NO as an autocrine mediator in the apoptosis of activated microglial cells: correlation between activation and apoptosis of microglial cells

Characterizing Microglial Signaling Dynamics During Inflammation Using Single-Cell Mass Cytometry

Microglia play a critical role in maintaining central nervous system (CNS) homeostasis and display remarkable plasticity in their response to inflammatory stimuli. However, the specific signaling profiles that microglia adopt during such challenges remain incompletely understood. Traditional transcriptomic approaches provide valuable insights, but fail to capture dynamic post-translational changes. In this study, we utilized time-resolved single-cell mass cytometry (CyTOF) to measure distinct signaling pathways activated in microglia upon exposure to bacterial and viral mimetics-lipopolysaccharide (LPS) and polyinosinic-polycytidylic acid (Poly(I:C)), respectively. Furthermore, we evaluated the immunomodulatory role of astrocytes on microglial signaling in mixed cultures. Microglia or mixed cultures derived from neonatal mice were treated with LPS or Poly(I:C) for 48 h. Cultures were stained with a panel of 33 metal-conjugated antibodies targeting signaling and identity markers. High-dimensional clustering analysis was used to identify emergent signaling modules. We found that LPS treatment led to more robust early activation of pp38, pERK, pRSK, and pCREB compared to Poly(I:C). Despite these differences, both LPS and Poly(I:C) upregulated the classical reactivity markers CD40 and CD86 at later time points. Strikingly, the presence of astrocytes significantly blunted microglial responses to both stimuli, particularly dampening CD40 upregulation. Our studies demonstrate that single-cell mass cytometry effectively captures the dynamic signaling landscape of microglia under pro-inflammatory conditions. This approach may pave the way for targeted therapeutic investigations of various neuroinflammatory disorders. Moreover, our findings underscore the necessity of considering cellular context, such as astrocyte presence, in interpreting microglial behavior during inflammation.

Characterization of an Immortalized Human Microglial Cell Line as a Tool for the Study of Diabetic Retinopathy

The complexity of the retinal structure reflects on the difficulty to describe its composite cell interactions. Microglia is responsible for the immune reaction to inflammatory stimuli during diabetic retinopathy (DR), but most studies still use rodent cells. We characterized a commercially available immortalized human microglial line and tested its susceptibility to inflammation, to study the interactions between the neuro-vascular retinal portions in species-specific models. After checking the expression of microglial markers, we tried lipopolysaccharide (LPS) stimulation and several pro-inflammatory cocktails to select the best combination able to induce a significant M1 (inflammatory) response. We measured M1 induction through the expression of pro- and anti-inflammatory molecules and performed morphologic and functional assays. Marker expression confirmed the human microglial derivation of these cells. Differently from rodents, LPS did not induce a M1 profile. The best pro-inflammatory stimulus was an interleukin-1β + tumor necrosis factor-α + interferon-γ cocktail, which induced morphology changes and increased proliferation, apoptosis, migration, reactive oxygen species, and the expression of inflammatory cytokines and miRNAs. In conclusion, this microglial line proved potentially useful to investigate the cascade of events leading to DR. In perspective, co-culture models involving microvascular cells will help in the understanding of multifaceted interactions of the neurovascular unit.

Cysteine Peptidase Cathepsin X as a Therapeutic Target for Simultaneous TLR3/4-mediated Microglia Activation

Microglia are resident macrophages in the central nervous system that are involved in immune responses driven by Toll-like receptors (TLRs). Microglia-mediated inflammation can lead to central nervous system disorders, and more than one TLR might be involved in these pathological processes. The cysteine peptidase cathepsin X has been recognized as a pathogenic factor for inflammation-induced neurodegeneration. Here, we hypothesized that simultaneous TLR3 and TLR4 activation induces synergized microglia responses and that these phenotype changes affect cathepsin X expression and activity. Murine microglia BV2 cells and primary murine microglia were exposed to the TLR3 ligand polyinosinic-polycytidylic acid (poly(I:C)) and the TLR4 ligand lipopolysaccharide (LPS), individually and simultaneously. TLR3 and TLR4 co-activation resulted in increased inflammatory responses compared to individual TLR activation, where poly(I:C) and LPS induced distinct patterns of proinflammatory factors together with different patterns of cathepsin X expression and activity. TLR co-activation decreased intracellular cathepsin X activity and increased cathepsin X localization at the plasma membrane with concomitant increased extracellular cathepsin X protein levels and activity. Inhibition of cathepsin X in BV2 cells by AMS36, cathepsin X inhibitor, significantly reduced the poly(I:C)- and LPS-induced production of proinflammatory cytokines as well as apoptosis. Additionally, inhibiting the TLR3 and TLR4 common signaling pathway, PI3K, with LY294002 reduced the inflammatory responses of the poly(I:C)- and LPS-activated microglia and recovered cathepsin X activity. We here provide evidence that microglial cathepsin X strengthens microglia activation and leads to subsequent inflammation-induced neurodegeneration. As such, cathepsin X represents a therapeutic target for treating neurodegenerative diseases related to excess inflammation.

Regenerative Effects of CDP-Choline: A Dose-Dependent Study in the Toxic Cuprizone Model of De- and Remyelination

Inflammatory attacks and demyelination in the central nervous system (CNS) are the key factors responsible for the damage of neurons in multiple sclerosis (MS). Remyelination is the natural regenerating process after demyelination that also provides neuroprotection but is often incomplete or fails in MS. Currently available therapeutics are affecting the immune system, but there is no substance that might enhance remyelination. Cytidine-S-diphosphate choline (CDP-choline), a precursor of the biomembrane component phospholipid phosphatidylcholine was shown to improve remyelination in two animal models of demyelination. However, the doses used in previous animal studies were high (500 mg/kg), and it is not clear if lower doses, which could be applied in human trials, might exert the same beneficial effect on remyelination. The aim of this study was to confirm previous results and to determine the potential regenerative effects of lower doses of CDP-choline (100 and 50 mg/kg). The effects of CDP-choline were investigated in the toxic cuprizone-induced mouse model of de- and remyelination. We found that even low doses of CDP-choline effectively enhanced early remyelination. The beneficial effects on myelin regeneration were accompanied by higher numbers of oligodendrocytes. In conclusion, CDP-choline could become a promising regenerative substance for patients with multiple sclerosis and should be tested in a clinical trial.

Cathepsin H deficiency decreases hypoxia-ischemia-induced hippocampal atrophy in neonatal mice through attenuated TLR3/IFN-β signaling

Background

Cathepsin H (CatH) is a lysosomal cysteine protease with a unique aminopeptidase activity. Its expression level is increased in activated immune cells including dendritic cells, macrophages, and microglia. We have previously reported that CatH deficiency impairs toll-like receptor 3 (TLR3)-mediated activation of interferon regulatory factor 3 (IRF3), and the subsequent secretion of interferon (IFN)-β from dendritic cells. Furthermore, there is increasing evidence that IFN-β secreted from microglia/macrophages has neuroprotective effects. These observations prompted further investigation into the effects of CatH deficiency on neuropathological changes.

Methods

In this study, neuropathological changes were examined using histochemical staining (both hematoxylin-eosin (H&E) and Nissl) of the hippocampus of wild-type (WT) and CatH-deficient (CatH^−/−) mice after hypoxia-ischemia (HI). The density and the localization of CatH and TLR3 were examined by immunofluorescent staining. CatH processing in microglia was assayed by pulse-chase experiments, while immunoblotting was used to examine TLR3 expression and IRF3 activation in microglia/macrophages in the presence of poly(I:C). Microglial cell death was examined by fluorescence-activated cell sorting (FACS), and primary astrocyte proliferation in the presence of IFN-β was examined using scratch wound assay.

