate receptors (iGluNRs). Moreover, binding of the Np65-specific extracellular domain to Ca²⁺-permeable GluA1-containing α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type ionotropic glutamate receptors (iGluA1Rs) was found to be required for long-term potentiation (LTP). However, the link between PMCA and iGluRs function to regulate cytosolic Ca²⁺ signals remained unclear. Here, we report that Np65 coordinates PMCA and iGluRs' functions to modulate the duration and amplitude of cytosolic Ca²⁺ transients in dendrites and spines of hippocampal neurons. Using live-cell Ca²⁺ imaging, acute pharmacological treatments, and GCaMP5G-expressing hippocampal neurons, we discovered that endogenous or Np65-promoted PMCA activity contributes to the restoration of basal Ca²⁺ levels and that this effect is dependent on iGluR activation. Super-resolution STED and confocal microscopy revealed that electrical stimulation increases the abundance of synaptic neuroplastin-PMCA complexes depending on iGluR activation and that low-rate overexpression of Np65 doubled PMCA levels and decreased cell surface levels of GluN2A and GluA1 in dendrites and Shank2-positive glutamatergic synapses. In neuroplastin-deficient hippocampi, we observed reduced PMCA and unchanged GluN2B levels, while GluN2A and GluA1 levels were imbalanced. Our electrophysiological data from hippocampal slices argues for an essential interplay of PMCA with GluN2A- but not with GluN2B-containing receptors upon induction of synaptic plasticity. Accordingly, we conclude that Np65 may interconnect PMCA with core players of glutamatergic neurotransmission to fine-tune the Ca²⁺ signal regulation in basal synaptic function and plasticity.

The promise of the TGF-β superfamily as a therapeutic target for Parkinson's disease

A large body of evidence underscore the regulatory role of TGF-β superfamily in the central nervous system. Components of the TGF-β superfamily modulate key events during embryonic brain development and adult brain tissue injury repair. With respect to Parkinson's disease (PD), TGF-ß signaling pathways are implicated in the differentiation, maintenance and synaptic function of the dopaminergic neurons, as well as in processes related to the activation state of astrocytes and microglia. In vitro and in vivo studies using toxin models, have interrogated on the dopaminotrophic and protective role of the TGF-β superfamily members. The evolution of genetic and animal models of PD that more closely recapitulate the disease condition has made possible the dissection of intracellular pathways in response to TGF-ß treatment. Although the first clinical trials using GDNF did not meet their primary endpoints, substantial work has been carried out to reappraise the TGF-β superfamily's clinical benefit.

Pharmacological depletion of microglia leads to a dose-dependent reduction in inflammation and senescence in the aged murine brain

Chronic macrophage activation was implicated as one of the main culprits for chronical, low-grade inflammation which significantly contributes to development of age-related diseases. Microglia as the brain macrophages have been recently implicated as key players in neuroinflammation and neurodegeneration in the aged brain. Microglial cell functions are indispensable in early development, however, activation or senescence of microglia in aging cells may be detrimental. Depletion of microglia using genetical or pharmacological approaches leads to opposite results regarding effects on brain cognition. In this study we pharmacologically depleted microglia using orally delivered low and high doses of the CSF1R inhibitor PLX5622 and assessed the expression levels of known inflammation markers (TNF-α, IL1-β, IL-6, IL-10), glia markers (Iba-1 and Gfap) and specific senescence marker p16^Ink4a in the aged murine brain. Our results indicate that treatment with low and high doses of PLX5622 leads to a dose-dependent depletion of microglial cells with similar levels in young and aged mice. We also show that treatment with low and high PLX5622 differentially affected cytokine levels in young and old brains. By using low doses we could achieve reduction in inflammation circumventing the astrocyte activation. Removal of microglia cells led to decreased expression of the senescence marker p16^Ink4a in the aged brain, indicating a relevant contribution of these cells to the expression of this marker and their senescent status in the healthy aging brain. Our results indicate that increased and detrimental brain inflammation in aged murine brain can be impaired by selectively reducing the microglial cell population.

Aged Microglia in Neurodegenerative Diseases: Microglia Lifespan and Culture Methods

Microglia have been recognized as macrophages of the central nervous system (CNS) that are regarded as a culprit of neuroinflammation in neurodegenerative diseases. Thus, microglia have been considered as a cell that should be suppressed for maintaining a homeostatic CNS environment. However, microglia ontogeny, fate, heterogeneity, and their function in health and disease have been defined better with advances in single-cell and imaging technologies, and how to maintain homeostatic microglial function has become an emerging issue for targeting neurodegenerative diseases. Microglia are long-lived cells of yolk sac origin and have limited repopulating capacity. So, microglial perturbation in their lifespan is associated with not only neurodevelopmental disorders but also neurodegenerative diseases with aging. Considering that microglia are long-lived cells and may lose their functional capacity as they age, we can expect that aged microglia contribute to various neurodegenerative diseases. Thus, understanding microglial development and aging may represent an opportunity for clarifying CNS disease mechanisms and developing novel therapies.

Astroglial cells as neuroendocrine targets in forebrain development: Implications for sex differences in psychiatric disease

Astroglial cells are the most abundant cell type in the mammalian brain. They are implicated in almost every aspect of brain physiology, including maintaining homeostasis, building and maintaining the blood brain barrier, and the development and maturation of neuronal networks. Critically, astroglia also express receptors for gonadal sex hormones, respond rapidly to gonadal hormones, and are able to synthesize hormones. Thus, they are positioned to guide and mediate sexual differentiation of the brain, particularly neuronal networks in typical and pathological conditions. In this review, we describe astroglial involvement in the organization and development of the brain, and consider known sex differences in astroglial responses to understand how astroglial cell-mediated organization may play a role in forebrain sexual dimorphisms in human populations. Finally, we consider how sexually dimorphic astroglial responses and functions in development may lead to sex differences in vulnerability for neuropsychiatric disorders.

