Glutamate receptor-mediated regulation of c-fos expression in cultured microglia

Chronic Variable Stress and Cafeteria Diet Combination Exacerbate Microglia and c-fos Activation but Not Experimental Anxiety or Depression in a Menopause Model

The menopause transition is a vulnerable period for developing both psychiatric and metabolic disorders, and both can be enhanced by stressful events worsening their effects. The present study aimed to evaluate whether a cafeteria diet (CAF) combined with chronic variable stress (CVS) exacerbates anxious- or depressive-like behavior and neuronal activation, cell proliferation and survival, and microglia activation in middle-aged ovariectomized (OVX) rats. In addition, body weight, lipid profile, insulin resistance, and corticosterone as an index of metabolic changes or hypothalamus-pituitary-adrenal (HPA) axis activation, and the serum pro-inflammatory cytokines IL-6, IL-β, and TNFα were measured. A CAF diet increased body weight, lipid profile, and insulin resistance. CVS increased corticosterone and reduced HDL. A CAF produced anxiety-like behaviors, whereas CVS induced depressive-like behaviors. CVS increased serum TNFα independently of diet. A CAF and CVS separately enhanced the percentage of Iba-positive cells in the hippocampus; the combination of factors further increased Iba-positive cells in the ventral hippocampus. A CAF and CVS increased the c-fos-positive cells in the hippocampus; the combination of factors increased the number of positive cells expressing c-fos in the ventral hippocampus even more. The combination of a CAF and CVS generates a slight neuroinflammation process and neuronal activation in a hippocampal region-specific manner and differentially affects the behavior.

The Role of Intracellular Ca2+ and Mitochondrial ROS in Small Aβ1-42 Oligomer-Induced Microglial Death

Alzheimer's disease (AD) is the most common form of dementia worldwide, and it contributes up to 70% of cases. AD pathology involves abnormal amyloid beta (Aβ) accumulation, and the link between the Aβ1-42 structure and toxicity is of major interest. NMDA receptors (NMDAR) are thought to be essential in Aβ-affected neurons, but the role of this receptor in glial impairment is still unclear. In addition, there is insufficient knowledge about the role of Aβ species regarding mitochondrial redox states in neurons and glial cells, which may be critical in developing Aβ-caused neurotoxicity. In this study, we investigated whether different Aβ1-42 species-small oligomers, large oligomers, insoluble fibrils, and monomers-were capable of producing neurotoxic effects via microglial NMDAR activation and changes in mitochondrial redox states in primary rat brain cell cultures. Small Aβ1-42 oligomers induced a concentration- and time-dependent increase in intracellular Ca2+ and necrotic microglial death. These changes were partially prevented by the NMDAR inhibitors MK801, memantine, and D-2-amino-5-phosphopentanoic acid (DAP5). Neither microglial intracellular Ca2+ nor viability was significantly affected by larger Aβ1-42 species or monomers. In addition, the small Aβ1-42 oligomers caused mitochondrial reactive oxygen species (mtROS)-mediated mitochondrial depolarization, glutamate release, and neuronal cell death. In microglia, the Aβ1-42-induced mtROS overproduction was mediated by intracellular calcium ions and Aβ-binding alcohol dehydrogenase (ABAD). The data suggest that the pharmacological targeting of microglial NMDAR and mtROS may be a promising strategy for AD therapy.

Molecular and neuroimmune pharmacology of S1P receptor modulators and other disease-modifying therapies for multiple sclerosis

Multiple sclerosis (MS) is a neurological, immune-mediated demyelinating disease that affects people in the prime of life. Environmental, infectious, and genetic factors have been implicated in its etiology, although a definitive cause has yet to be determined. Nevertheless, multiple disease-modifying therapies (DMTs: including interferons, glatiramer acetate, fumarates, cladribine, teriflunomide, fingolimod, siponimod, ozanimod, ponesimod, and monoclonal antibodies targeting ITGA4, CD20, and CD52) have been developed and approved for the treatment of MS. All the DMTs approved to date target immunomodulation as their mechanism of action (MOA); however, the direct effects of some DMTs on the central nervous system (CNS), particularly sphingosine 1-phosphate (S1P) receptor (S1PR) modulators, implicate a parallel MOA that may also reduce neurodegenerative sequelae. This review summarizes the currently approved DMTs for the treatment of MS and provides details and recent advances in the molecular pharmacology, immunopharmacology, and neuropharmacology of S1PR modulators, with a special focus on the CNS-oriented, astrocyte-centric MOA of fingolimod.