Results

WT mice displayed severe atrophy in association with neuronal death and moderate astrogliosis in the hippocampus following neonatal HI. Somewhat surprisingly, CatH^−/− mice showed marked neuronal death without severe atrophy in the hippocampus following HI. Furthermore, there was notable microglia/macrophages cell death and strong astrogliosis in the hippocampus. The TLR3 and phosphorylated IRF3 expression level in the hippocampus or splenocytes (mainly splenic macrophages); from CatH^−/− mice was lower than in WT mice. In vitro experiments demonstrated that recombinant IFN-β suppressed HI-induced microglial cell death and astrocyte proliferation.

Conclusion

These observations suggest that CatH plays a critical role in the proteolytic maturation and stabilization of TLR3, which is necessary for IFN-β production. Therefore, impaired TLR3/IFN-β signaling resulting from CatH deficiency may induce microglial cell death after activation and astrogliosis/glial scar formation in the hippocampus following HI injury, leading to suppression of hippocampal atrophy.

Interleukin-13 Propagates Prothrombin Kringle-2-Induced Neurotoxicity in Hippocampi In Vivo via Oxidative Stress

The present study investigated expression of endogenous interleukin-13 (IL-13) and its possible function in the hippocampus of prothrombin kringle-2 (pKr-2)-lesioned rats. Here we report that intrahippocampal injection of pKr-2 revealed a significant loss of NeuN-immunopositive (NeuN⁺) and Nissl⁺ cells in the hippocampus at 7 days after pKr-2. In parallel, pKr-2 increased IL-13 levels, which reached a peak at 3 days post pKr-2 and sustained up to 7 days post pKr-2. IL-13 immunoreactivity was seen exclusively in activated microglia/macrophages and neutrophils, but not in neurons or astrocytes. In experiments designed to explore the involvement of IL-13 in neurodegeneration, IL-13 neutralizing antibody (IL-13Nab) significantly increased survival of NeuN⁺ and Nissl⁺ cells. Accompanying neuroprotection, immunohistochemical analysis indicated that IL-13Nab inhibited pKr-2-induced expression of inducible nitric oxide synthase and myeloperoxidase within activated microglia/macrophages and neutrophils, possibly resulting in attenuation of reactive oxygen species (ROS) generation and oxidative damage of DNA and protein. The current findings suggest that the endogenous IL-13 expressed in pKr-2 activated microglia/macrophages and neutrophils might be harmful to hippocampal neurons via oxidative stress.

Subanesthetic isoflurane abates ROS-activated MAPK/NF-κB signaling to repress ischemia-induced microglia inflammation and brain injury

Isoflurane (ISO) elicits protective effects on ischemia-induced brain injury. We investigated whether sub-anesthetic (0.7%) ISO post-conditioning attenuates the inflammation and apoptosis in oxygen-glucose deprivation (OGD)-insulted co-cultures (microglia and neurons) in vitro and the brain injury of the middle cerebral arterial occlusion (MCAO) rat. We demonstrated that ISO augmented the viability of OGD-treated microglia and neurons. ISO reduced the expression and activation of COX2 and iNOS in OGD-challenged microglia. ISO repressed the production of tumor necrosis factor-α, interleukin (IL)-1β, IL-6, IL-8, and monocyte chemoattractant protein-1 in OGD-exposed microglia. ISO also decreased nucleosomal fragmentation and caspase-3 activity but increased mitochondrial membrane potential in OGD-stimulated microglia and neurons. Mechanistically, ISO suppressed OGD-induced microglial inflammation by blocking ROS-regulated p38 MAPK/NF-κB signaling pathway and hampered OGD-triggered microglial apoptosis in a ROS- or NO-dependent fashion. In vivo results with MCAO rats were partly consistent with the in vitro observation. These findings indicate that sub-anesthetic ISO post-conditioning abates the inflammation and apoptosis in OGD-stimulated rat microglia and the apoptosis of OGD-exposed neurons and the brain injuries of MCAO rats, suggesting it as a potentially effective therapeutic approach for ischemic brain damages.

Isoliquiritigenin Reduces LPS-Induced Inflammation by Preventing Mitochondrial Fission in BV-2 Microglial Cells

Excessive microglial cell activation in the brain can lead to the production of various neurotoxic factors (e.g., pro-inflammatory cytokines, nitric oxide) which can, in turn, initiate neurodegenerative processes. Recent research has been reported that mitochondrial dynamics regulate the inflammatory response of lipopolysaccharide (LPS). Isoliquiritigenin (ISL) is a compound found in Glycyrrhizae radix with anti-inflammatory and antioxidant properties. In this study, we investigated the function of ISL on the LPS-induced pro-inflammatory response in BV-2 microglial cells. We showed that ISL reduced the LPS-induced increase in pro-inflammatory mediators (e.g., nitric oxide and pro-inflammatory cytokines) via the inhibition of ERK/p38/NF-κB activation and the generation of reactive oxygen species (ROS). Furthermore, ISL inhibited the excessive mitochondrial fission induced by LPS, regulating mitochondrial ROS generation and pro-inflammatory response by suppressing the calcium/calcineurin pathway to dephosphorylate Drp1 at the serine 637 residue. Interestingly, the ISL pretreatment reduced the number of apoptotic cells and levels of cleaved caspase3/PARP, compared to LPS-treated cells. Our findings suggested that ISL ameliorated the pro-inflammatory response of microglia by inhibiting dephosphorylation of Drp1 (Ser637)-dependent mitochondrial fission. This study provides the first evidence for the effects of ISL against LPS-induced inflammatory response related and its link to mitochondrial fission and the calcium/calcineurin pathway. Consequently, we also identified the protective effects of ISL against LPS-induced microglial apoptosis, highlighting the pharmacological role of ISL in microglial inflammation-mediated neurodegeneration.

Cathepsin regulation on microglial function

Microglia, the resident mononuclear phagocyte population in the brain, have long been implicated in the pathology of neurodegenerative age-associated disorders. However, activated microglia have now been identified as homeostatic keepers in the brain, because they are involved in the initiation and resolution of neuropathology. The complex roles of activated microglia appear to be linked to change from inflammatory and neurotoxic to anti-inflammatory and neuroprotective phenotypes. Increased expression and secretion of various cathepsins support roles of activated microglia in chronic neuroinflammation, the neurotoxic M1-like polarization and neuronal death. Moreover, changes in expression and localization of microglial cathepsin B play a critical role in the acceleration of the brain aging. Beyond the role as brain-resident macrophages, many lines of evidence have shown that microglia have essential roles in the maturation and maintenance of neuronal circuits in the developing and adult brain. Cathepsin S secreted from microglia induces the diurnal variation of spine density of cortical neurons though proteolytic modification of peri-synaptic extracellular matrix molecules. In this review, I highlight the emerging roles of cathepsins that support the roles of microglia in both normal healthy and pathological brains. In addition, I discuss cathepsin inhibitors as potential therapeutic targets for brain disorders.

