Regulation of microglial migration, phagocytosis, and neurite outgrowth by HO-1/CO signaling

Olanzapine attenuates amyloid-β-induced microglia-mediated progressive neurite lesions

The accumulation of amyloid-β (Aβ) in the brain is the first pathological mechanism to initiate Alzheimer's disease (AD) pathogenesis. However, the precise role of Aβ in the disease progression remains unclear. Through decades of research, prolonged inflammation has emerged as an important core pathology in AD. Previously, a study has demonstrated the neurotoxic effect of Aβ-induced neuroinflammation in neuron-glia co-culture at 72 h. Here, we hypothesise that initial stage Aβ may trigger microglial inflammation, synergistically contributing to the progression of neurite lesions relevant to AD progression. In the present study, we aimed to determine whether olanzapine, an antipsychotic drug possessing anti-inflammatory properties, can ameliorate Aβ-induced progressive neurite lesions. Our study reports that Aβ induces neurite lesions with or without inflammatory microglial cells in vitro. More intriguingly, the present study revealed that Aβ exacerbates neurite lesions in synergy with microglia. Moreover, the time course study revealed that Aβ promotes microglia-mediated neurite lesions by eliciting the secretion of pro-inflammatory cytokines. Furthermore, our study shows that olanzapine at lower doses prevents Aβ-induced microglia-mediated progressive neurite lesions. The increase in pro-inflammatory cytokines induced by Aβ is attenuated by olanzapine administration, associated with a reduction in microglial inflammation. Finally, this study reports that microglial senescence induced by Aβ was rescued by olanzapine. Thus, our study provides the first evidence that 1 µM to 5 µM of olanzapine can effectively prevent Aβ-induced microglia-mediated progressive neurite lesions by modulating microglial inflammation. These observations reinforce the potential of targeting microglial remodelling to slow disease progression in AD.

The Ketogenic Diet and Neuroinflammation: The Action of Beta-Hydroxybutyrate in a Microglial Cell Line

The ketogenic diet (KD), a diet high in fat and protein but low in carbohydrates, is gaining much interest due to its positive effects, especially in neurodegenerative diseases. Beta-hydroxybutyrate (BHB), the major ketone body produced during the carbohydrate deprivation that occurs in KD, is assumed to have neuroprotective effects, although the molecular mechanisms responsible for these effects are still unclear. Microglial cell activation plays a key role in the development of neurodegenerative diseases, resulting in the production of several proinflammatory secondary metabolites. The following study aimed to investigate the mechanisms by which BHB determines the activation processes of BV2 microglial cells, such as polarization, cell migration and expression of pro- and anti-inflammatory cytokines, in the absence or in the presence of lipopolysaccharide (LPS) as a proinflammatory stimulus. The results showed that BHB has a neuroprotective effect in BV2 cells, inducing both microglial polarization towards an M2 anti-inflammatory phenotype and reducing migratory capacity following LPS stimulation. Furthermore, BHB significantly reduced expression levels of the proinflammatory cytokine IL-17 and increased levels of the anti-inflammatory cytokine IL-10. From this study, it can be concluded that BHB, and consequently the KD, has a fundamental role in neuroprotection and prevention in neurodegenerative diseases, presenting new therapeutic targets.

Effects of neuronal cell adhesion molecule L1 and nanoparticle surface modification on microglia