From Determining Brain Insulin Resistance in a Sporadic Alzheimer's Disease Model to Exploring the Region-Dependent Effect of Intranasal Insulin

Impaired response to insulin has been linked to many neurodegenerative disorders like Alzheimer's disease (AD). Animal model of sporadic AD has been developed by intracerebroventricular (icv) administration of streptozotocin (STZ), which given peripherally causes insulin resistance. Difficulty in demonstrating insulin resistance in this model led to our aim: to determine brain regional and peripheral response after intranasal (IN) administration of insulin in control and STZ-icv rats, by exploring peripheral and central metabolic parameters. One month after STZ-icv or vehicle-icv administration to 3-month-old male Wistar rats, cognitive status was determined after which rats received 2 IU of fast-acting insulin aspart intranasally (CTR + INS; STZ + INS) or saline only (CTR and STZ). Rats were sacrificed 2 h after administration and metabolic and glutamatergic parameters were measured in plasma, CSF, and the brain. Insulin and STZ increased amyloid-β concentration in plasma (CTR + INS and STZ vs CTR), while there was no effect on glucose and insulin plasma and CSF levels. INS normalized the levels of c-fos in temporal cortex of STZ + INS vs STZ (co-localized with neurons), while hypothalamic c-fos was found co-localized with the microglial marker. STZ and insulin brain region specifically altered the levels and activity of proteins involved in cell metabolism and glutamate signaling. Central changes found after INS in STZ-icv rats suggest hippocampal and cortical insulin sensitivity. Altered hypothalamic metabolic parameters of STZ-icv rats were not normalized by INS, indicating possible hypothalamic insulin insensitivity. Brain insulin sensitivity depends on the affected brain region and presence of metabolic dysfunction induced by STZ-icv administration.

Loss of cannabinoid receptor 2 promotes α-Synuclein-induced microglial synaptic pruning in nucleus accumbens by modulating the pCREB-c-Fos signaling pathway and complement system

The disruption of nucleus accumbens (NAc) function impacts mood and learning behavior in α-Synucleinopathy, in which microglial synaptic pruning plays a pivotal role in modulating the neuropathologic progression. Available literature documents that in microglia, the activation of cannabinoid receptor 2 (CB2R) decreases inflammation, but it remains obscured regarding the roles of CB2R in microglia-mediated synaptic pruning in the NAc during the neuropathological progression of α-Synucleinopathy. We adopted the fibrillar α-Synuclein (α-Syn) treatment to characterize the effect of genetic CB2R deletion on microglial function and the signaling pathway. CB2R knockout (CB2^-/-) mice and wild-type (CB2^+/+) mice were divided into the α-Syn or saline treatment groups. Biochemical and microscopy approaches, including immunofluorescence, real-time PCR, and western blotting, were employed to assess the changes in homeostasis of synaptic pruning in NAc under the α-Syn-induced microglia. Moreover, the underlying mechanisms of CB2R on α-Syn induced microglial activity was assessed in vitro. After the injection of α-Syn into the NAc, distinct microglial morphological changes and M1 phenotype transformation were observed between CB2^-/- and CB2^+/+ mice. Meanwhile, after the α-Syn treatment, CB2^-/- mice showed an increased upregulation of CD68 protein and IL-1β mRNA but decreased brain-derived neurotrophic factor (BDNF) and TGF-β mRNA compared with CB2^+/+ mice. Additionally, CB2^-/- microglia after the treatment showed a highly enriched complement 3a receptor (C3aR) producing excessive pruning of cholinergic synapses but less engulfment of dopaminergic synapses. Mechanistically, the loss of CB2R function in the α-Syn stimulation triggered c-Fos activation in microglia, but not in neurons. Further inhibition of microglial CB2R functions under α-Syn stimulation activated the phosphorylated cAMP-response element-binding protein (pCREB)-c-Fos, which was closely related to the C3aR upregulation. Our results reveal a critical and mechanistic role of CB2R in altering the microglial function and its value in the homeostasis of synaptic circuits in the NAc under the α-Syn pathology.

cFOS expression in the prefrontal cortex correlates with altered cerebral metabolism in developing germ-free mice

Introduction

The microbiota plays a critical role in modulating various aspects of host physiology, particularly through the microbiota-gut-brain (MGB) axis. However, the mechanisms that transduce and affect gut-to-brain communication are still not well understood. Recent studies have demonstrated that dysbiosis of the microbiome is associated with anxiety and depressive symptoms, which are common complications of metabolic syndrome. Germ-free (GF) animal models offer a valuable tool for studying the causal effects of microbiota on the host.