Effect of Glycine on BV-2 Microglial Cells Treated with Interferon-γ and Lipopolysaccharide

Microglia are first-line defense antigen-presenting phagocytes in the central nervous system. Activated microglial cells release pro-inflammatory cytokines and can trigger an oxidative burst. The amino acid glycine exerts anti-inflammatory, immunomodulatory and cytoprotective effects and influences cell volume regulation. This study aimed to investigate the role of glycine in the modulation of inflammatory processes in mouse BV-2 microglial cells. Inflammatory stress was induced by lipopolysaccharide/interferon-γ (LPS/IFN-γ) treatment for 24 h in the absence or presence of 1 or 5 mM glycine. Cells were analyzed by flow cytometry for cell volume, side scatter, apoptosis/necrosis and expression of activation-specific surface markers. Apoptosis progression was monitored by life cell imaging. Reduced glutathione/oxidized glutathione (GSH/GSSG) ratios and release of the pro-inflammatory cytokines IL-6 and TNF-α were measured using luminescence-based assays and ELISA, respectively. We found that LPS/IFN-γ-induced apoptosis was decreased and the fraction of living cells was increased by glycine. Expression of the surface markers CD11b, CD54 and CD80 was dose-dependently increased, while IL-6 and TNF-α release was not altered compared to LPS/IFN-γ-treated cells. We showed that in BV-2 microglial cells glycine improves viability and counteracts deleterious responses to LPS/IFN-γ, which might be relevant in neurodegenerative processes associated with inflammation, like Alzheimer’s or Parkinson’s disease.

Panton-Valentine Leukocidin Colocalizes with Retinal Ganglion and Amacrine Cells and Activates Glial Reactions and Microglial Apoptosis

Experimental models have established Panton-Valentine leukocidin (PVL) as a potential critical virulence factor during Staphylococcus aureus endophthalmitis. In the present study, we aimed to identify retinal cell targets for PVL and to analyze early retinal changes during infection. After the intravitreous injection of PVL, adult rabbits were euthanized at different time points (30 min, 1, 2, 4 and 8 h). PVL location in the retina, expression of its binding receptor C5a receptor (C5aR), and changes in Müller and microglial cells were analyzed using immunohistochemistry, Western blotting and RT-qPCR. In this model of PVL eye intoxication, only retinal ganglion cells (RGCs) expressed C5aR, and PVL was identified on the surface of two kinds of retinal neural cells. PVL-linked fluorescence increased in RGCs over time, reaching 98% of all RGCs 2 h after PVL injection. However, displaced amacrine cells (DACs) transiently colocalized with PVL. Müller and microglial cells were increasingly activated after injection over time. IL-6 expression in retina increased and some microglial cells underwent apoptosis 4 h and 8 h after PVL infection, probably because of abnormal nitrotyrosine production in the retina.

Inhibition of cathepsin X reduces the strength of microglial-mediated neuroinflammation

Inflammation plays a central role in the processes associated with neurodegeneration. The inflammatory response is mediated by activated microglia that release inflammatory mediators to the neuronal environment. Microglia-derived lysosomal cathepsins, including cathepsin X, are increasingly recognized as important mediators of the inflammation involved in lipopolysaccharide (LPS)-induced neuroinflammation. The current study was undertaken to investigate the role of cathepsin X and its molecular target, γ-enolase, in neuroinflammation and to elucidate the underlying mechanism. We determined that the exposure of activated BV2 and EOC 13.31 cells to LPS led to increased levels of cathepsin X protein and activity in the culture supernatants in a concentration- and time-dependent manner. In contrast, LPS stimulation of these two cells reduced the release of active γ-enolase in a manner regulated by the cathepsin X activity. Cathepsin X inhibitor AMS36 significantly reduced LPS-induced production of nitric oxide, reactive oxygen species and the pro-inflammatory cytokines interleukin-6 and tumor necrosis factor-α from BV2 cells. Inhibition of cathepsin X suppressed microglial activation through the reduced caspase-3 activity, together with diminished microglial cell death and apoptosis, and also through inhibition of the activity of the mitogen-activated protein kinases. Further, SH-SY5Y treatment with culture supernatants of activated microglial cells showed that cathepsin X inhibition reduces microglia-mediated neurotoxicity. These results indicate that up-regulated expression and increased release and activity of microglial cathepsin X leads to microglia activation-mediated neurodegeneration. Cathepsin X inhibitor caused neuroprotection via its inhibition of the activation of microglia. Cathepsin X could thus be a potential therapeutic target for neuroinflammatory disorders.

Mir143-BBC3 cascade reduces microglial survival via interplay between apoptosis and autophagy: Implications for methamphetamine-mediated neurotoxicity

BBC3 (BCL2 binding component 3) is a known apoptosis inducer; however, its role in microglial survival remains poorly understood. In addition to the classical transcription factor TRP53, Mir143 is involved in BBC3 expression at the post-transcriptional level. Here, we identify unique roles of Mir143-BBC3 in mediating microglial survival via the regulation of the interplay between apoptosis and autophagy. Autophagy inhibition accelerated methamphetamine-induced apoptosis, whereas autophagy induction attenuated the decrease in microglial survival. Moreover, anti-Mir143-dependent BBC3 upregulation reversed the methamphetamine-induced decrease in microglial survival via the regulation of apoptosis and autophagy. The in vivo relevance of these findings was confirmed in mouse models, which demonstrated that the microinjection of anti-Mir143 into the hippocampus ameliorated the methamphetamine-induced decrease in microglia as well as that observed in heterozygous Mir143(+/-) mice. These findings provide new insight regarding the specific contributions of Mir143-BBC3 to microglial survival in the context of drug abuse.

Low-dose ribavirin treatments attenuate neuroinflammatory activation of BV-2 Cells by interfering with inducible nitric oxide synthase

Microglia play a key role in defending central nervous system from various internal and external threats. However, their excessive and/or chronic activation is associated with deleterious effects in a variety of neurodegenerative diseases. Previously, we have shown that ribavirin when applied in clinically relevant dosage (10 μM) modulates activated microglia in complex fashion inducing both anti- and proinflammatory effects, simultaneously causing cytotoxicity. Here, we examined potential of low-dose ribavirin (0.1 and 1 μM) to modulate activated BV-2 microglia. Morphological and functional activation of BV-2 cells was achieved with lipopolysaccharide (LPS) stimulation. Our results demonstrated that low-dose ribavirin did not induce cell death, while 10 μM ribavirin promoted LPS induced apoptosis. We determined that 1 μM ribavirin was equally efficient in deactivation of LPS induced morphological changes as 10 μM ribavirin treatment. Ribavirin showed halfway success in reducing markers of functional activation of microglia. Namely, none of the doses had effect on LPS triggered production of proinflammatory cytokine tumor necrosis factor alpha. On the other hand, low-dose ribavirin proved its effectiveness in reduction of another inflammatory mediator, nitric oxide, by inhibiting inducible form of nitric oxide synthase. Our results imply that low-dose ribavirin may alleviate nitrosative stress during neuroinflammation.

Paeonol attenuates inflammation-mediated neurotoxicity and microglial activation

Chronic activation of microglial cells endangers neuronal survival through the release of various proinflammatory and neurotoxic factors. The root of Paeonia lactiflora Pall has been considered useful for the treatment of various disorders in traditional oriental medicine. Paeonol, found in the root of Paeonia lactiflora Pall, has a wide range of pharmacological functions, including anti-oxidative, anti-inflammatory and neuroprotective activities. The objective of this study was to examine the efficacy of paeonol in the repression of inflammation-induced neurotoxicity and microglial cell activation. Organotypic hippocampal slice cultures and primary microglial cells from rat brain were stimulated with bacterial lipopolysaccharide. Paeonol pretreatment was performed for 30 minutes prior to lipopolysaccharide addition. Cell viability and nitrite (the production of nitric oxide), tumor necrosis factor-alpha and interleukin-1beta products were measured after lipopolysaccharide treatment. In organotypic hippocampal slice cultures, paeonol blocked lipopolysaccharide-related hippocampal cell death and inhibited the release of nitrite and interleukin-1beta. Paeonol was effective in inhibiting nitric oxide release from primary microglial cells. It also reduced the lipopolysaccharide-stimulated release of tumor necrosis factor-alpha and interleukin-1β from microglial cells. Paeonol possesses neuroprotective activity in a model of inflammation-induced neurotoxicity and reduces the release of neurotoxic and proinflammatory factors in activated microglial cells.