Microelectrode arrays for neural recording suffer from low yield and stability partly due to the inflammatory host responses. A neuronal cell adhesion molecule L1 coating has been shown to promote electrode-neuron integration, reduce microglia activation and improve recording. Coupling L1 to surface via a nanoparticle (NP) base layer further increased the protein surface density and stability. However, the exact L1-microglia interaction in these coatings has not been studied. Here we cultured primary microglia on L1 modified surfaces (with and without NP) and characterized microglia activation upon phorbol myristate acetate (PMA) and lipopolysaccharide (LPS) stimulation. Results showed L1 coatings reduced microglia's superoxide production in response to PMA and presented intrinsic antioxidant properties. Meanwhile, L1 decreased iNOS, NO, and pro-inflammatory cytokines (TNF alpha, IL-6, IL-1 beta), while increased anti-inflammatory cytokines (TGF beta 1, IL-10) in LPS stimulated microglia. Furthermore, L1 increased Arg-1 expression and phagocytosis upon LPS stimulation. Rougher NP surface showed lower number of microglia attached per area than their smooth counterpart, lower IL-6 release and superoxide production, and higher intrinsic reducing potential. Finally, we examined the effect of L1 and nanoparticle modifications on microglia response in vivo over 8 weeks with 2-photon imaging. Microglial coverage on the implant surface was found to be lower on the L1 modified substrates relative to unmodified, consistent with the in vitro observation. Our results indicate L1 significantly reduces superoxide production and inflammatory response of microglia and promotes wound healing, while L1 immobilization via a nanoparticle base layer brings added benefit without adverse effects. STATEMENT OF SIGNIFICANCE: Surface modification of microelectrode arrays with L1 has been shown to reduce microglia coverage on neural probe surface in vivo and improves neural recording, but the specific mechanism of action is not fully understood. The results in this study show that surface bound L1 reduces superoxide production from cultured microglia via direct reduction reaction and signaling pathways, increases anti-inflammatory cytokine release and phagocytosis in response to PMA or LPS stimulation. Additionally, roughening the surface with nanoparticles prior to L1 immobilization further increased the benefit of L1 in reducing microglia activation and oxidative stress. Together, our findings shed light on the mechanisms of action of nanotextured and neuroadhesive neural implant coatings and guide future development of seamless tissue interface.

Beneficial and Detrimental Roles of Heme Oxygenase-1 in the Neurovascular System

Heme oxygenase (HO) has both beneficial and detrimental effects via its metabolites, including carbon monoxide (CO), biliverdin or bilirubin, and ferrous iron. HO-1 is an inducible form of HO that is upregulated by oxidative stress, nitric oxide, CO, and hypoxia, whereas HO-2 is a constitutive form that regulates vascular tone and homeostasis. In brains injured by trauma, ischemia-reperfusion, or Alzheimer’s disease (AD), the long-term expression of HO-1 can be detected, which can lead to cytotoxic ferroptosis via iron accumulation. In contrast, the transient induction of HO-1 in the peri-injured region may have regenerative potential (e.g., angiogenesis, neurogenesis, and mitochondrial biogenesis) and neurovascular protective effects through the CO-mediated signaling pathway, the antioxidant properties of bilirubin, and the iron-mediated ferritin synthesis. In this review, we discuss the dual roles of HO-1 and its metabolites in various neurovascular diseases, including age-related macular degeneration, ischemia-reperfusion injury, traumatic brain injury, Gilbert’s syndrome, and AD.

Repair Mechanisms of the Neurovascular Unit after Ischemic Stroke with a Focus on VEGF

The functional neural circuits are partially repaired after an ischemic stroke in the central nervous system (CNS). In the CNS, neurovascular units, including neurons, endothelial cells, astrocytes, pericytes, microglia, and oligodendrocytes maintain homeostasis; however, these cellular networks are damaged after an ischemic stroke. The present review discusses the repair potential of stem cells (i.e., mesenchymal stem cells, endothelial precursor cells, and neural stem cells) and gaseous molecules (i.e., nitric oxide and carbon monoxide) with respect to neuroprotection in the acute phase and regeneration in the late phase after an ischemic stroke. Commonly shared molecular mechanisms in the neurovascular unit are associated with the vascular endothelial growth factor (VEGF) and its related factors. Stem cells and gaseous molecules may exert therapeutic effects by diminishing VEGF-mediated vascular leakage and facilitating VEGF-mediated regenerative capacity. This review presents an in-depth discussion of the regeneration ability by which endogenous neural stem cells and endothelial cells produce neurons and vessels capable of replacing injured neurons and vessels in the CNS.