Methods

We employed gene expression and nuclear magnetic resonance (NMR)-based metabolomic techniques to investigate the relationships between brain plasticity and immune gene expression, peripheral immunity, and cerebral and liver metabolism in GF and specific pathogen-free (SPF) mice.

Results

Our principal findings revealed that brain acetate (p = 0.012) was significantly reduced in GF relative to SPF mice, whereas glutamate (p = 0.0013), glutamine (p = 0.0006), and N-acetyl aspartate (p = 0.0046) metabolites were increased. Notably, cFOS mRNA expression, which was significantly decreased in the prefrontal cortex of GF mice relative to SPF mice (p = 0.044), correlated with the abundance of a number of key brain metabolites altered by the GF phenotype, including glutamate and glutamine.

Discussion

These results highlight the connection between the GF phenotype, altered brain metabolism, and immediate-early gene expression. The study provides insight into potential mechanisms by which microbiota can regulate neurotransmission through modulation of the host's brain and liver metabolome, which may have implications for stress-related psychiatric disorders such as anxiety.

Immediate Early Gene c-fos in the Brain: Focus on Glial Cells

The c-fos gene was first described as a proto-oncogene responsible for the induction of bone tumors. A few decades ago, activation of the protein product c-fos was reported in the brain after seizures and other noxious stimuli. Since then, multiple studies have used c-fos as a brain activity marker. Although it has been attributed to neurons, growing evidence demonstrates that c-fos expression in the brain may also include glial cells. In this review, we collect data showing that glial cells also express this proto-oncogene. We present evidence demonstrating that at least astrocytes, oligodendrocytes, and microglia express this immediate early gene (IEG). Unlike neurons, whose expression changes used to be associated with depolarization, glial cells seem to express the c-fos proto-oncogene under the influence of proliferation, differentiation, growth, inflammation, repair, damage, plasticity, and other conditions. The collected evidence provides a complementary view of c-fos as an activity marker and urges the introduction of the glial cell perspective into brain activity studies. This glial cell view may provide additional information related to the brain microenvironment that is difficult to obtain from the isolated neuron paradigm. Thus, it is highly recommended that detection techniques are improved in order to better differentiate the phenotypes expressing c-fos in the brain and to elucidate the specific roles of c-fos expression in glial cells.

Gene expression changes in the brain of a Cushing's syndrome mouse model

Excess glucocorticoid exposure affects emotional and cognitive brain functions. The extreme form, Cushing's syndrome, is adequately modelled in the AdKO_2.0 mouse, consequential to adrenocortical hypertrophy and hypercorticosteronemia. We previously reported that the AdKO_2.0 mouse brain undergoes volumetric changes that resemble closely those of Cushing's syndrome human patients, as well as changes in expression of glial related marker proteins. In the present work, the expression of genes related to glial and neuronal cell populations and functions was assessed in regions of the anterior brain, hippocampus, amygdala and hypothalamus. Glucocorticoid target genes were consistently regulated, including CRH mRNA suppression in the hypothalamus and induction in amygdala and hippocampus, even if glucocorticoid receptor protein was downregulated. Expression of glial genes was also affected in the AdKO_2.0 mouse brain, indicating a different activation status in glial cells. Generic markers for neuronal cell populations, and cellular integrity were only slightly affected. Our findings highlight the vulnerability of glial cell populations to chronic high levels of circulating glucocorticoids.

Transcriptome analysis reveals the contribution of oligodendrocyte and radial glia-derived cues for maintenance of microglia identity

Microglia are increasingly being recognized as druggable targets in neurodegenerative disorders, and good in vitro models are crucial to address cell biological questions. Major challenges are to recapitulate the complex microglial morphology and their in vivo transcriptome. We have therefore exposed primary microglia from adult rhesus macaques to a variety of different culture conditions including exposure to soluble factors as M-CSF, IL-34, and TGF-β as well as serum replacement approaches, and compared their morphologies and transcriptomes to those of mature, homeostatic in vivo microglia. This enabled us to develop a new, partially serum-free, monoculture protocol, that yields high numbers of ramified cells. We also demonstrate that exposure of adult microglia to M-CSF or IL-34 induces similar transcriptomes, and that exposure to TGF-β has much less pronounced effects than it does on rodent microglia. However, regardless of culture conditions, the transcriptomes of in vitro and in vivo microglia remained substantially different. Analysis of differentially expressed genes inspired us to perform 3D-spherical coculture experiments of microglia with oligodendrocytes and radial glia. In such spheres, microglia signature genes were strongly induced, even in the absence of neurons and astrocytes. These data reveal a novel role for oligodendrocyte and radial glia-derived cues in the maintenance of microglial identity, providing new anchor points to study microglia in health and disease.