Lipopolysaccharide treatment arrests the cell cycle of BV-2 microglial cells in G₁ phase and protects them from UV light-induced apoptosis

We previously reported that an optimal dose of lipopolysaccharide (LPS) markedly extends the lifespan of murine primary-cultured microglia by suppressing cell death pathways. In this study, we investigated the effects of LPS pretreatment on UV light-induced apoptosis of cells from the microglial cell line BV-2. More than half of BV-2 cells were apoptotic, and procaspase-3 was cleaved into its active form at 3 h of UV irradiation. In contrast, in BV-2 cells treated with LPS for 24 h, UV irradiation caused neither apoptosis nor procaspase-3 cleavage. LPS treatment arrested the cell cycle in G1 phase and upregulated cyclin-dependent kinase inhibitor p21(Waf1/Cip1) and growth arrest and DNA damage-inducible (GADD) 45α in BV-2 cells. When p21(Waf1/Cip1) and GADD45α were knocked down by small interfering RNA, procaspase-3 was cleaved into its active form to induce apoptosis. Our findings suggest that LPS inhibits UV-induced apoptosis in BV-2 cells through arrest of the cell cycle in G1 phase by upregulation of p21(Waf1/Cip1) and GADD45α. Excessive activation of microglia may play a critical role in the exacerbation of neurodegeneration, therefore, normalizing the precise regulation of apoptosis may be a new strategy to prevent the deterioration caused by neurodegenerative disorders.

From the nucleus to the plasma membrane: translocation of the nuclear proteins histone H3 and lamin B1 in apoptotic microglia

Nuclear autoantibodies have been found in patients with autoimmune diseases. One possible source for nuclear antigens are apoptotic cells. However, the mechanism of how apoptotic cells make nuclear factors accessible to the immune system is still elusive. In the present study, we investigated the redistribution of nuclear components after UV irradiation in the microglial cell line BV-2 and in primary mouse microglia at the ultrastructural level. We used transmission electron microscopy-coupled electron energy loss spectroscopy (EELS) to measure phosphorus as an indicator for nucleic acids and immunogold labeling to detect histone H3 and lamin B1 in apoptotic cells. EELS revealed elevated concentrations of phosphorus in nuclear and cytoplasmic condensed chromatin compared to the remaining cytoplasm. Furthermore, immunolabeling of lamin B1 and histone H3 was detected in apoptotic microglia not only in the nucleus, but also in the cytoplasm, and even at the plasma membrane. Confocal images of apoptotic microglia, which were not previously permeabilized, showed patches of histone H3 and lamin B1 labeling at the cell surface. The pan-caspase inhibitor Z-VAD-FMK (carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone) prevented the occurrence of cytoplasmic condensed chromatin in apoptotic microglia. Our findings indicate that nuclear components leak from the nucleus into the cytoplasm in apoptotic microglia. At least histone H3 and lamin B1 reach the cell surface, this may promote autoreactive processes.

Basal CD38/cyclic ADP-ribose-dependent signaling mediates ATP release and survival of microglia by modulating connexin 43 hemichannels

It is necessary to investigate the mechanisms underlying ATP release from neural cells, because extracellular ATP plays multiple important biological roles in the brain. CD38 is an ectoenzyme that consumes NAD(+) to produce cyclic ADP-ribose (cADPR), a potent agonist of ryanodine receptors. Our previous study showed that CD38 reductions led to microglial apoptosis. In this study, we used both murine microglial BV2 cells and primary microglial cultures as cellular models to test our hypothesis that basal CD38/cyclic ADP-ribose (CD38/cADPR)-dependent signaling plays a key role in ATP release, which mediates basal survival of microglia. We found that inhibition of CD38/cADPR-dependent signaling by CD38 silencing or 8-Bromo-cADPR, a ryanodine receptor antagonist, produced significant ATP release from BV2 microglia. Cx43 small interfering RNA and Cx43 hemichannel blocker 18-α-glycyrrhetinic acid completely prevented the CD38 silencing or 8-Bromo-cADPR-induced ATP release. Prevention of the ATP release could also be due to P2X7 receptor antagonists. Our study has further suggested a key role of ATP release in the microglial apoptosis induced by decreased CD38/cADPR-dependent signaling. In addition, by using primary microglial cultures, we found that 8-Bromo-cADPR also induced significant ATP release, which could be attenuated by 18-α-glycyrrhetinic acid. 8-Bromo-cADPR was also found to induce death of primary microglial cultures. In conclusion, our results have suggested novel roles of basal activation of CD38/cADPR-dependent signaling in mediating microglial functions and survival: It mediates ATP release from microglia by modulating Cx43 hemichannels, which can significantly affect microglial survival.

Paeoniflorin, a monoterpene glycoside, attenuates lipopolysaccharide-induced neuronal injury and brain microglial inflammatory response

Chronic activation of microglial cells endangers neuronal survival through the release of various proinflammatory and neurotoxic factors. Paeoniflorin (PF), a water-soluble monoterpene glycoside found in the root of Paeonia lactiflora Pall, has a wide range of pharmacological functions, such as anti-oxidant, anti-inflammatory, and anti-cancer effects. Neuroprotective potential of PF has also been demonstrated in animal models of neuropathologies. Here, we have examined the efficacy of PF in the repression of inflammation-induced neurotoxicity and microglial inflammatory response. In organotypic hippocampal slice cultures, PF significantly blocked lipopolysaccharide (LPS)-induced hippocampal cell death and productions of nitric oxide (NO) and interleukin (IL)-1β. PF also inhibited the LPS-stimulated productions of NO, tumor necrosis factor-α, and IL-1β from primary microglial cells. These results suggest that PF possesses neuroprotective activity by reducing the production of proinflammatory factors from activated microglial cells.

Prevention of inflammation-mediated neurotoxicity by butylidenephthalide and its role in microglial activation

Microglial cells are the prime effectors in immune and inflammatory responses of the central nervous system (CNS). During pathological conditions, the activation of these cells helps restore CNS homeostasis. However, chronic microglial activation endangers neuronal survival through the release of various proinflammatory molecules and neurotoxins. Thus, negative regulators of microglial activation have been considered as potential therapeutic candidates to target neurodegeneration, such as that in Alzheimer's and Parkinson's diseases. The rhizome of Ligusticum chuanxiong Hort. (Ligusticum wallichii Franch) has been widely used for the treatment of vascular diseases in traditional oriental medicine. Butylidenephthalide (BP), a major bioactive component from L. chuanxiong, has been reported to have a variety of pharmacological activities, including vasorelaxant, anti-anginal, anti-platelet and anti-cancer effects. The aim of this study was to examine whether BP represses microglial activation. In rat brain microglia, BP significantly inhibited the lipopolysaccharide (LPS)-induced production of nitric oxide (NO), tumour necrosis factor-α and interleukin-1β. In organotypic hippocampal slice cultures, BP clearly blocked the effect of LPS on hippocampal cell death and inhibited LPS-induced NO production in culture medium. These results newly suggest that BP provide neuroprotection by reducing the release of various proinflammatory molecules from activated microglia.