Artemisinin improves neurocognitive deficits associated with sepsis by activating the AMPK axis in microglia

Sepsis is life-threatening organ dysfunction due to dysregulated systemic inflammatory and immune response to infection, often leading to cognitive impairments. Growing evidence shows that artemisinin, an antimalarial drug, possesses potent anti-inflammatory and immunoregulatory activities. In this study we investigated whether artemisinin exerted protective effect against neurocognitive deficits associated with sepsis and explored the underlying mechanisms. Mice were injected with LPS (750 μg · kg-1 · d-1, ip, for 7 days) to establish an animal model of sepsis. Artemisinin (30 mg · kg-1 · d-1, ip) was administered starting 4 days prior LPS injection and lasting to the end of LPS injection. We showed that artemisinin administration significantly improved LPS-induced cognitive impairments assessed in Morris water maze and Y maze tests, attenuated neuronal damage and microglial activation in the hippocampus. In BV2 microglial cells treated with LPS (100 ng/mL), pre-application of artemisinin (40 μΜ) significantly reduced the production of proinflammatory cytokines (i.e., TNF-α, IL-6) and suppressed microglial migration. Furthermore, we revealed that artemisinin significantly suppressed the nuclear translocation of NF-κB and the expression of proinflammatory cytokines by activating the AMPKα1 pathway; knockdown of AMPKα1 markedly abolished the anti-inflammatory effects of artemisinin in BV2 microglial cells. In conclusion, atemisinin is a potential therapeutic agent for sepsis-associated neuroinflammation and cognitive impairment, and its effect is probably mediated by activation of the AMPKα1 signaling pathway in microglia.

Regenerative Potential of Carbon Monoxide in Adult Neural Circuits of the Central Nervous System

Regeneration of adult neural circuits after an injury is limited in the central nervous system (CNS). Heme oxygenase (HO) is an enzyme that produces HO metabolites, such as carbon monoxide (CO), biliverdin and iron by heme degradation. CO may act as a biological signal transduction effector in CNS regeneration by stimulating neuronal intrinsic and extrinsic mechanisms as well as mitochondrial biogenesis. CO may give directions by which the injured neurovascular system switches into regeneration mode by stimulating endogenous neural stem cells and endothelial cells to produce neurons and vessels capable of replacing injured neurons and vessels in the CNS. The present review discusses the regenerative potential of CO in acute and chronic neuroinflammatory diseases of the CNS, such as stroke, traumatic brain injury, multiple sclerosis and Alzheimer’s disease and the role of signaling pathways and neurotrophic factors. CO-mediated facilitation of cellular communications may boost regeneration, consequently forming functional adult neural circuits in CNS injury.

Inhibition of inflammatory mediators and cell migration by 1,2,3,4-tetrahydroquinoline derivatives in LPS-stimulated BV2 microglial cells via suppression of NF-κB and JNK pathway

Novel 1,2,3,4-tetrahydroquinoline derivatives with N-alkanoyl, N-benzoyl, or chlorobenzoyl substituents were designed and synthesized to inhibit nuclear factor-kappa B (NF-κB) known to be involved in the regulation of many immune and inflammatory responses. These compounds have been previously reported to inhibit NF-κB transcriptional activity in Raw 267.4 macrophage cells and exhibit cytotoxicities to several human cancer cell lines (Jo et al., ACS Med. Chem. Lett. 7 (2016) 385-390). Accumulating evidence indicated that NF-κB is also involved in neuroinflammation implicated in many neurodegenerative diseases. Thus, the present study investigated effects of 1,2,3,4-tetrahydroquinoline derivatives on LPS-stimulated inflammatory mediators and cell migration using BV2 microglial cells as a model. We found that seven compounds tested in this study inhibited LPS-induced pro-inflammatory mediators including interleukin-6, tumor necrosis factor-α, and nitric oxide in concentration-dependent manners. Among these compounds, ELC-D-2 exhibited the most potent inhibition without showing significant cytotoxicity. We also found that ELC-D-2 attenuated levels of LPS-induced inducible nitric oxide synthase and cyclooxygenase-2. Moreover, ELC-D-2 inhibited nuclear translocation of NF-κB by suppressing inhibitor of kappa Bα phosphorylation. Furthermore, ELC-D-2 inhibited LPS-induced activation of c-Jun N-terminal kinase (JNK), which was associated with suppression of inflammatory mediators and migration of LPS-treated BV2 cells. Collectively, our findings demonstrate that ELC-D-2 inhibits LPS-induced pro-inflammatory mediators and cell migration by suppressing NF-κB translocation and JNK phosphorylation in BV2 microglial cells. These results suggest that ELC-D-2 might have a beneficial impact on various brain disorders in which neuroinflammation involving microglial activation plays a crucial role in the pathogenesis of these diseases.