Identification of Socially-activated Neurons

Determining the neuronal circuitry responsible for specific behaviors is a major focus in the field of neurobiology. Activity-dependent immediate early genes (IEGs), transcribed and translated shortly after neurons discharge action potentials, have been used extensively to either identify or gain genetic access to neurons and brain regions involved in such behaviors. By using immunohistochemistry for the protein product of the IEG c-Fos combined with retrograde labeling of specific neuronal populations, precise experimental timing, and identical data acquisition and processing, we present a method to quantitatively identify specific neuronal subpopulations that were active during social encounters. We have previously used this method to show a stronger recruitment of ventral hippocampal neurons that project to the medial prefrontal cortex, compared to those that project to the lateral hypothalamus, following social interactions. After optimization of surgeries for the injection of retrograde tracers, this method will be useful for the identification and mapping of neuronal populations engaged in many different behaviors.

Microglial NMDA receptors drive pro-inflammatory responses via PARP-1/TRMP2 signaling

Chronic neuroinflammation driven by microglia is a characteristic feature associated with numerous neurodegenerative diseases. While acute inflammation can assist with recovery and repair, prolonged microglial pro-inflammatory responses are known to exacerbate neurodegenerative processes. Yet, detrimental outcomes of extended microglial activation are counterbalanced by beneficial outcomes including phagocytosis and release of trophic factors promoting neuronal viability. Our past work has shown that the nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) is a key signaling hub driving pro-inflammatory microglia responses, but the signaling pathway maintaining PARP-1 activation remains elusive. While best understood for its role in promoting DNA repair, our group has shown that PARP-1 activity can be stimulated via Ca²⁺ influx-dependent ERK1/2-mediated phosphorylation. However, to date, the route of Ca²⁺ entry responsible for stimulating PARP-1 has not been identified. A likely candidate is via Ca²⁺ -permeable transient receptor potential melastatin 2 (TRPM2) channels activated downstream of PARP-1 in a cascade that involves ADP-ribose (ADPR) production by poly(ADP-ribose) glycohydrolase (PARG). Here we demonstrate that NMDA receptor (NMDAR) stimulation in primary cultured microglia induces their proliferation, morphological activation and release of pro-inflammatory mediators. These responses were contingent on the recruitment of PARP-1, PARG and Ca²⁺ permeable TRPM2 channels. Furthermore, we show that Ca²⁺ influx is necessary to activate PARP-1/TRPM2 signaling, in an ERK1/2-dependent, but DNA damage independent, manner. Our findings, showing that PARP-1/TRPM2 mediate the pro-inflammatory effects of NMDAR stimulation, provides a unifying mechanism linking elevated glutamate levels to chronic neuroinflammation.

A Functionally Defined In Vivo Astrocyte Population Identified by c-Fos Activation in a Mouse Model of Multiple Sclerosis Modulated by S1P Signaling: Immediate-Early Astrocytes (ieAstrocytes)

Astrocytes have prominent roles in central nervous system (CNS) function and disease, with subpopulations defined primarily by morphologies and molecular markers often determined in cell culture. Here, we identify an in vivo astrocyte subpopulation termed immediate-early astrocytes (ieAstrocytes) that is defined by functional c-Fos activation during CNS disease development. An unbiased screen for CNS cells showing c-Fos activation during experimental autoimmune encephalomyelitis (EAE), a mouse model for multiple sclerosis (MS), was developed by using inducible, TetTag c-Fos reporter mice that label activated cells with a temporally stable, nuclear green fluorescent protein (GFP). Four-dimensional (3D over time) c-Fos activation maps in the spinal cord were produced by combining tissue clearing (iDISCO) and confocal microscopy that identified onset and expansion of GFP+ cell populations during EAE. More than 95% of the GFP+ cells showed glial fibrillary acidic protein (GFAP) immunoreactivity-in contrast to absent or rare labeling of neurons, microglia, and infiltrating immune cells-which constituted ieAstrocytes that linearly increased in number with progression of EAE. ieAstrocyte formation was reduced by either astrocyte-specific genetic removal of sphingosine 1-phosphate receptor 1 (S1P1) or pharmacological inhibition by fingolimod (FTY720), an FDA-approved MS medicine that can functionally antagonize S1P1. ieAstrocytes thus represent a functionally defined subset of disease-linked astrocytes that are the first and predominant CNS cell population activated during EAE, and that track with disease severity in vivo. Their reduction by a disease-modifying agent supports their therapeutic relevance to MS and potentially other neuroinflammatory and neurodegenerative diseases.