Regulation of oxidative stress in long-lived lipopolysaccharide-activated microglia

1. Previously, we reported that an optimal dose of lipopolysaccharide (LPS) markedly extends the life span of mouse primary-cultured microglia by suppressing apoptotic and autophagic cell death pathways. The aim of the present study was to assess how these cells protect themselves against reactive oxygen species (ROS) generated by LPS treatment. 2. The study was conducted in microglia obtained from murine neonate brain, which are destined to die within a few days under ordinary culture conditions. 3. The generation of ROS was maximal after 15 h LPS treatment (1 ng/mL LPS and 100 ng/mL LPS). The expression of inducible nitric oxide (NO) synthase protein was significantly increased by Day 1 of LPS treatment and was followed by the production of NO. The expression of either Cu/Zn- or Mn-superoxide dismutase protein (SOD) was also increased by 16 h and Day 1 of LPS treatment. LPS did not affect the expression of Cu/Zn- and Mn-SOD proteins, nor did it extend the life span of microglia that had mutated Toll-like receptor (TLR) 4. 4. The findings of the present study suggest that SODs function as a potent barrier to overcome ROS generated in primary-cultured microglia following LPS treatment and that TLR4 may be significantly involved in inducing these proteins. The microglia may be able to protect themselves against oxidative stress, allowing them to live for more than 1 month. Because long-lived microglia may play a critical role in the exacerbation of neurodegeneration, bringing activated microglia back to their resting stage could be a new and promising strategy to inhibit the deterioration underlying neurodegenerative disorders.

Baicalin attenuates proinflammatory cytokine production in oxygen-glucose deprived challenged rat microglial cells by inhibiting TLR4 signaling pathway

Baicalin, a flavonoid compound isolated from Scutellariae radix, has been shown to possess a number of pharmacological effects. The aim of the present study was to observe the inhibitory effects of baicalin on the activation of microglial cells induced by oxygen-glucose deprivation (OGD) and the specific mechanisms by which these effects are mediated. Cultured rat primary microglial cells were exposed to baicalin at final concentrations of 10 μg/ml, 20 μg/ml and 40 μg/ml during 4h of OGD. The effects of baicalin on (i) cell viability; (ii) secretion of proinflammatory cytokines; (iii) Tlr4 mRNA expression; (iv) p-c-jun, p-ERK1/2, p-JNK, p-p38, TRAF6 and p-IκB-α levels; and (v) co-localization of TLR4 and MyD88 were evaluated using the Cell Counting Kit-8 (CCK-8), enzyme-linked immunosorbent assays (ELISA), reverse transcription-polymerase chain reaction (RT-PCR), western blot and double-labeled immunofluorescence staining, respectively. OGD increased cell viability and release of TNF-α, IL-1β, IL-6 and IL-8, these effects were suppressed by baicalin. Baicalin also attenuated the OGD-induced increases in Tlr4 mRNA expression. In addition, high dose of baicalin reduced TRAF6 levels remarkably. Furthermore, baicalin also downregulated phosphorylation of IκB-α, c-jun, ERK1/2, JNK, p38 and inhibited the OGD-induced transfer of MyD88 from cytoplasm to membrane in microglial cells. The results show that baicalin can inhibit OGD-induced production of inflammatory factors in microglial cells by attenuating inflammatory factors and regulating the TLR4 signaling pathways.

Molecular effects of activated BV-2 microglia by mitochondrial toxin 1-methyl-4-phenylpyridinium

Microglia plays an important role in inflammation-mediated neurodegeneration. Compelling evidence supports the hypothesis that microglial activation contributes to the pathogenesis of various neurodegenerative diseases. However, little is known about the molecular outcome of activated microglia. In this report, we investigate the molecular consequences of MPP(+) toxin-induced activated BV-2 microglia. Intoxication of specific mitochondrial toxin methyl-4-phenylpyridinium iodide ion (MPP(+)) to BV-2 cells induced significant mitochondrial dysfunction and increased the reactive oxygen species generation, caspase-3 activation, and poly ADP ribose polymerase proteolysis. Further, MAC-1 immunostaining in the midbrain of mice revealed a decrease in activated microglia at day 4 after intoxication with MPP(+). From this study, it was confirmed that BV-2 microglia respond to the mitochondrial toxin MPP(+) which may lead to apoptotic cell death. Understanding of the mechanistic basis of apoptotic elimination of activated microglia may help to develop new strategies for the treatment of neurodegenerative diseases.

Daxx mediates activation-induced cell death in microglia by triggering MST1 signalling

Microglia, the resident macrophages of the mammalian central nervous system, migrate to sites of tissue damage or infection and become activated. Although the persistent secretion of inflammatory mediators by the activated cells contributes to the pathogenesis of various neurological disorders, most activated microglia eventually undergo apoptosis through the process of activation-induced cell death (AICD). The molecular mechanism of AICD, however, has remained unclear. Here, we show that Daxx and mammalian Ste20-like kinase-1 (MST1) mediate apoptosis elicited by interferon-γ (IFN-γ) in microglia. IFN-γ upregulated the expression of Daxx, which in turn mediated the homodimerization, activation, and nuclear translocation of MST1 and apoptosis in microglial cells. Depletion of Daxx or MST1 by RNA interference also attenuated IFN-γ-induced cell death in primary rat microglia. Furthermore, the extent of IFN-γ-induced death of microglia in the brain of MST1-null mice was significantly reduced compared with that apparent in wild-type mice. Our results thus highlight new functions of Daxx and MST1 that they are the key mediators of microglial cell death initiated by the proinflammatory cytokine IFN-γ.

Expression of mutant alpha-synuclein modulates microglial phenotype in vitro

Background

Increased reactive microglia are a histological characteristic of Parkinson's disease (PD) brains, positively correlating with levels of deposited α-synuclein protein. This suggests that microglial-mediated inflammatory events may contribute to disease pathophysiology. Mutations in the gene coding for α-synuclein lead to a familial form of PD. Based upon our prior findings that α-synuclein expression regulates microglial phenotype we hypothesized that expression of mutant forms of the protein may contribute to the reactive microgliosis characteristic of PD brains.

Methods

To quantify the effects of wild type and mutant α-synuclein over-expression on microglial phenotype a murine microglial cell line, BV2, was transiently transfected to express human wild type (WT), and mutant α-synuclein (A30P and A53T) proteins. Transfected cells were used to assess changes in microglia phenotype via Western blot analysis, ELISA, phagocytosis, and neurotoxicity assays.

Results

As expected, over-expression of α-synuclein induced a reactive phenotype in the transfected cells. Expression of α-synuclein increased protein levels of cycloxygenase-2 (Cox-2). Transfected cells demonstrated increased secretion of the proinflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6), as well as increased nitric oxide production. Transfected cells also had impaired phagocytic ability correlating with decreased protein levels of lysosomal-associated membrane protein 1 (LAMP-1). In spite of the increased cytokine secretion profile, the transfected cells did not exhibit increased neurotoxic ability above control non-transfected BV2 cells in neuron-microglia co-cultures.

Conclusions

These data demonstrated that over-expression of α-synuclein drives microglial cells into a form of reactive phenotype characterized by elevated levels of arachidonic acid metabolizing enzymes, cytokine secretion, and reactive nitrogen species secretion all superimposed upon impaired phagocytic potential.

Indirubin-3'-oxime inhibits inflammatory activation of rat brain microglia

Microglial cells play critical roles in the immune and inflammatory responses of the brain. Under pathological conditions, the activation of microglia helps to restore brain homeostasis. However, chronic microglial activation endangers neuronal survival through the release of various proinflammatory and neurotoxic factors. As such, regulators of microglial activation have been considered as potential therapeutic candidates to reduce the risk of neurodegeneration associated with neurodegenerative diseases, including Alzheimer's and, Parkinson's diseases. Indirubin-3'-oxime, a potent inhibitor of cyclin-dependent kinases and glycogen synthase kinase-3β, has been shown to have neuroprotective potential. The specific aim of this study was to examine the efficacy of indirubin-3'-oxime in the repression of microglial activation. Indirubin-3'-oxime was shown to effectively inhibit lipopolysaccharide (LPS)-induced nitric oxide release from cultured rat brain microglia. This compound reduced the LPS-stimulated productions of tumor necrosis factor-α, interleukin-1β, prostaglandin E(2), and intracellular reactive oxygen species and also effectively reduced LPS-elicited NF-κB activation. In organotypic hippocampal slice cultures, indirubin-3'-oxime blocked LPS-related hippocampal cell death. These results suggest that indirubin-3'-oxime provides neuroprotection by reducing the productions of various neurotoxic molecules in activated microglia.