The Interaction of TRAF6 With Neuroplastin Promotes Spinogenesis During Early Neuronal Development

Correct brain wiring depends on reliable synapse formation. Nevertheless, signaling codes promoting synaptogenesis are not fully understood. Here, we report a spinogenic mechanism that operates during neuronal development and is based on the interaction of tumor necrosis factor receptor-associated factor 6 (TRAF6) with the synaptic cell adhesion molecule neuroplastin. The interaction between these proteins was predicted in silico and verified by co-immunoprecipitation in extracts from rat brain and co-transfected HEK cells. Binding assays show physical interaction between neuroplastin’s C-terminus and the TRAF-C domain of TRAF6 with a K_d value of 88 μM. As the two proteins co-localize in primordial dendritic protrusions, we used young cultures of rat and mouse as well as neuroplastin-deficient mouse neurons and showed with mutagenesis, knock-down, and pharmacological blockade that TRAF6 is required by neuroplastin to promote early spinogenesis during in vitro days 6-9, but not later. Time-framed TRAF6 blockade during days 6–9 reduced mEPSC amplitude, number of postsynaptic sites, synapse density and neuronal activity as neurons mature. Our data unravel a new molecular liaison that may emerge during a specific window of the neuronal development to determine excitatory synapse density in the rodent brain.

TGF-β1 Provides Neuroprotection via Inhibition of Microglial Activation in 3-Acetylpyridine-Induced Cerebellar Ataxia Model Rats

Cerebellar ataxias (CAs) consist of a heterogeneous group of neurodegenerative diseases hallmarked by motor deficits and deterioration of the cerebellum and its associated circuitries. Neuroinflammatory responses are present in CA brain, but how neuroinflammation may contribute to CA pathogenesis remain unresolved. Here, we investigate whether transforming growth factor (TGF)-β1, which possesses anti-inflammatory and neuroprotective properties, can ameliorate the microglia-mediated neuroinflammation and thereby alleviate neurodegeneration in CA. In the current study, we administered TGF-β1 via the intracerebroventricle (ICV) in CA model rats, by intraperitoneal injection of 3-acetylpyridine (3-AP), to reveal the neuroprotective role of TGF-β1. The TGF-β1 administration after 3-AP injection ameliorated motor impairments and reduced the calbindin-positive neuron loss and apoptosis in the brain stem and cerebellum. Meanwhile, 3-AP induced microglial activation and inflammatory responses in vivo, which were determined by morphological alteration and an increase in expression of CD11b, enhancement of percentage of CD40 + and CD86 + microglial cells, upregulation of pro-inflammatory mediators, tumor necrosis factor (TNF)-α and interleukin (IL)-1β, and a downregulation of neurotrophic factor, insulin-like growth factor (IGF)-1 in the brain stem and cerebellum. TGF-β1 treatment significantly prevented all the changes caused by 3-AP. In addition, in vitro experiments, TGF-β1 directly attenuated 3-AP-induced microglial activation and inflammatory responses in primary cultures. Purkinje cell exposure to supernatants of primary microglia that had been treated with TGF-β1 reduced neuronal loss and apoptosis induced by 3-AP-treated microglial supernatants. Furthermore, the protective effect was similar to those treated with TNF-α-neutralizing antibody. These findings suggest that TGF-β1 protects against neurodegeneration in 3-AP-induced CA rats via inhibiting microglial activation and at least partly TNF-α release.

Glial gene networks associated with alcohol dependence

Chronic alcohol abuse alters the molecular structure and function of brain cells. Recent work suggests adaptations made by glial cells, such as astrocytes and microglia, regulate physiological and behavioral changes associated with addiction. Defining how alcohol dependence alters the transcriptome of different cell types is critical for developing the mechanistic hypotheses necessary for a nuanced understanding of cellular signaling in the alcohol-dependent brain. We performed RNA-sequencing on total homogenate and glial cell populations isolated from mouse prefrontal cortex (PFC) following chronic intermittent ethanol vapor exposure (CIE). Compared with total homogenate, we observed unique and robust gene expression changes in astrocytes and microglia in response to CIE. Gene co-expression network analysis revealed biological pathways and hub genes associated with CIE in astrocytes and microglia that may regulate alcohol-dependent phenotypes. Astrocyte identity and synaptic calcium signaling genes were enriched in alcohol-associated astrocyte networks, while TGF-β signaling and inflammatory response genes were disrupted by CIE treatment in microglia gene networks. Genes related to innate immune signaling, specifically interferon pathways, were consistently up-regulated across CIE-exposed astrocytes, microglia, and total homogenate PFC tissue. This study illuminates the cell-specific effects of chronic alcohol exposure and provides novel molecular targets for studying alcohol dependence.

Stress and aging act through common mechanisms to elicit neuroinflammatory priming

Over the course of an animal's lifespan, there is a protracted breakdown in basic homeostatic functions. Stressors (both psychological and physiological) can accelerate this process and compromise multiple homeostatic mechanisms. For example, both stress and aging can modulate neuroinflammatory function and cause a primed phenotype resulting in a heightened neuroinflammatory profile upon immune activation. Microglia, the brain's resident myeloid cell, produce "silent" immune machinery in response to stress and aging that does not cause immediate immune activation; rather, these changes prime the cell for a subsequent immune insult. Primed microglia exhibit a hyperinflammatory response upon immune activation that can exacerbate pathology. In this review, we will explore parallels between stress- and aging-induced neuroinflammatory priming. First, we will provide a background on the basic principles of neuroimmunology. Next, we will discuss evidence that neuroinflammatory responses become primed in the context of both stress and aging. We will also describe cell-specific contributions to neuroinflammatory priming with a focus on microglia. Finally, common mechanisms underlying priming in the context of stress and aging will be discussed: these mechanisms include glucocorticoid signaling; accumulation of danger signals; dis-inhibition of microglia; and breakdown of circadian rhythms. Overall, there are multifarious parallels between stress- and aging-elicited neuroinflammatory priming, suggesting that stress may promote a form of premature aging. Further unravelling mechanisms underlying priming could lead to improved treatments for buffering against stress- and aging-elicited behavioral pathologies.

Impact of Aging in Microglia-Mediated D-Serine Balance in the CNS

A mild chronic inflammatory state, like that observed in aged individuals, affects microglial function, inducing a dysfunctional phenotype that potentiates neuroinflammation and cytotoxicity instead of neuroprotection in response to additional challenges. Given that inflammatory activation of microglia promotes increased release of D-serine, we postulate that age-dependent inflammatory brain environment leads to microglia-mediated changes on the D-serine-regulated glutamatergic transmission. Furthermore, D-serine dysregulation, in addition to affecting synaptogenesis and synaptic plasticity, appears also to potentiate NMDAR-dependent excitotoxicity, promoting neurodegeneration and cognitive impairment. D-serine dysregulation promoted by microglia could have a role in age-related cognitive impairment and in the induction and progression of neurodegenerative processes like Alzheimer's disease.