The Role of Ion Channels in Microglial Activation and Proliferation - A Complex Interplay between Ligand-Gated Ion Channels, K(+) Channels, and Intracellular Ca(2.)

Microglia are often referred to as the immune cells of the brain. They are most definitely involved in immune responses to invading pathogens and inflammatory responses to tissue damage. However, recent results suggest microglia are vital for normal functioning of the brain. Neuroinflammation, as well as more subtle changes, in microglial function has been implicated in the pathogenesis of many brain diseases and disorders. Upon sensing alterations in their local environment, microglia change their shape and release factors that can modify the excitability of surrounding neurons. During neuroinflammation, microglia proliferate and release NO, reactive oxygen species, cytokines and chemokines. If inflammation resolves then their numbers normalize again via apoptosis. Microglia express a wide array of ion channels and different types are implicated in all of the cellular processes listed above. Modulation of microglial ion channels has shown great promise as a therapeutic strategy in several brain disorders. In this review, we discuss recent advances in our knowledge of microglial ion channels and their roles in responses of microglia to changes in the extracellular milieu.

Dieckol Attenuates Microglia-mediated Neuronal Cell Death via ERK, Akt and NADPH Oxidase-mediated Pathways

Excessive microglial activation and subsequent neuroinflammation lead to synaptic loss and dysfunction as well as neuronal cell death, which are involved in the pathogenesis and progression of several neurodegenerative diseases. Thus, the regulation of microglial activation has been evaluated as effective therapeutic strategies. Although dieckol (DEK), one of the phlorotannins isolated from marine brown alga Ecklonia cava, has been previously reported to inhibit microglial activation, the molecular mechanism is still unclear. Therefore, we investigated here molecular mechanism of DEK via extracellular signal-regulated kinase (ERK), Akt and nicotinamide adenine dinuclelotide phosphate (NADPH) oxidase-mediated pathways. In addition, the neuroprotective mechanism of DEK was investigated in microglia-mediated neurotoxicity models such as neuron-microglia co-culture and microglial conditioned media system. Our results demonstrated that treatment of anti-oxidant DEK potently suppressed phosphorylation of ERK in lipopolysaccharide (LPS, 1 µg/ml)-stimulated BV-2 microglia. In addition, DEK markedly attenuated Akt phosphorylation and increased expression of gp91^phox, which is the catalytic component of NADPH oxidase complex responsible for microglial reactive oxygen species (ROS) generation. Finally, DEK significantly attenuated neuronal cell death that is induced by treatment of microglial conditioned media containing neurotoxic secretary molecules. These neuroprotective effects of DEK were also confirmed in a neuron-microglia co-culture system using enhanced green fluorescent protein (EGFP)-transfected B35 neuroblastoma cell line. Taken together, these results suggest that DEK suppresses excessive microglial activation and microglia-mediated neuronal cell death via downregulation of ERK, Akt and NADPH oxidase-mediated pathways.

Microglia toxicity in preterm brain injury

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Microglia responses in the preterm human brain in association with injury.

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Microglia responses in animal models of preterm brain injury.

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Mechanisms of microglia toxicity from in vitro primary microglia cell culture experiments.

Microglia are the resident phagocytic cells of the central nervous system. During brain development they are also imperative for apoptosis of excessive neurons, synaptic pruning, phagocytosis of debris and maintaining brain homeostasis. Brain damage results in a fast and dynamic microglia reaction, which can influence the extent and distribution of subsequent neuronal dysfunction. As a consequence, microglia responses can promote tissue protection and repair following brain injury, or become detrimental for the tissue integrity and functionality. In this review, we will describe microglia responses in the human developing brain in association with injury, with particular focus on the preterm infant. We also explore microglia responses and mechanisms of microglia toxicity in animal models of preterm white matter injury and in vitro primary microglia cell culture experiments.