Targeting the PI3K/Akt cell survival pathway to induce cell death of HIV-1 infected macrophages with alkylphospholipid compounds

BACKGROUND

HIV-1 infected macrophages and microglia are long-lived viral reservoirs persistently producing viral progenies. HIV-1 infection extends the life span of macrophages by promoting the stress-induced activation of the PI3K/Akt cell survival pathway. Importantly, various cancers also display the PI3K/Akt activation for long-term cell survival and outgrowth, and Akt inhibitors have been extensively searched as anti-cancer agents. This led us to investigate whether Akt inhibitors could antagonize long-term survival and cytoprotective phenotype of HIV-1 infected macrophages.

PRINCIPAL FINDINGS

Here, we examined the effect of one such class of drugs, alkylphospholipids (ALPs), on cell death and Akt pathway signals in human macrophages and a human microglial cell line, CHME5, infected with HIV-1 BaL or transduced with HIV-1 vector, respectively. Our findings revealed that the ALPs, perifosine and edelfosine, specifically induced the death of HIV-1 infected primary human macrophages and CHME5 cells. Furthermore, these two compounds reduced phosphorylation of both Akt and GSK3β, a downstream substrate of Akt, in the transduced CHME5 cells. Additionally, we observed that perifosine effectively reduced viral production in HIV-1 infected primary human macrophages. These observations demonstrate that the ALP compounds tested are able to promote cell death in both HIV-1 infected macrophages and HIV-1 expressing CHME5 cells by inhibiting the action of the PI3K/Akt pathway, ultimately restricting viral production from the infected cells.

SIGNIFICANCE

This study suggests that Akt inhibitors, such as ALP compounds, may serve as potential anti-HIV-1 agents specifically targeting long-living HIV-1 macrophages and microglia reservoirs.

The role of p38 MAPK in valproic acid induced microglia apoptosis

Valproic acid (VPA), a widely prescribed drug for seizures and bipolar disorder, induces apoptosis in microglia, but the underlying mechanism by which microglia apoptosis in response to VPA is not yet known. In this study, we found that the mitochondrial pathway played an important role in VPA-induced apoptosis in both BV-2 microglia and mouse primary microglial cells. In addition, VPA increased the level of phospho-p38 mitogen-activated protein kinase (MAPK), but had no effects on phospho-ERK and phospho-JNK MAPKs. Moreover, p38 inhibitor SB203580 strongly inhibited VPA-induced apoptosis and caspase-3 activation. Taken together, our results clearly demonstrated that VPA could induce apoptosis of microglia via p38 MAPK and mitochondrial apoptosis pathway.

Glycogen synthase kinase-3 and p38 MAPK are required for opioid-induced microglia apoptosis

Opioids have been widely applied in clinics as one of the most potent pain relievers for centuries, but their abuse has deleterious physiological effects beyond addiction. We previously reported that opioids inhibit cell growth and trigger apoptosis in lymphocytes. However, the underlying mechanism by which microglia apoptosis in response to opioids is not yet known. In this study, we show that morphine induces microglia apoptosis and caspase-3 activation in an opioid-receptor dependent manner. Morphine decreased the levels of microglia phosphorylated Akt (p-Akt) and p-GSK-3β (glycogen synthase kinase-3 beta) in an opioid-receptor dependent manner. More interestingly, GSK-3β inhibitor SB216763 significantly increases morphine-induced apoptosis in both BV-2 microglia and mouse primary microglial cells. Moreover, co-treatment of microglia with SB216763 and morphine led to a significant synergistic effect on the level of phospho-p38 mitogen-activated protein kinase (MAPK). In addition, inhibition of p38 MAPK by its specific inhibitor SB203580 significantly inhibited morphine-induced apoptosis and caspase-3 activation. Taken together, our data clearly demonstrates that morphine-induced apoptosis in microglial cells, which is mediated via GSK-3β and p38 MAPK pathways.

NF-κB as a common signaling pathway in ganglioside-induced autophagic cell death and activation of astrocytes

We have previously shown that gangliosides induce autophagic cell death of brain astrocytes. As gangliosides are also known to induce inflammatory activation of astrocytes, we hypothesized that a canonical inflammatory signaling pathway NF-κB might be involved in the ganglioside-induced astrocyte cell death and activation. Using cultured mouse astrocytes and C6 rat glioma cell line, we determined the role of NF-κB in autophagic cell death and nitric oxide (NO) production in astrocytes. Gangliosides induced iNOS/GFAP expression and NF-κB activation. IKK inhibitor SC-514 and NF-κB inhibitor PDTC reduced ganglioside-induced astrocyte activation and cell death. Moreover, inhibition of NF-κB pathway also attenuated autophagy of astrocytes. Rho subfamily of small G proteins antagonized the ganglioside-induced astrocyte cell death as well as activation pathways. Taken together, IKK/NF-κB may constitute one of the common signaling pathways in ganglioside-induced astrocyte activation and autophagic cell death, and may play an important role in the ganglioside intracellular signaling that regulates astrocyte physiology and pathology.

Innate immunity and neuroinflammation in the CNS: the role of microglia in Toll-like receptor-mediated neuronal injury

Microglia are key players of the immune response in the central nervous system (CNS) and, being the resident innate immune cells, they are responsible for the early control of infections and for the recruitment of cells of the adaptive immune system required for pathogen clearance. The innate and adaptive immune responses triggered by microglia include the release of proinflammatory mediators. Although an efficient immune response is required for the defense against invading pathogens, an inflammatory response in the CNS may also lead to tissue injury and neurodegeneration. Engagement of Toll-like receptors (TLRs), a major family of pattern recognition receptors that mediate innate immunity but also link with the adaptive immune response, provides an important mechanism by which microglia are able to sense both pathogen- and host-derived ligands within the CNS. Although there is an increasing body of evidence that TLR signaling mediates beneficial effects in the CNS, it has become clear that TLR-induced activation of microglia and the release of proinflammatory molecules are responsible for neurotoxic processes in the course of various CNS diseases. Thus, the functional outcome of TLR-induced activation of microglia in the CNS depends on a subtle balance between protective and harmful effects. This review focuses on the neurodegenerative effects of TLR signaling in the CNS.

Down-regulation of lipocalin 2 contributes to chemoresistance in glioblastoma cells

Malignant gliomas are the most common primary brain tumor and have a poor clinical prognosis. 1, 3-Bis (2-chloroethyl)-1-nitrosourea (BCNU) is an alkylating agent that is commonly used in glioma therapy. However, BCNU chemotherapy often fails due to drug resistance. To gain better understanding of molecular mechanisms underlying the drug resistance of glioma, a BCNU-resistant variant (C6R) of C6 rat glioma cells was selected and characterized. The established C6R cells were resistant to BCNU-induced cell death and cell cycle arrest as confirmed by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide reduction assay and flow cytometric analysis of DNA content. C6R cells showed an increased expression of common drug resistance-related genes such as O6-methylguanine-DNA methyltransferase and multiple drug resistance 1. In contrast, C6R cells showed a decreased expression of glial fibrillary acidic protein, therefore, displaying shorter cellular processes compared with parental C6 cells. More importantly, in conjunction with the morphological changes, the expression of lipocalin-2 (lcn2), a 25-kDa secreted proapoptotic protein, was markedly reduced in the BCNU-resistant C6R cells. However, there was no significant change in the expression of lcn2 receptors. Addition of recombinant LCN2 protein or introduction of lcn2 cDNA significantly increased the sensitivity of C6 cells and human glioma cells to BCNU or other anticancer drugs, while knockdown of lcn2 expression by antisense cDNA transfection decreased the sensitivity. When lcn2 was re-expressed in C6R cells, the BCNU sensitivity was restored. Lcn2 enhanced BCNU-induced Akt dephosphorylation providing a molecular basis of apoptosis sensitization. These results suggest that LCN2 protein may be involved in glioma drug resistance and may provide a new approach to sensitizing glioblastoma to chemotherapy.