Ibrutinib suppresses LPS-induced neuroinflammatory responses in BV2 microglial cells and wild-type mice

Background

The FDA-approved small-molecule drug ibrutinib is an effective targeted therapy for patients with chronic lymphocytic leukemia (CLL). Ibrutinib inhibits Bruton’s tyrosine kinase (BTK), a kinase involved in B cell receptor signaling. However, the potential regulation of neuroinflammatory responses in the brain by ibrutinib has not been comprehensively examined.

Methods

BV2 microglial cells were treated with ibrutinib (1 μM) or vehicle (1% DMSO), followed by lipopolysaccharide (LPS; 1 μg/ml) or PBS. RT-PCR, immunocytochemistry, and subcellular fractionation were performed to examine the effects of ibrutinib on neuroinflammatory responses. In addition, wild-type mice were sequentially injected with ibrutinib (10 mg/kg, i.p.) or vehicle (10% DMSO, i.p.), followed by LPS (10 mg/kg, i.p.) or PBS, and microglial and astrocyte activations were assessed using immunohistochemistry.

Results

Ibrutinib significantly reduced LPS-induced increases in proinflammatory cytokine levels in BV2 microglial and primary microglial cells but not in primary astrocytes. Ibrutinib regulated TLR4 signaling to alter LPS-induced proinflammatory cytokine levels. In addition, ibrutinib significantly decreased LPS-induced increases in p-AKT and p-STAT3 levels, suggesting that ibrutinib attenuates LPS-induced neuroinflammatory responses by inhibiting AKT/STAT3 signaling pathways. Interestingly, ibrutinib also reduced LPS-induced BV2 microglial cell migration by inhibiting AKT signaling. Moreover, ibrutinib-injected wild-type mice exhibited significantly reduced microglial/astrocyte activation and COX-2 and IL-1β proinflammatory cytokine levels.

Conclusions

Our data provide insights on the mechanisms of a potential therapeutic strategy for neuroinflammation-related diseases.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1308-0) contains supplementary material, which is available to authorized users.

Neuroprotection and neuroregeneration of retinal ganglion cells after intravitreal carbon monoxide release

PURPOSE

Retinal ischemia induces apoptosis leading to neurodegeneration and vision impairment. Carbon monoxide (CO) in gaseous form showed cell-protective and anti-inflammatory effects after retinal ischemia-reperfusion-injury (IRI). These effects were also demonstrated for the intravenously administered CO-releasing molecule (CORM) ALF-186. This article summarizes the results of intravitreally released CO to assess its suitability as a neuroprotective and neuroregenerative agent.

METHODS

Water-soluble CORM ALF-186 (25 μg), PBS, or inactivated ALF (iALF) (all 5 μl) were intravitreally applied into the left eyes of rats directly after retinal IRI for 1 h. Their right eyes remained unaffected and were used for comparison. Retinal tissue was harvested 24 h after intervention to analyze mRNA or protein expression of Caspase-3, pERK1/2, p38, HSP70/90, NF-kappaB, AIF-1 (allograft inflammatory factor), TNF-α, and GAP-43. Densities of fluorogold-prelabeled retinal ganglion cells (RGC) were examined in flat-mounted retinae seven days after IRI and were expressed as mean/mm2. The ability of RGC to regenerate their axon was evaluated two and seven days after IRI using retinal explants in laminin-1-coated cultures. Immunohistochemistry was used to analyze the different cell types growing out of the retinal explants.

RESULTS

Compared to the RGC-density in the contralateral right eyes (2804±214 RGC/mm2; data are mean±SD), IRI+PBS injection resulted in a remarkable loss of RGC (1554±159 RGC/mm2), p<0.001. Intravitreally injected ALF-186 immediately after IRI provided RGC protection and reduced the extent of RGC-damage (IRI+PBS 1554±159 vs. IRI+ALF 2179±286, p<0.001). ALF-186 increased the IRI-mediated phosphorylation of MAP-kinase p38. Anti-apoptotic and anti-inflammatory effects were detectable as Caspase-3, NF-kappaB, TNF-α, and AIF-1 expression were significantly reduced after IRI+ALF in comparison to IRI+PBS or IRI+iALF. Gap-43 expression was significantly increased after IRI+ALF. iALF showed effects similar to PBS. The intrinsic regenerative potential of RGC-axons was induced to nearly identical levels after IRI and ALF or iALF-treatment under growth-permissive conditions, although RGC viability differed significantly in both groups. Intravitreal CO further increased the IRI-induced migration of GFAP-positive cells out of retinal explants and their transdifferentiation, which was detected by re-expression of beta-III tubulin and nestin.