Calcium ion influx in microglial cells: physiological and therapeutic significance

Microglial cells, the immunocompetent cells of the central nervous system (CNS), exhibit a resting phenotype under healthy conditions. In response to injury, however, they transform into an activated state, which is a hallmark feature of many CNS diseases. Factors or agents released from the neurons, blood vessels, and/or astrocytes could activate these cells, leading to their functional and structural modifications. Microglial cells are well equipped to sense environmental changes within the brain under both physiological and pathological conditions. Entry of calcium ions (Ca(2+)) plays a critical role in the process of microglial transformation; several channels and receptors have been identified on the surface of microglial cells. These include store-operated channel, Orai1, and its sensor protein, stromal interaction molecule 1 (STIM1), in microglial cells, and their functions are modulated under pathological stimulations. Transient receptor potential (TRP) channels and voltage- and ligand-gated channels (ionotropic and metabotropic receptors) are also responsible for Ca(2+) influx into the microglial cells. An elevation of intracellular Ca(2+) concentration subsequently regulates microglial cell functions by activating a diverse array of Ca(2+)-sensitive signaling cascades. Perturbed Ca(2+) homeostasis contributes to the progression of a number of CNS disorders. Thus, regulation of Ca(2+) entry into microglial cells could be a pharmacological target for several CNS-related pathological conditions. This Review addresses the recent insights into microglial cell Ca(2+) influx mechanisms, their roles in the regulation of functions, and alterations of Ca(2+) entry in specific CNS disorders.

Neuroprotective effects of Gua Lou Gui Zhi decoction against glutamate-induced apoptosis in BV-2 cells

Gua Lou Gui Zhi decoction (GLGZD), a traditional Chinese medicine consisting of different herbal medicines, has been used for centuries in the treatment of muscular spasticity following stroke, epilepsy or spinal cord injury. However, the precise mechanisms involved remain poorly understood. In the present study, we investigated the neuroprotective effects of GLGZD on glutamate-induced apoptosis in cultured BV-2 cells, as well as the underlying mechanisms. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was applied to assess the viability of the cells. An Annexin V/propidium iodide (PI) assay was utilized to analyze cellular apoptosis. Mitochondrial membrane potential (MMP) was evaluated by flow cytometry and laser scanning confocal microscopy. The gene and protein expression of the apoptosis-related genes, Bcl-2 and Bax, was analyzed by RT-PCR and western blot analysis, respectively. Furthermore, the expression of cleaved caspase-3 protein was detected by immunofluorescence. Glutamate treatment induced the loss of BV-2 cell viability, which was associated with an increase in the apoptotic rate, as well as an increase in the Bax/Bcl-2 ratio and the extracellular levels of cleaved caspase-3. Treatment with GLGZD significantly reversed these phenotypes, with its maximum protective effects observed at the concentration of 1,000 µg/ml. These results indicate that GLGZD protects BV-2 cells from glutamate-induced cytotoxicity. These protective effects may be ascribed to its anti-apoptotic activities, in part, associated with the decrease in the Bax/Bcl-2 ratio and caspase-3 expression, as well as with the stability of high mitochondrial membrane potential.

Neurotransmitter signaling in the pathophysiology of microglia

Microglial cells are the resident immune cells of the central nervous system. In the resting state, microglia are highly dynamic and control the environment by rapidly extending and retracting motile processes. Microglia are closely associated with astrocytes and neurons, particularly at the synapses, and more recent data indicate that neurotransmission plays a role in regulating the morphology and function of surveying/resting microglia, as they are endowed with receptors for most known neurotransmitters. In particular, microglia express receptors for ATP and glutamate, which regulate microglial motility. After local damage, the release of ATP induces microgliosis and activated microglial cells migrate to the site of injury, proliferate, and phagocytose cells, and cellular compartments. However, excessive activation of microglia could contribute to the progression of chronic neurodegenerative diseases, though the underlying mechanisms are still unclear. Microglia have the capacity to release a large number of substances that can be detrimental to the surrounding neurons, including glutamate, ATP, and reactive oxygen species. However, how altered neurotransmission following acute insults or chronic neurodegenerative conditions modulates microglial functions is still poorly understood. This review summarizes the relevant data regarding the role of neurotransmitter receptors in microglial physiology and pathology.