Comparing Effects of Transforming Growth Factor β1 on Microglia From Rat and Mouse: Transcriptional Profiles and Potassium Channels

The cytokine, transforming growth factor β1 (TGFβ1), is up-regulated after central nervous system (CNS) injuries or diseases involving microglial activation, and it has been proposed as a therapeutic agent for treating neuroinflammation. Microglia can produce and respond to TGFβ1. While rats and mice are commonly used for studying neuroinflammation, very few reports directly compare them. Such studies are important for improving pre-clinical studies and furthering translational progress in developing therapeutic interventions. After intracerebral hemorrhage (ICH) in the rat striatum, the TGFβ1 receptor was highly expressed on microglia/macrophages within the hematoma. We recently found species similarities and differences in response to either a pro-inflammatory (interferon-γ, IFN-γ, +tumor necrosis factor, TNF-α) or anti-inflammatory interleukin-4 (IL-4) stimulus. Here, we assessed whether rat and mouse microglia differ in their responses to TGFβ1. Microglia were isolated from Sprague-Dawley rats and C57BL/6 mice and treated with TGFβ1. We quantified changes in expression of >50 genes, in their morphology, proliferation, apoptosis and in three potassium channels that are considered therapeutic targets. Many inflammatory mediators, immune receptors and modulators showed species similarities, but notable differences included that, for some genes, only one species responded (e.g., Il4r, Il10, Tgfbr2, colony-stimulating factor receptor (Csf1r), Itgam, suppressor of cytokine signaling 1 (Socs1), toll-like receptors 4 (Tlr4), P2rx7, P2ry12), and opposite responses were seen for others (Tgfb1, Myc, Ifngr1). In rat only, TGFβ1 affected microglial morphology and proliferation, but there was no apoptosis in either species. In both species, TGFβ1 dramatically increased Kv1.3 channel expression and current (no effects on Kir2.1). KCa3.1 showed opposite species responses: the current was low in unstimulated rat microglia and greatly increased by TGFβ1 but higher in control mouse cells and decreased by TGFβ1. Finally, we compared TGFβ1 and IL10 (often considered similar anti-inflammatory stimuli) and found many different responses in both species. Overall, the numerous species differences should be considered when characterizing neuroinflammation and microglial activation in vitro and in vivo, and when targeting potassium channels.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

Signaling by the transforming growth factor β (TGF-β) family is necessary for proper neural development and function throughout life. Sequential waves of activation, inhibition, and reactivation of TGF-β family members regulate numerous elements of the nervous system from the earliest stages of embryogenesis through adulthood. This review discusses the expression, regulation, and function of TGF-β family members in the central nervous system at various developmental stages, beginning with induction and patterning of the nervous system to their importance in the adult as modulators of inflammatory response and involvement in degenerative diseases.

TGF-β1 Neuroprotection via Inhibition of Microglial Activation in a Rat Model of Parkinson's Disease

Transforming growth factor (TGF)-β1 is a pleiotropic cytokine with immunosuppressive and anti-inflammatory properties. Recently we have shown that TGF-β1 pretreatment in vitro protects against 1-methyl-4-phenylpyridinium (MPP⁺)-induced dopaminergic neuronal loss that characterizes in Parkinson's disease (PD). Herein, we aimed to demonstrate that TGF-β1 administration in vivo after MPP⁺ toxicity has neuroprotection that is achieved by a mediation of microglia. A rat model of PD was prepared by injecting MPP⁺ unilaterally in the striatum. At 14 days after MPP⁺ injection, TGF-β1 was administrated in the right lateral cerebral ventricle. Primary ventral mesencephalic (VM) neurons and cerebral cortical microglia were treated by MPP⁺, respectively, and TGF-β1 was applied to neuronal or microglial cultures at 1 h after MPP⁺ treatment. As expected, MPP⁺ resulted in decrease in TGF-β1 production in the substantia nigra and in primary VM neurons and microglia. TGF-β1 intracerebroventricular administration alleviated MPP⁺-induced PD-like changes in pathology, motor coordination and behavior. Meanwhile, TGF-β1 ameliorated MPP⁺-induced microglial activation and inflammatory cytokine production in vivo. Interestingly, TGF-β1 treatment was not able to ameliorate MPP⁺-induced dopaminergic neuronal loss and caspase-3/9 activation in mono-neuron cultures, but TGF-β1 alleviated MPP⁺-induced microglial activation and inflammatory cytokine production in microglia-enriched cultures. This effect of TGF-β1 inhibiting microglial inflammatory response was blocked by Smad3 inhibitor SIS3. Importantly, neuronal exposure to supernatants of primary microglia that had been treated with TGF-β1 reduced dopaminergic neuronal loss and caspase-3/9 activation induced by MPP⁺-treated microglial supernatants. These findings establish that TGF-β1 exerts neuroprotective property in PD by inhibiting microglial inflammatory response via Smad3 signaling.

Spinal versus brain microglial and macrophage activation traits determine the differential neuroinflammatory responses and analgesic effect of minocycline in chronic neuropathic pain

Substantial evidence indicates involvement of microglia/macrophages in chronic neuropathic pain. However, the temporal-spatial features of microglial/macrophage activation and their pain-bound roles remain elusive. Here, we evaluated microglia/macrophages and the subtypes in the lumbar spinal cord (SC) and prefrontal cortex (PFC), and analgesic-anxiolytic effect of minocycline at different stages following spared nerve injury (SNI) in rats. While SNI enhanced the number of spinal microglia/macrophages since post-operative day (POD)3, pro-inflammatory MHCII⁺ spinal microglia/macrophages were unexpectedly less abundant in SNI rats than shams on POD21. By contrast, less abundant anti-inflammatory CD172a (SIRPα)⁺ microglia/macrophages were found in the PFC of SNI rats. Interestingly in naïve rats, microglial/macrophage expression of CD11b/c, MHCII and MHCII⁺/CD172a⁺ ratio were higher in the SC than the cortex. Consistently, multiple immune genes involved in anti-inflammation, phagocytosis, complement activation and M2 microglial/macrophage polarization were upregulated in the spinal dorsal horn and dorsal root ganglia but downregulated in the PFC of SNI rats. Furthermore, daily intrathecal minocycline treatment starting from POD0 for two weeks alleviated mechanical allodynia most robustly before POD3 and attenuated anxiety on POD9. Although minocycline dampened spinal MHCII⁺ microglia/macrophages until POD13, it failed to do so on cortical microglia/macrophages, indicating that dampening only spinal inflammation may not be enough to alleviate centralized pain at the chronic stage. Taken together, our data provide the first evidence that basal microglial/macrophage traits underlie differential region-specific responses to SNI and minocycline treatment, and suggest that drug treatment efficiently targeting not only spinal but also brain inflammation may be more effective in treating chronic neuropathic pain.