CONCLUSION

Intravitreal CORM ALF-186 protected RGC after IRI and stimulated their axons to regenerate in vitro. ALF conveyed anti-apoptotic, anti-inflammatory, and growth-associated signaling after IRI. CO's role in neuroregeneration and its effect on retinal glial cells needs further investigation.

Heme Oxygenase-1 Induction by Carbon Monoxide Releasing Molecule-3 Suppresses Interleukin-1β-Mediated Neuroinflammation

Neurodegenerative disorders and brain damage are initiated by excessive production of reactive oxygen species (ROS), which leads to tissue injury, cellular death and inflammation. In cellular anti-oxidant systems, heme oxygenase-1 (HO-1) is an oxidative-sensor protein induced by ROS generation or carbon monoxide (CO) release. CO releasing molecules (CORMs), including CORM-3, exert anti-oxidant and anti-inflammatory effects. However, the molecular mechanisms of CORM-3-induced HO-1 expression and protection against interleukin (IL)-1β-induced inflammatory responses have not been fully elucidated in rat brain astrocytes (RBA-1). To study the regulation of CORM-3-induced HO-1 expression, signaling pathways, promoter activity, mRNA and protein expression were assessed following treatment with pharmacological inhibitors and gene-specific siRNA knockdown. We found that CORM-3 mediated HO-1 induction via transcritional and translational processes. Furthermore, CORM-3-induced HO-1 expression was mediated by phosphorylation of several protein kinases, such as c-Src, Pyk2, protein kinase Cα (PKCα) and p42/p44 mitogen-activated protein kinase (MAPK), which were inhibited by respective pharmacological inhibitors or by gene-specific knockdown with siRNA transfections. Next, we found that CORM-3 sequentially activated the c-Src/Pyk2/PKCα/p42/p44 MAPK pathway, thereby up-regulating mRNA for the activator protein (AP)-1 components c-Jun and c-Fos; these effects were attenuated by an AP-1 inhibitor (Tanshinone IIA; TSIIA) and other relevant inhibitors. Moreover, CORM-3-induced upregulation of HO-1 attenuated the IL-1β-induced cell migration and matrix metallopeptidase-9 mRNA expression in RBA-1 cells. These effects were reversed by an matrix metalloproteinase (MMP)2/9 inhibitor or by transfection with HO-1 siRNA.

Phosphoinositide 3-kinase γ ties chemoattractant- and adrenergic control of microglial motility

Microglial motility is tightly controlled by multitude of agonistic and antagonistic factors. Chemoattractants, released after infection or damage of the brain, provoke directed migration of microglia to the pathogenic incident. In contrast, noradrenaline and other stress hormones have been shown to suppress microglial movement. Here we asked for the signaling reactions involved in the positive and negative control of microglial motility. Using pharmacological and genetic approaches we identified the lipid kinase activity of phosphoinositide 3-kinase species γ (PI3Kγ) as an essential mediator of microglial migration provoked by the complement component C5a and other chemoattractants. Inhibition of PI3Kγ lipid kinase activity by protein kinase A was disclosed as mechanism causing suppression of microglial migration by noradrenaline. Together these data characterize PI3Kγ as a nodal point in the control of microglial motility.