Metabotropic glutamate receptors inhibit microglial glutamate release

Pro-inflammatory stimuli evoke an export of glutamate from microglia that is sufficient to contribute to excitotoxicity in neighbouring neurons. Since microglia also express various glutamate receptors themselves, we were interested in the potential feedback of glutamate on this system. Several agonists of mGluRs (metabotropic glutamate receptors) were applied to primary rat microglia, and the export of glutamate into their culture medium was evoked by LPS (lipopolysaccharide). Agonists of group-II and -III mGluR ACPD [(1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid] and L-AP4 [L-(+)-2-amino-4-phosphonobutyric acid] were both capable of completely blocking the glutamate export without interfering with the production of NO (nitric oxide); the group-I agonist tADA (trans-azetidine-2,4-dicarboxylic acid) was ineffective. Consistent with the possibility of feedback, inhibition of mGluR by MSPG [(R,S)-α-2-methyl-4sulfonophenylglycine] potentiated glutamate export. As the group-II and -III mGluR are coupled to Gα_i-containing G-proteins and the inhibition of adenylate cyclase, we explored the role of cAMP in this effect. Inhibition of cAMP-dependent protein kinase [also known as protein kinase A (PKA)] by H89 mimicked the effect of ACPD, and the mGluR agonist had its actions reversed by artificially sustaining cAMP through the PDE (phosphodiesterase) inhibitor IBMX (isobutylmethylxanthine) or the cAMP mimetic dbcAMP (dibutyryl cAMP). These data indicate that mGluR activation attenuates a potentially neurotoxic export of glutamate from activated microglia and implicate cAMP as a contributor to this aspect of microglial action.

Neuroprotective effect of methyl lucidone against microglia-mediated neurotoxicity

Excessive microglial activation-mediated neurotoxicity has been implicated in playing a crucial role in the pathogenesis of stroke and neurodegenerative diseases. Therefore, much attention has been paid to therapeutic strategies aimed at suppressing neurotoxic microglial activation. The microglial regulatory mechanism of methyl lucidone, a cyclopentenedione isolated from the stem bark of Lindera erythrocarpa Makino, was investigated in the present study. Methyl lucidone treatment (0.1-10 μM) significantly inhibited lipopolysaccharide (LPS, 100 ng/ml, 24 h)-stimulated nitric oxide (NO) production in a dose-dependent manner in both primary cortical microglia and BV-2 cell line. Moreover, it strongly inhibited LPS-stimulated secretion of pro-inflammatory cytokines, such as interleukin 6 (IL-6) and tumor necrosis factor α (TNF-α). Methyl lucidone treatment markedly induced down-regulation of LPS-induced nuclear translocation of nuclear factor κB (NF-κB) through preventing the degradation of the inhibitory protein IκBα. In addition, phosphorylation of Akt and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK) and p38 kinases were also suppressed by methyl lucidone. The cell viabilities of HT-22 neurons were significantly attenuated by treatment of the conditioned media containing neurotoxic secretary molecules from LPS-stimulated microglia. However, methyl lucidone significantly blocked neuronal cell death induced by microglial conditioned media. These neuroprotective effects of methyl lucidone were also confirmed in a neuron-microglia co-culture system using EGFP-transfected B35 neuroblastoma cell line. Taken together, these results suggest that methyl lucidone may have a neuroprotective potential via inhibition of neurotoxic microglial activation implicated in neurodegeneration.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Immediate early gene expression invoked by electrical intracochlear stimulation in some but not all types of neurons in the rat auditory brainstem

Specific patterns of sensory activity may induce plastic remodeling of neurons and the communication network they form in the adult mammalian brain. Among the indicators for the initiation of neuronal remodeling is the expression of immediate early genes (IEGs). The IEGs c-fos and egr-1 encode transcription factors. Following spectrally and temporally precisely defined unilateral electrical intracochlear stimulation (EIS) that corresponded in strength to physiological acoustic stimuli and lasted for 2 h under anesthesia, we characterized those neuronal cell types in ventral (VCN) and dorsal cochlear nucleus (DCN), lateral superior olive (LSO) and central nucleus of the inferior colliculus (CIC) of the rat brain that expressed IEGs. We found that EIS affected only specific types of neurons. Whereas sub-populations of glutamatergic and glycinergic cells responded in all four regions, GABAergic neurons failed to do so except in DCN. Combining immunocytochemistry with axonal tracing, neurons participating in major ascending pathways, commissural cells of VCN and certain types of neurons of the descending auditory system were seen to respond to EIS with IEG expression. By contrast, principal LSO cells projecting to the contralateral CIC as well as collicular efferents of the DCN did not. In total, less than 50% of the identified neurons turned up expression of the IEGs studied. The pattern of IEG expression caused by unilateral EIS led us to suggest that dominant sensory activity may quickly initiate a facilitation of central pathways serving the active ear at the expense of those serving the unstimulated ear.