Insensitivity of astrocytes to interleukin 10 signaling following peripheral immune challenge results in prolonged microglial activation in the aged brain

Immune-activated microglia from aged mice produce exaggerated levels of cytokines. Despite high levels of microglial interleukin (IL)-10 in the aged brain, neuroinflammation was prolonged and associated with depressive-like deficits. Because astrocytes respond to IL-10 and, in turn, attenuate microglial activation, we investigated if astrocyte-mediated resolution of microglial activation was impaired with age. Here, aged astrocytes had a dysfunctional profile with higher glial fibrillary acidic protein, lower glutamate transporter expression, and significant cytoskeletal re-arrangement. Moreover, aged astrocytes had reduced expression of growth factors and IL-10 receptor-1 (IL-10R1). After in vivo lipopolysaccharide immune challenge, aged astrocytes had a molecular signature associated with reduced responsiveness to IL-10. This IL-10 insensitivity of aged astrocytes resulted in a failure to induce IL-10R1 and transforming growth factor β and resolve microglial activation. In addition, adult astrocytes reduced microglial activation when co-cultured ex vivo, whereas aged astrocytes did not. Consistent with the aging studies, IL-10R(KO) astrocytes did not augment transforming growth factor β after immune challenge and failed to resolve microglial activation. Collectively, a major cytokine-regulatory loop between activated microglia and astrocytes is impaired in the aged brain.

Venlafaxine treatment after endothelin-1-induced cortical stroke modulates growth factor expression and reduces tissue damage in rats

Neuromodulators, such as antidepressants, may contribute to neuroprotection by modulating growth factor expression to exert anti-inflammatory effects and to support neuronal plasticity after stroke. Our objective was to study whether early treatment with venlafaxine, a serotonin-norepinephrine reuptake inhibitor, modulates growth factor expression and positively contributes to reducing the volume of infarcted brain tissue resulting in increased functional recovery. We studied the expression of BDNF, FGF2 and TGF-β1 by examining their mRNA and protein levels and cellular distribution using quantitative confocal microscopy at 5 days after venlafaxine treatment in control and infarcted brains. Venlafaxine treatment did not change the expression of these growth factors in sham rats. In infarcted rats, BDNF mRNA and protein levels were reduced, while the mRNA and protein levels of FGF2 and TGF-β1 were increased. Venlafaxine treatment potentiated all of the changes that were induced by cortical stroke alone. In particular, increased levels of FGF2 and TGF-β1 were observed in astrocytes at 5 days after stroke induction, and these increases were correlated with decreased astrogliosis (measured by GFAP) and increased synaptophysin immunostaining at twenty-one days after stroke in venlafaxine-treated rats. Finally, we show that venlafaxine reduced infarct volume after stroke resulting in increased functional recovery, which was measured using ladder rung motor tests, at 21 days after stroke. Our results indicate that the early oral administration of venlafaxine positively contributes to neuroprotection during the acute and late events that follow stroke.

The biphasic function of microglia in ischemic stroke

Microglia are brain resident macrophages originated from primitive progenitor cells in the yolk sac. Microglia can be activated within hours and recruited to the lesion site. Traditionally, microglia activation is considered to play a deleterious role in ischemic stroke, as inhibition of microglia activation attenuates ischemia induced brain injury. However, increasing evidence show that microglia activation is critical for attenuating neuronal apoptosis, enhancing neurogenesis, and promoting functional recovery after cerebral ischemia. Differential polarization of microglia could likely explain the biphasic role of microglia in ischemia. We comprehensively reviewed the mechanisms involved in regulating microglia activation and polarization. The latest discoveries of microRNAs in modulating microglia function are discussed. In addition, the interaction between microglia and other cells including neurons, astrocytes, oligodendrocytes, and stem cells were also reviewed. Future therapies targeting microglia may not exclusively aim at suppressing microglia activation, but also at modulating microglia polarization at different stages of ischemic stroke. More work is needed to elucidate the cellular and molecular mechanisms of microglia polarization under ischemic environment. The roles of microRNAs and transplanted stem cells in mediating microglia activation and polarization during brain ischemia also need to be further studied.

Microglia Function in the Normal Brain

The activation of microglia has been recognized for over a century by their morphological changes. Long slender microglia acquire a short sturdy ramified shape when activated. During the past 20 years, microglia have been accepted as an essential cellular component for understanding the pathogenic mechanism of many brain diseases, including neurodegenerative diseases. More recently, functional studies and imaging in mouse models indicate that microglia are active in the healthy central nervous system. It has become evident that microglia release several signal molecules that play key roles in the crosstalk among brain cells, i.e., astrocytes and oligodendrocytes with neurons, as well as with regulatory immune cells. Recent studies also reveal the heterogeneous nature of microglia diverse functions depending on development, previous exposure to stimulation events, brain region of residence, or pathological state. Subjects to approach by future research are still the unresolved questions regarding the conditions and mechanisms that render microglia protective, capable of preventing or reducing damage, or deleterious, capable of inducing or facilitating the progression of neuropathological diseases. This novel knowledge will certainly change our view on microglia as therapeutic target, shifting our goal from their general silencing to the generation of treatments able to change their activation pattern.