Lipopolysaccharide extends the lifespan of mouse primary-cultured microglia

Microglial activation has been implicated in the recognition and phagocytic removal of degenerating neurons; however, this process must be tightly regulated in the central nervous system, because prolonged activation could damage normal neurons. We report that mouse primary-cultured microglia, which are destined to die within a few days under ordinary culture conditions, can live for more than 1 month when kept activated by lipopolysaccharide (LPS) treatment. Primary-cultured microglia treated with sublethal doses of LPS remained viable, without any measurable increase in apoptotic or necrotic cell death. LPS-treated microglia had an arborescent shape, with enlarged somata and thickened cell bodies. Although the amount of intracellular ATP in these microglia was reduced by 2 h after the start of LPS treatment, this had no effect on the viability of the cells. LPS treatment of microglia increased the antiapoptotic factor Bcl-xL protein level at day 1, although the level of the proapoptotic Bcl-associated X-protein was unaffected. Furthermore, the level of microtubule-associated light chain 3, a marker protein for autophagy, decreased at 3 h after exposure to LPS. These data show that the optimal dose of LPS suppresses the induction of both apoptosis and autophagy in primary-cultured microglia, allowing the cells to stay alive for more than 1 month. Because long-lived microglia may play critical roles in the exacerbation of neurodegeneration, our findings suggest that inducing a resting stage in active microglia could be a new and promising strategy to inhibit the deterioration of neurodegenerative disease.

Lipocalin-2 is an autocrine mediator of reactive astrocytosis

Astrocytes, the most abundant glial cell type in the brain, provide metabolic and trophic support to neurons and modulate synaptic activity. In response to a brain injury, astrocytes proliferate and become hypertrophic with an increased expression of intermediate filament proteins. This process is collectively referred to as reactive astrocytosis. Lipocalin 2 (lcn2) is a member of the lipocalin family that binds to small hydrophobic molecules. We propose that lcn2 is an autocrine mediator of reactive astrocytosis based on the multiple roles of lcn2 in the regulation of cell death, morphology, and migration of astrocytes. lcn2 expression and secretion increased after inflammatory stimulation in cultured astrocytes. Forced expression of lcn2 or treatment with LCN2 protein increased the sensitivity of astrocytes to cytotoxic stimuli. Iron and BIM (Bcl-2-interacting mediator of cell death) proteins were involved in the cytotoxic sensitization process. LCN2 protein induced upregulation of glial fibrillary acidic protein (GFAP), cell migration, and morphological changes similar to characteristic phenotypic changes termed reactive astrocytosis. The lcn2-induced phenotypic changes of astrocytes occurred through a Rho-ROCK (Rho kinase)-GFAP pathway, which was positively regulated by nitric oxide and cGMP. In zebrafishes, forced expression of rat lcn2 gene increased the number and thickness of cellular processes in GFAP-expressing radial glia cells, suggesting that lcn2 expression in glia cells plays an important role in vivo. Our results suggest that lcn2 acts in an autocrine manner to induce cell death sensitization and morphological changes in astrocytes under inflammatory conditions and that these phenotypic changes may be the basis of reactive astrocytosis in vivo.

Dual role of CD38 in microglial activation and activation-induced cell death

Microglia, the resident immune cells of the CNS, are normally quiescent but become activated after infection or injury. Their properties then change, and they promote both repair and damage processes. The extent of microglial activation is regulated, in part, by activation-induced cell death (AICD). Although many apoptotic aspects of the microglial AICD mechanism have been elucidated, little is known about the connection between the activation step and the death process. Using mouse primary microglial cultures, we show that the ectoenzyme CD38, via its calcium-mobilizing metabolite cyclic-ADP-ribose (cADPR), helps promote microglial activation and AICD induced by LPS plus IFN-gamma (LPS/IFN-gamma), suggesting that CD38 links the two processes. Accordingly, CD38 expression and activity, as well as the intracellular calcium concentration ([Ca2+]i) in the primary microglia were increased by LPS/IFN-gamma treatment. Moreover, CD38 deficiency or treatment with cADPR antagonists conferred partial resistance to LPS/IFN-gamma-induced AICD and also reduced [Ca2+]i. Microglial activation, indicated by induced expression of NO synthase-2 mRNA and production of NO, secretion and mRNA expression of TNF-alpha and IL-12 p40, and expression of IL-6 mRNA, was attenuated by CD38 deficiency or cADPR-antagonist treatment. The observed effects of CD38 on microglial activation are probably mediated via a cADPR-dependent increase in [Ca2+]i and the effect on AICD by regulation of NO production. Our results thus suggest that CD38 significantly affects regulation of the amount and function of activated microglia, with important consequences for injury and repair processes in the brain.

Multiple mechanisms that prevent excessive brain inflammation

Inflammation of the injured brain has a double-edged effect. Inflammation protects the brain from infection, but it aggravates injury. Furthermore, brain inflammation is considered a risk factor for neurodegenerative disorders, such as Alzheimer's and Parkinson's diseases. Emerging evidence supports the activation of negative regulatory mechanisms during this process to prevent prolonged and extensive inflammation. The inflammatory stimulators themselves or products of inflammatory cells may induce the expression of negative feedback regulators, such as suppressor of cytokine signaling (SOCS)-family proteins, antioxidant enzymes, and antiinflammatory cytokines. Furthermore, death of activated microglia (major inflammatory cells in the brain) may regulate brain inflammation. Astrocytes, the most abundant cells in the brain, may also act in preventing microglial overactivation. Therefore, we propose that the extent and duration of brain inflammation is tightly regulated through the cooperation of multiple mechanisms to maximize antipathogenic effects and minimize tissue damage.

TLR2 and caspase-8 are essential for group B Streptococcus-induced apoptosis in microglia

Microglia, the resident innate immune cells of the CNS, detect invading pathogens via various receptors, including the TLR. Microglia are involved in a number of neurodegenerative diseases in which their activation may be detrimental to neurons. It is largely unknown how this potentially deleterious action can be countered on a cellular level. We previously found that the interaction of TLR2 with group B Streptococcus (GBS), the most important pathogen in neonatal bacterial meningitis, activates microglia that in turn generate neurotoxic NO. We report in this study that GBS not only activates microglia, but also induces apoptosis in these cells via TLR2 and the TLR-adaptor molecule MyD88. Soluble toxic mediators, such as NO, are not responsible for this form of cell death. Instead, interaction of GBS with TLR2 results in formation and activation of caspase-8, a process that involves the transcription factor family Ets. Whereas caspase-8 plays an essential role in GBS-induced microglial apoptosis, caspase-3 is dispensable in this context. We suggest that TLR2- and caspase-8-mediated microglial apoptosis constitutes an autoregulatory mechanism that limits GBS-induced overactivation of the innate immune system in the CNS.