Cell cycle-dependent regulation of kainate-induced inward currents in microglia

Microglia are reported to have alpha-amino-hydroxy-5-methyl-isoxazole-4-propionate/kainate (KA) types. However, only small population of primary cultured rat microglia (approximately 20%) responded to KA. In the present study, we have attempted to elucidate the regulatory mechanism of responsiveness to KA in GMIR1 rat microglial cell line. When the GMIR1 cells were plated at a low density in the presence of granulocyte macrophage colony-stimulating factor, the proliferation rate increased and reached the peak after 2 days in culture and then gradually decreased because of density-dependent inhibition. At cell proliferation stage, approximately 80% of the GMIR1 cells exhibited glutamate (Glu)- and KA-induced inward currents at cell proliferation stage, whereas only 22.5% of the cells showed responsiveness to Glu and KA at cell quiescent stage. Furthermore, the mean amplitudes of inward currents induced by Glu and KA at cell proliferation stage (13.8+/-3.0 and 8.4+/-0.6 pA) were significantly larger than those obtained at cell quiescent stage (4.7+/-0.8 and 6.2+/-1.2 pA). In the GMIR1 cells, KA-induced inward currents were markedly inhibited by (RS)-3-(2-carboxybenzyl) willardiine (UBP296), a selective antagonist for KA receptors. The KA-responsive cells also responded to (RS)-2-amino-3-(3-hydroxy-5-tert-butylisoxazol-4-yl) propanoic acid (ATPA), a selective agonist for GluR5, in both GMIR1 cells and primary cultured rat microglia. Furthermore, mRNA levels of the KA receptor subunits, GluR5 and GluR6, at the cell proliferation stage were significantly higher than those at the cell quiescent stage. Furthermore, the immunoreactivity for GluR6/7 was found to increase in activated microglia in the post-ischemic hippocampus. These results strongly suggest that microglia have functional KA receptors mainly consisting of GluR5 and GluR6, and the expression levels of these subunits are closely regulated by the cell cycle mechanism.

Glutamate activatesc-fos in glial cells via a novel mechanism involving the glutamate receptor subtype mGlu5 and the transcriptional repressor DREAM

Activation of c-fos in brain is related to coupling of neuronal activity to gene expression, but also to pathological conditions such as seizures or excitotoxicity-induced cell death. Glutamate activates c-fos in neurons through the calcium-dependent phosphorylation of CREB by ERK and/or CaMKIV kinase pathways downstream NMDA-receptors. In glial cells, however, the activation of c-fos by glutamate is poorly understood. Because glial cells actively modulate neuronal excitability and the brain's response to injury, we studied the mechanisms by which glutamate activates c-fos in rat cortical glial cells. Glutamate potently induced c-fos mRNA in a calcium-dependent manner, as demonstrated by using the calcium chelator BAPTA-AM. Glutamate-induced c-fos mRNA expression was not sensitive to inhibitors of ERK, p38(MAPK), or CaMK pathways, indicating that glial c-fos is activated by a distinct mechanism. Thapsigargin abolished the glutamate effect on c-fos mRNA, indicating ER calcium mobilization. Additionally, glutamate induction of c-fos mRNA was sensitive to the mGluR5 antagonist MPEP but not the NMDA-R antagonist MK-801. In luciferase reporter assays, DRE, which actively represses c-fos by binding the calcium-binding transcriptional repressor DREAM, was activated by glutamate, whereas SRE and CRE were not. Finally, glutamate caused the nuclear export of DREAM in astrocytes, and transfection of astrocytes with a mutant variant of DREAM that constitutively binds DNA inhibited glutamate-induced c-Fos expression. These findings are in sharp contrast to the mechanism described in neurons and suggest a novel pathway activated by glutamate in glial cells that employs mGluR5, ER calcium, and the derepression of c-fos at the DRE.