Role of TGFβ signaling in the pathogenesis of Alzheimer's disease

Aging is the main risk factor for Alzheimer’s disease (AD); being associated with conspicuous changes on microglia activation. Aged microglia exhibit an increased expression of cytokines, exacerbated reactivity to various stimuli, oxidative stress, and reduced phagocytosis of β-amyloid (Aβ). Whereas normal inflammation is protective, it becomes dysregulated in the presence of a persistent stimulus, or in the context of an inflammatory environment, as observed in aging. Thus, neuroinflammation can be a self-perpetuating deleterious response, becoming a source of additional injury to host cells in neurodegenerative diseases. In aged individuals, although transforming growth factor β (TGFβ) is upregulated, its canonical Smad3 signaling is greatly reduced and neuroinflammation persists. This age-related Smad3 impairment reduces protective activation while facilitating cytotoxic activation of microglia through several cellular mechanisms, potentiating microglia-mediated neurodegeneration. Here, we critically discuss the role of TGFβ-Smad signaling on the cytotoxic activation of microglia and its relevance in the pathogenesis of AD. Other protective functions, such as phagocytosis, although observed in aged animals, are not further induced by inflammatory stimuli and TGFβ1. Analysis in silico revealed that increased expression of receptor scavenger receptor (SR)-A, involved in Aβ uptake and cell activation, by microglia exposed to TGFβ, through a Smad3-dependent mechanism could be mediated by transcriptional co-factors Smad2/3 over the MSR1 gene. We discuss that changes of TGFβ-mediated regulation could at least partially mediate age-associated microglia changes, and, together with other changes on inflammatory response, could result in the reduction of protective activation and the potentiation of cytotoxicity of microglia, resulting in the promotion of neurodegenerative diseases.

Microglial cell dysregulation in brain aging and neurodegeneration

Aging is the main risk factor for neurodegenerative diseases. In aging, microglia undergoes phenotypic changes compatible with their activation. Glial activation can lead to neuroinflammation, which is increasingly accepted as part of the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD). We hypothesize that in aging, aberrant microglia activation leads to a deleterious environment and neurodegeneration. In aged mice, microglia exhibit an increased expression of cytokines and an exacerbated inflammatory response to pathological changes. Whereas LPS increases nitric oxide (NO) secretion in microglia from young mice, induction of reactive oxygen species (ROS) predominates in older mice. Furthermore, there is accumulation of DNA oxidative damage in mitochondria of microglia during aging, and also an increased intracellular ROS production. Increased ROS activates the redox-sensitive nuclear factor kappa B, which promotes more neuroinflammation, and can be translated in functional deficits, such as cognitive impairment. Mitochondria-derived ROS and cathepsin B, are also necessary for the microglial cell production of interleukin-1β, a key inflammatory cytokine. Interestingly, whereas the regulatory cytokine TGFβ1 is also increased in the aged brain, neuroinflammation persists. Assessing this apparent contradiction, we have reported that TGFβ1 induction and activation of Smad3 signaling after inflammatory stimulation are reduced in adult mice. Other protective functions, such as phagocytosis, although observed in aged animals, become not inducible by inflammatory stimuli and TGFβ1. Here, we discuss data suggesting that mitochondrial and endolysosomal dysfunction could at least partially mediate age-associated microglial cell changes, and, together with the impairment of the TGFβ1-Smad3 pathway, could result in the reduction of protective activation and the facilitation of cytotoxic activation of microglia, resulting in the promotion of neurodegenerative diseases.

Potential neuroprotective role of transforming growth factor β1 (TGFβ1) in the brain

TGFβ1 is a growth factor that is known to be expressed in most neurodegenerative diseases and after vascular accidents in the brain. TGFβ1 downregulates the activity of activated microglia and promotes astrogliosis. It also prevents cell death by a known mechanism dependant on astrocytes and the secretion of the plasminogen activator inhibitor 1 (PAI-1). This mechanism can provide light on what is the mechanism of action of TGFβ1 as a protective factor and it can provide the pharmacological principles in which this pathway could be used with therapeutic purposes. TGFβ1 is upregulated in most neurodegenerative diseases, however, its expression appears dramatically blocked in Huntington's disease, the fastest of those diseases in progress after the onset. This fact suggests that TGFβ1 slows down the neurodegenerative process, preventing tissue damage and neural apoptotic death. However, the exact details of TGFβ1 action are still unknown and the physiological roles on the diseases are still mysterious. Interestingly, all the data regarding the roles of TGFβ1 in health and disease have been also confirmed with the use of transgenic knockouts and TGFβ1 overexpressing mice. What possibly came as a surprise from the study of TGFβ1 overexpressing models is that combining its neuroprotective and antiproliferative effects, this cytokine generates a significant disruption in the hippocampal circuitry with its consequent learning and memory deficit.

Age-dependent changes on TGFβ1 Smad3 pathway modify the pattern of microglial cell activation

Aging is the main risk factor for Alzheimer's disease. Among other characteristics, it shows changes in inflammatory signaling that could affect the regulation of glial cell activation. We have shown that astrocytes prevent microglial cell cytotoxicity by mechanisms mediated by TGFβ1. However, whereas TGFβ1 is increased, glial cell activation persists in aging. To understand this apparent contradiction, we studied TGFβ1-Smad3 signaling during aging and their effect on microglial cell function. TGFβ1 induction and activation of Smad3 signaling in the hippocampus by inflammatory stimulation was greatly reduced in adult mice. We evaluated the effect of TGFβ1-Smad3 pathway on the regulation of nitric oxide (NO) and reactive oxygen species (ROS) secretion, and phagocytosis of microglia from mice at different ages with and without in vivo treatment with lipopolysaccharide (LPS) to induce an inflammatory status. NO secretion was only induced on microglia from young mice exposed to LPS, and was potentiated by inflammatory preconditioning, whereas in adult mice the induction of ROS was predominant. TGFβ1 modulated induction of NO and ROS production in young and adult microglia, respectively. Modulation was partially dependent on Smad3 pathway and was impaired by inflammatory preconditioning. Phagocytosis was induced by inflammation and TGFβ1 only in microglia cultures from young mice. Induction by TGFβ1 was also prevented by Smad3 inhibition. Our findings suggest that activation of the TGFβ1-Smad3 pathway is impaired in aging. Age-related impairment of TGFβ1-Smad3 can reduce protective activation while facilitating cytotoxic activation of microglia, potentiating microglia-mediated neurodegeneration.

Reciprocal modulation between microglia and astrocyte in reactive gliosis following the CNS injury

Reactive gliosis, also known as glial scar formation, is an inflammatory response characterized by the proliferation of microglia and astrocytes as well as astrocytic hypertrophy following injury in the central nervous system (CNS). The glial scar forms a physical and molecular barrier to isolate the injured area from adjacent normal nervous tissue for re-establishing the integrity of the CNS. It prevents the further spread of cellular damage but represents an obstacle to regrowing axons. In this review, we integrated the current findings to elucidate the tightly reciprocal modulation between activated microglia and astrocytes in reactive gliosis and proposed that modification of cellular response to the injury or cellular reprogramming in the glial scar could lead advances in axon regeneration and functional recovery after the CNS injury.