Developmental stage of oligodendrocytes determines their response to activated microglia in vitro

Background

Oligodendrocyte progenitor cells (OPCs) and mature oligodendrocytes are both lost in central nervous system injury and disease. Activated microglia may play a role in OPC and oligodendrocyte loss or replacement, but it is not clear how the responses of OPCs and oligodendrocytes to activated microglia differ.

Methods

OPCs and microglia were isolated from rat cortex. OPCs were induced to differentiate into oligodendrocytes with thyroid hormone in defined medium. For selected experiments, microglia were added to OPC or oligodendrocyte cultures. Lipopolysaccharide was used to activate microglia and microglial activation was confirmed by TNFα ELISA. Cell survival was assessed with immunocytochemistry and cell counts. OPC proliferation and oligodendrocyte apoptosis were also assessed.

Results

OPCs and oligodendrocytes displayed phenotypes representative of immature and mature oligodendrocytes, respectively. Activated microglia reduced OPC survival, but increased survival and reduced apoptosis of mature oligodendrocytes. Activated microglia also underwent cell death themselves.

Conclusion

Activated microglia may have divergent effects on OPCs and mature oligodendrocytes, reducing OPC survival and increasing mature oligodendrocyte survival. This may be of importance because activated microglia are present in several disease states where both OPCs and mature oligodendrocytes are also reacting to injury. Activated microglia may simultaneously have deleterious and helpful effects on different cells after central nervous system injury.

A dual role of lipocalin 2 in the apoptosis and deramification of activated microglia

Activated microglia are thought to undergo apoptosis as a self-regulatory mechanism. To better understand molecular mechanisms of the microglial apoptosis, apoptosis-resistant variants of microglial cells were selected and characterized. The expression of lipocalin 2 (lcn2) was significantly down-regulated in the microglial cells that were resistant to NO-induced apoptosis. lcn2 expression was increased by inflammatory stimuli in microglia. The stable expression of lcn2 as well as the addition of rLCN2 protein augmented the sensitivity of microglia to the NO-induced apoptosis, while knockdown of lcn2 expression using short hairpin RNA attenuated the cell death. Microglial cells with increased lcn2 expression were more sensitive to other cytotoxic agents as well. Thus, inflammatory activation of microglia may lead to up-regulation of lcn2 expression, which sensitizes microglia to the self-regulatory apoptosis. Additionally, the stable expression of lcn2 in BV-2 microglia cells induced a morphological change of the cells into the round shape with a loss of processes. Treatment of primary microglia cultures with the rLCN2 protein also induced the deramification of microglia. The deramification of microglia was closely related with the apoptosis-prone phenotype, because other deramification-inducing agents such as cAMP-elevating agent forskolin, ATP, and calcium ionophore also rendered microglia more sensitive to cell death. Taken together, our results suggest that activated microglia may secrete LCN2 protein, which act in an autocrine manner to sensitize microglia to the self-regulatory apoptosis and to endow microglia with an amoeboid form, a canonical morphology of activated microglia in vivo.

Activation of microglial cells by ceruloplasmin

Ceruloplasmin (Cp) is the major copper transport protein in plasma and catalyzes the conversion of toxic ferrous iron to the safer ferric iron. As an acute-phase protein, Cp is induced during inflammation. It is synthesized primarily in the liver and is expressed in several other tissues, including the brain. Elevated Cp levels have been observed in the brain of patients with neurodegenerative conditions, including Alzheimer's, Parkinson's, and Huntington's diseases. However, the exact role(s) of Cp in inflammatory and neuropathological conditions remains unclear. Microglia are the prime effector cells involved in immune and inflammatory responses in the central nervous system (CNS). They are activated during pathological conditions to restore CNS homeostasis, but chronic microglial activation endangers neuronal survival. Consequently, it is important to identify the regulators of microglial activation and the underlying mechanisms. We sought to examine whether Cp might modulate microglial activation. We observed that Cp induced nitric oxide (NO) release and inducible NO synthase mRNA expression in BV2 microglial cells and rat brain microglia. Cp also increased levels of mRNAs encoding tumor necrosis factor-alpha, interleukin-1beta, cyclooxygenase-2, and NADPH oxidase. Treatment of BV2 cells and primary microglia with Cp induced phosphorylation of p38 MAP kinase. Moreover, Cp induced nuclear factor (NF)-kappaB activation, showing a more sustained pattern than seen with bacterial lipopolysaccharide. Cp-stimulated NO induction was significantly attenuated by a p38 inhibitor, SB203580, and the NF-kappaB inhibitor SN50. Cp induced secretion of TNF-alpha and prostaglandin E(2) in primary microglial cultures. These results suggest that Cp may play an important role in neuropathological conditions by stimulating various proinflammatory and neurotoxic molecules in microglia.

Heme oxygenase-1 mediates cytoprotective effects of immunostimulation in microglia

Microglia are brain-resident immune cells playing a pivotal role in the neuroinflammation. Previously, it has been shown that immunostimulation protects microglial cells against nitric oxide toxicity. Herein, we report that heme oxygenase-1 (HO-1) mediates the protective effects of immunostimulation. Pro-inflammatory activation of BV-2 microglial cells with endotoxin lipopolysaccharide (LPS) conferred a protection against various cytotoxic stimuli, whereas anti-inflammatory cytokines such as IL-4 and IL-10 were without effects. The LPS-induced cytoprotection was accompanied by HO-1 induction. The cytoprotective effect of LPS treatment was significantly attenuated by co-treatment with a HO-1 inhibitor, zinc protoporphyrin. Adenoviral expression of HO-1 in microglial cells was similarly cytoprotective, indicating that HO-1 mediates the cytoprotective effects of pro-inflammatory stimulation. Additional experiments revealed the involvement of carbon monoxide (CO) and iron, products of HO-1-mediated heme degradation, in the cytoprotective effect of LPS. Taken together, our results suggest that immunostimulation of microglia with LPS provides cytoprotective effects via HO-1 induction followed by the generation of CO and iron.

Janus faces of microglia in multiple sclerosis

Multiple sclerosis (MS) is the most common cause of neurological disability in young adults. The disease is characterized by inflammatory reactions, demyelination and axonal loss in the brain, spinal cord and optic nerves. Microglia seem to play an important role in the inflammatory processes in MS, since they are found in actively demyelinating lesions. Their role in the differentiation of T cells could led to the expansion of inflammation and tissue destruction. However, microglia are also involved in the termination of an inflammatory response and produce protective factors. To be able to therapeutically manipulate microglia, their exact function in the onset and development of MS needs to be clarified. This review provides an overview of the functions of the most important microglia-associated molecules in MS, being CD40, B7-1 and B7-2, interferon-gamma, tumor necrosis factor-alpha, chemokines, prostanoids, and nitric oxide.

The microglial gene regulatory network activated by interferon-gamma

We have analysed the microglial pathway stimulated by interferon-gamma (IFN-gamma) using an in silico approach employing a database of eukaryotic molecular interactions and a microarray dataset validated by quantitative real-time PCR (qRT-PCR). Following IFN-gamma stimulation, production of neuroprotective factors by microglia was found to be reduced while caspase 1 and serping1 which are involved in cell death cascades are up-regulated suggesting a safeguarding mechanism. Extracellular matrix interactions and intracellular protein degradation are altered in concert with these changes. The regulatory network of IFN-gamma responsive microglial genes is outlined in detail and differentially expressed genes are mapped to their respective cellular compartments. A pathway approach to the analysis of microarray data is advocated since overlaying pathway and actual expression data as shown here greatly facilitates understanding the biological meaning of a gene regulatory network. In addition, genes of similar function that are differentially regulated are less likely to be false positives than single unrelated genes.