Astrocytes inhibit nitric oxide-dependent Ca(2+) dynamics in activated microglia: involvement of ATP released via pannexin 1 channels

Under inflammatory conditions, microglia exhibit increased levels of free intracellular Ca(2+) and produce high amounts of nitric oxide (NO). However, whether NO, Ca(2+) dynamics, and gliotransmitter release are reciprocally modulated is not fully understood. More importantly, the effect of astrocytes in the potentiation or suppression of such signaling is unknown. Our aim was to address if astrocytes could regulate NO-dependent Ca(2+) dynamics and ATP release in LPS-stimulated microglia. Griess assays and Fura-2AM time-lapse fluorescence images of microglia revealed that LPS produced an increased basal [Ca(2+) ]i that depended on the sequential activation of iNOS, COXs, and EP1 receptor. TGFβ1 released by astrocytes inhibited the abovementioned responses and also abolished LPS-induced ATP release by microglia. Luciferin/luciferase assays and dye uptake experiments showed that release of ATP from LPS-stimulated microglia occurred via pannexin 1 (Panx1) channels, but not connexin 43 hemichannels. Moreover, in LPS-stimulated microglia, exogenous ATP triggered activation of purinergic P2Y1 receptors resulting in Ca(2+) release from intracellular stores. Interestingly, TGFβ1 released by astrocytes inhibited ATP-induced Ca(2+) response in LPS-stimulated microglia to that observed in control microglia. Finally, COX/EP1 receptor signaling and activation of P2 receptors via ATP released through Panx1 channels were critical for the increased NO production in LPS-stimulated microglia. Thus, Ca(2+) dynamics depended on the inflammatory profile of microglia and could be modulated by astrocytes. The understanding of mechanisms underlying glial cell regulatory crosstalk could contribute to the development of new treatments to reduce inflammatory cytotoxicity in several brain pathologies.

Scavenger receptor class A ligands induce secretion of IL1β and exert a modulatory effect on the inflammatory activation of astrocytes in culture

Class-A scavenger receptor (SR-A) is expressed by microglia, and we show here that it is also expressed by astrocytes, where it participates on their inflammatory activation. Astrocytes play a key role on the inflammatory response of the central nervous system, secreting several soluble mediators like cytokines and radical species. Exposure to SR ligands activated MAPKs and NF-κB signaling and increased production of IL1β and nitric oxide (NO). IL1β classically an inflammatory cytokine surprisingly did not increase but inhibited LPS+IFNγ-induced NO production by astrocytes. Our results suggest that SRs expressed by astrocytes participate in the modulation of inflammatory activation.

TGFβ signalling plays an important role in IL4-induced alternative activation of microglia

BACKGROUND

Microglia are the resident immune cells of the central nervous system and are accepted to be involved in a variety of neurodegenerative diseases. Several studies have demonstrated that microglia, like peripheral macrophages, exhibit two entirely different functional activation states, referred to as classical (M1) and alternative (M2) activation. TGFβ is one of the most important anti-inflammatory cytokines and its effect on inhibiting microglia or macrophage classical activation has been extensively studied. However, the role of TGFβ during alternative activation of microglia has not been described yet.

METHODS

To investigate the role of TGFβ in IL4-induced microglia alternative activation, both, BV2 as well as primary microglia from new born C57BL/6 mice were used. Quantitative RT-PCR and western blots were performed to detect mRNA and protein levels of the alternative activation markers Arginase1 (Arg1) and Chitinase 3-like 3 (Ym1) after treatment with IL4, TGFβ or both. Endogenous TGFβ release after IL4 treatment was evaluated using the mink lung epithelial cell (MLEC) assay and a direct TGFβ2 ELISA. TGFβ receptor type I inhibitor and MAPK inhibitor were applied to address the involvement of TGFβ signalling and MAPK signalling in IL4-induced alternative activation of microglia.

RESULTS

TGFβ enhances IL4-induced microglia alternative activation by strongly increasing the expression of Arg1 and Ym1. This synergistic effect on Arg1 induction is almost completely blocked by the application of the MAPK inhibitor, PD98059. Further, treatment of primary microglia with IL4 increased the expression and secretion of TGFβ2, suggesting an involvement of endogenous TGFβ in IL4-mediated microglia activation process. Moreover, IL4-mediated induction of Arg1 and Ym1 is impaired after blocking the TGFβ receptor I indicating that IL4-induced microglia alternative activation is dependent on active TGFβ signalling. Interestingly, treatment of primary microglia with TGFβ alone results in up regulation of the IL4 receptor alpha, indicating that TGFβ increases the sensitivity of microglia for IL4 signals.

CONCLUSIONS

Taken together, our data reveal a new role for TGFβ during IL4-induced alternative activation of microglia and consolidate the essential functions of TGFβ as an anti-inflammatory molecule and immunoregulatory factor for microglia.

Modulation of interferon-γ-induced glial cell activation by transforming growth factor β1: a role for STAT1 and MAPK pathways

Overactivated glial cells can produce neurotoxic oxidant molecules such as nitric oxide (NO·) and superoxide anion (O(2)·(-)). We have previously reported that transforming growth factor β1 (TGFβ1) released by hippocampal cells modulates interferon-γ (IFNγ)-induced production of O(2)·(-) and NO· by glial cells. However, underlying molecular mechanisms are not completely understood, thereby, the aim of this work was to study the effect of TGFβ1 on IFNγ-induced signaling pathways. We found that costimulation with TGFβ1 decreased IFNγ-induced phosphorylation of signal transducer and activator of transcription-type-1 (STAT1) and extracellular signal-regulated kinase (ERK), which correlated with a reduced O(2)·(-) and NO· production in mixed and purified glial cultures. Moreover, IFNγ caused a decrease in TGFβ1-mediated phosphorylation of P38, whereas pre-treatment with ERK and P38 inhibitors decreased IFNγ-induced phosphorylation of STAT1 on serine727 and production of radical species. These results suggested that modulation of glial activation by TGFβ1 is mediated by deactivation of MAPKs. Notably, TGFβ1 increased the levels of MAPK phosphatase-1 (MKP-1), whose participation in TGFβ1-mediated modulation was confirmed by MKP-1 siRNA transfection in mixed and purified glial cultures. Our results indicate that the cross-talk between IFNγ and TGFβ1 might regulate the activation of glial cells and that TGFβ1 modulated IFNγ-induced production of neurotoxic oxidant molecules through STAT1, ERK, and P38 pathways.

The effect of ribavirin on reactive astrogliosis in experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is an animal model of CNS inflammatory and demyelinating disease multiple sclerosis. Microglia and astrocytes represent two related cell types involved in the brain pathology in EAE. Accumulations of hypertrophic reactive astrocytes, intensely stained with glial fibrillary acidic protein (GFAP), which also expressed vimentin, are prominent features of EAE lesions. Recent studies from our laboratory reported that ribavirin attenuated the disease process in EAE by reducing clinical and histological manifestations. EAE was induced in genetically susceptible Dark Agouti rats with syngeneic spinal cord homogenate in complete Freund's adjuvant. Real time PCR and immunohistochemistry were used for determination of GFAP and vimentin gene and tissue expression. We have observed the increased gene and tissue expression of GFAP and vimentin in EAE rats. Ribavirin treatment significantly decreased the number of reactive astrocytes at the peak of disease. At the end of the disease, we have observed reactive GFAP(+) and vimentin(+) astrocytes in both immunized and ribavirin-treated groups, accompanied by increased level of GFAP mRNA. The present study indicates that ribavirin may have the ability to attenuate astrocyte proliferation and glial scaring at the peak of the disease and modulate the astroglial response to EAE during the time-course of the disease.

Alzheimer's disease: redox dysregulation as a common denominator for diverse pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia and a progressive neurodegeneration that appears to result from multiple pathogenic mechanisms (including protein misfolding/aggregation, involved in both amyloid β-dependent senile plaques and tau-dependent neurofibrillary tangles), metabolic and mitochondrial dysfunction, excitoxicity, calcium handling impairment, glial cell dysfunction, neuroinflammation, and oxidative stress. Oxidative stress, which could be secondary to several of the other pathophysiological mechanisms, appears to be a major determinant of the pathogenesis and progression of AD. The identification of oxidized proteins common for mild cognitive impairment and AD suggests that key oxidation pathways are triggered early and are involved in the initial progression of the neurodegenerative process. Abundant data support that oxidative stress, also considered as a main factor for aging, the major risk factor for AD, can be a common key element capable of articulating the divergent nature of the proposed pathogenic factors. Pathogenic mechanisms influence each other at different levels. Evidence suggests that it will be difficult to define a single-target therapy resulting in the arrest of progression or the improvement of AD deterioration. Since oxidative stress is present from early stages of disease, it appears as one of the main targets to be included in a clinical trial. Exploring the articulation of AD pathogenic mechanisms by oxidative stress will provide clues for better understanding the pathogenesis and progression of this dementing disorder and for the development of effective therapies to treat this disease.

Induction of nitric oxide synthase-2 expression and measurement of nitric oxide production in enriched primary cortical astrocyte cultures

Astrocytes produce numerous mediators under conditions of inflammation in the central nervous system. One such mediator is nitric oxide (NO) derived from nitric oxide synthase-2 (NOS-2), the high output, inducible NOS isoform. Expression of NOS-2 and production of NO can be stimulated in astrocyte cultures by combinations of cytokines and lipopolysaccharide, a gram-negative bacterial endotoxin. This chapter details methods to induce and analyze NOS-2 expression and NO production in astrocyte cultures.

Astrocytes and microglia: responses to neuropathological conditions

Activated astrocytes and microglia, hallmark of neurodegenerative diseases release different factors like array of pro and anti-inflammatory cytokines, free radicals, anti-oxidants, and neurotrophic factors during neurodegeneration which further contribute to neuronal death as well as in survival mechanisms. Astrocytes act as double-edged sword exerting both detrimental and neuroprotective effects while microglial cells are attributed more in neurodegenerative mechanisms. The dual and insufficient knowledge about the precise role of glia in neurodegeneration showed the need for further investigations and thorough review of the function of glia in neurodegeneration. In this review, we consolidate and categorize the glia-released factors which contribute in degenerative and protective mechanisms during neuropathological conditions.

Cross talk between activation of microglia and astrocytes in pathological conditions in the central nervous system

Microglia and astrocytes in the central nervous system are now recognized as active participants in various pathological conditions such as trauma, stroke, or chronic neurodegenerative disorders. Their activation is closely related with the development and severity of diseases. Interestingly, activation of microglia and astrocytes occurs with a spatially and temporarily distinct pattern. The present review explores the cross talk in the process of their activation. Microglia, activated earlier than astrocytes, promote astrocytic activation. On the other hand, activated astrocytes not only facilitate activation of distant microglia, but also inhibit microglial activities. Molecules contributing to their intercommunication include interleukin-1 (IL-1), adenosine triphosphate (ATP), and transforming growth factor beta (TGF-β). A better understanding about the cross talk between activation of microglia and astrocytes would be helpful to elucidate the role of glial cells in pathological conditions, which could accelerate the development of treatment for various diseases.

TGF-β and LPS modulate ADP-induced migration of microglial cells through P2Y1 and P2Y12 receptor expression

Nucleotides act as early signals for microglial recruitment to sites of CNS injury. As microglial motility and activation can be influenced by several local factors at the site of the lesion, we investigated the effects of interferon-gamma, lipopolysaccharide (LPS) or transforming growth factor-β (TGF-β) addition to mixed glial cell cultures, on microglial migration in response to ADP, P2Y12 and P2Y1 mRNA expression as well as on the expression of an array of genes associated with the process of microglial activation. First, we demonstrated, by pharmacological inhibition and by using small interfering RNAs, that in addition to P2Y12, P2Y1 is involved in ADP-stimulated microglial migration. The ability of specific agonists to induce Ca(2+) mobilization further confirmed the expression of functional P2Y receptors in microglia. Then, we found that migratory capability and expression of both P2Y receptors were abrogated in microglial cells from LPS-stimulated mixed glial cultures, while TGF-β increased ADP-induced migration and the expression of P2Y12 and P2Y1 receptors. Interferon-gamma did not influence receptor expression or microglial migration. Finally, the patterns of gene expression induced in microglia by LPS or TGF-β treatment of mixed glial cultures were clearly distinct. LPS induced a set of classical pro-inflammatory genes, whereas TGF-β increased the expression of genes associated with atypical microglial phenotype, namely arginase-1 and TGF-β genes. These results imply that both P2Y1 and P2Y12 may guide microglia toward the lesion. They also suggest that the modulation of microglial purinergic receptors expression by local factors, through direct and/or astrocyte-mediated actions, may represent a novel mechanism affecting neuroinflammatory response.

Role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination of the CNS

As the resident innate immune cells of the central nervous system (CNS), microglia fulfil a critical role in maintaining tissue homeostasis and in directing and eliciting molecular responses to CNS damage. The human disease Multiple Sclerosis and animal models of inflammatory demyelination are characterized by a complex interplay between degenerative and regenerative processes, many of which are regulated and mediated by microglia. Cellular communication between microglia and other neural and immune cells is controlled to a large extent by the activity of cytokines. Here we review the role of cytokines as mediators and regulators of microglial activity in inflammatory demyelination, highlighting their importance in potentiating cell damage, promoting neuroprotection and enhancing cellular repair in a context-dependent manner.

Aging-dependent changes of microglial cells and their relevance for neurodegenerative disorders

Among multiple structural and functional brain changes, aging is accompanied by an increase of inflammatory signaling in the nervous system as well as a dysfunction of the immune system elsewhere. Although the long-held view that aging involves neurocognitive impairment is now dismissed, aging is a major risk factor for neurodegenerative diseases such as Alzheimer;s disease, Parkinson;s disease and Huntington's disease, among others. There are many age-related changes affecting the brain, contributing both to certain declining in function and increased frailty, which could singly and collectively affect neuronal viability and vulnerability. Among those changes, both inflammatory responses in aged brains and the altered regulation of toll like receptors, which appears to be relevant for understanding susceptibility to neurodegenerative processes, are linked to pathogenic mechanisms of several diseases. Here, we review how aging and pro-inflammatory environment could modulate microglial phenotype and its reactivity and contribute to the genesis of neurodegenerative processes. Data support our idea that age-related microglial cell changes, by inducing cytotoxicity in contrast to neuroprotection, could contribute to the onset of neurodegenerative changes. This view can have important implications for the development of new therapeutic approaches.

Microglial cells and Parkinson's disease

Chronic inflammation mediated by microglial cells is the fundamental process contributing to the death of dopamine (DA)-producing neurons in the brain. Production of inflammatory products by these microglial cells characterizes the slow destructive process in Parkinson's disease (PD). The activation of microglial cells and the generation of pro-inflammatory cytokines that characterize PD are mediated by several different signaling pathways, with the activation of the respiratory burst by microglial cells being a critical event in the ultimate toxicity of DA-neurons. The work on our lab is concerned with understanding the mechanisms of activation, response, and therapeutic targets of microglial cells, with the aim to provide more effective treatments for PD and other inflammatory diseases of the CNS.

Potent anti-inflammatory and neuroprotective effects of TGF-beta1 are mediated through the inhibition of ERK and p47phox-Ser345 phosphorylation and translocation in microglia

TGF-beta1 is one of the most potent endogenous immune modulators of inflammation. The molecular mechanism of its anti-inflammatory effect on the activation of the transcription factor NF-kappaB has been well-studied; however, the potential effects of TGF-beta1 on other proinflammatory signaling pathways is less clear. In this study, using the well-established LPS and the 1-methyl-4-phenylpyridinium-mediated models of Parkinson's disease, we demonstrate that TGF-beta1 exerts significant neuroprotection in both models via its anti-inflammatory properties. The neuroprotective effects of TGF-beta1 are mainly attributed to its ability to inhibit the production of reactive oxygen species from microglia during their activation or reactivation. Moreover, we demonstrate that TGF-beta1 inhibited LPS-induced NADPH oxidase (PHOX) subunit p47phox translocation from the cytosol to the membrane in microglia within 10 min. Mechanistic studies show that TGF-beta1 fails to protect dopaminergic neurons in cultures from PHOX knockout mice, and significantly reduced LPS-induced translocation of the PHOX cytosolic subunit p47phox to the cell membrane. In addition, LPS-induced ERK phosphorylation and subsequent Ser345 phosphorylation on p47phox were significantly inhibited by TGF-beta1 pretreatment. Taken together, our results show that TGF-beta1 exerted potent anti-inflammatory and neuroprotective properties, either through the prevention of the direct activation of microglia by LPS, or indirectly through the inhibition of reactive microgliosis elicited by 1-methyl-4-phenylpyridinium. The molecular mechanisms of TGF-beta1-mediated anti-inflammatory properties is through the inhibition of PHOX activity by preventing the ERK-dependent phosphorylation of Ser345 on p47phox in microglia to reduce oxidase activities induced by LPS.

Glial cell dysregulation: a new perspective on Alzheimer disease

Alzheimer disease (AD) is a major cause of dementia. Several mechanisms have been postulated to explain its pathogenesis, beta-amyloid (A beta toxicity, cholinergic dysfunction, Tau hyper-phosphorylation, oxidative damage, synaptic dysfunction and inflammation secondary to senile plaques, among others. Glial cells are the major producers of inflammatory mediators, and cytotoxic activation of glial cells is linked to several neurodegenerative diseases; however, whether inflammation is a consequence or the cause of neurodegeneration is still unclear. I propose that inflammation and cellular stress associated with aging are key events in the development of AD through the induction of glial dysfunction. Dysregulated inflammatory response can elicit glial cell activation by compounds which are normally poorly reactive. Inflammation can also be the major cause of defective handling of A beta and the amyloid precursor protein (APP). Here I review evidence that support the proposal that dysfunctional glia and the resulting neuroinflammation can explain many features of AD. Evidence supports the notion that damage caused by inflammation is not only a primary cause of neurodegeneration but also an inducer for the accumulation of A beta in AD. Dysfunctional glia can result in impaired neuronal function in AD, as well as in many progressive neurodegenerative disorders. We show that microglial cell activation is enhanced under pro-inflammatory conditions, indicating that glial cell responses to A beta related proteins can be critically dependent on the priming of glial cells by pro-inflammatory factors.