Modulation of growth cone filopodial length by carbon monoxide

Carbon monoxide (CO) is physiologically produced via heme degradation by heme oxygenase enzymes. Whereas CO has been identified as an important physiological signaling molecule, the roles it plays in neuronal development and regeneration are poorly understood. During these events, growth cones guide axons through a rich cellular environment to locate target cells and establish synaptic connections. Previously, we have shown that another gaseous signaling molecule, nitric oxide (NO), has potent effects on growth cone motility. With NO and CO sharing similar cellular targets, we wanted to determine whether CO affected growth cone motility as well. We assessed how CO affected growth cone filopodial length and determined the signaling pathway by which this effect was mediated. Using two well-characterized neurons from the freshwater snail, Helisoma trivolvis, it was found that the CO donor, carbon monoxide releasing molecule-2 (CORM-2), increased filopodial length. CO utilized a signaling pathway that involved the activation of soluble guanylyl cyclase, protein kinase G, and ryanodine receptors. While increases in filopodial length often occur from robust increases in intracellular calcium levels, the timing in which CO increased filopodial length corresponded with low basal calcium levels in growth cones. Taken together with findings of a heme oxygenase-like protein in the Helisoma nervous system, these results provide evidence for CO as a modulator of growth cone motility and implicate CO as a neuromodulatory signal during neuronal development and/or regeneration. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 677-690, 2017.

Carbon monoxide treatment reduces microglial activation in the ischemic rat retina

PURPOSE

Ischemia and reperfusion (I/R) injury damages retinal neurons. Retinal injury is accompanied by activation of microglia, which scavenge the dead or dying neurons, but increasing evidence now indicates that amoeboid-shaped microglia cells activated in the brain after ischemia have neurotoxic and damaging properties in their own right. A previous study showed that postconditioning with carbon monoxide (CO) protects retinal ganglion cells (RGCs) after I/R through anti-apoptotic and anti-inflammatory mechanisms. The present study was designed to investigate and quantify the activation of retinal microglia after I/R with and without CO postconditioning.

METHODS

Adult Sprague-Dawley rats underwent retinal ischemia by increasing the ocular pressure to 120 mmHg for 1 h through a needle inserted into the anterior chamber. Reperfusion was induced by removing the needle. After I/R, one group of animals was kept in a CO (250 ppm) atmosphere for 1 h; the other group was kept in room air (Air). At 1, 2, 3, and 7 days after I/R, the eyes were enucleated and fixed. Intracardiac blood was analyzed for systemic effects of CO or I/R. Retinal cross sections were taken from the middle third of the eye and were stained with anti-Iba-1. Microglia cells were graded as amoeboid or ramified phenotypes according to morphologic criteria. Retinal thicknesses were determined.

RESULTS

Evaluation of retinal tissue revealed a significant reduction of amoeboid microglia cells after I/R + CO when compared to the I/R + Air group. The peak number of amoeboid microglia was observed at day 2 post-I/R + Air. This rise was attenuated by CO postconditioning (815 versus 572 cells/mm² for I/R + Air versus I/R + CO, respectively; p = 0.005). CO reduced and further postponed the peak in the numbers of amoeboid and ramified microglia cells in ischemic eyes and prevented microglial activation in the contralateral eyes. I/R-induced leucocytosis was inhibited by CO inhalation. The reduction of retinal thickness after I/R was more serious after Air inhalation when compared to the CO group.

CONCLUSIONS

Numerous activated microglia cells appear in the inner retina after I/R, and CO-treatment significantly attenuates this glial response. Antagonism of microglial activation may be a further neuroprotective effect of CO, apart from its direct anti-apoptotic capacity.

Regulation of Microglial Phagocytosis by RhoA/ROCK-Inhibiting Drugs

Inflammation within the central nervous system (CNS) is a major component of many neurodegenerative diseases. The underlying mechanisms of neuronal loss are not fully understood, but the activation of CNS resident phagocytic microglia seems to be a significant element contributing to neurodegeneration. At the onset of inflammation, high levels of microglial phagocytosis may serve as an essential prerequisite for creating a favorable environment for neuronal regeneration. However, the excessive and long-lasting activation of microglia and the augmented engulfment of neurons have been suggested to eventually govern widespread neurodegeneration. Here, we investigated in a functional assay of acute inflammation how the small GTPase RhoA and its main target the Rho kinase (ROCK) influence microglial phagocytosis of neuronal debris. Using BV-2 microglia and human NT2 model neurons, we demonstrate that the pain reliever Ibuprofen decreases RhoA activation and microglial phagocytosis of neuronal cell fragments. Inhibition of the downstream effector ROCK with the small-molecule agents Y-27632 and Fasudil reduces the engulfment of neuronal debris and attenuates the production of the inflammatory mediator nitric oxide during stimulation with lipopolysaccharide. Our results support a therapeutic potential for RhoA/ROCK-inhibiting agents as an effective treatment of excessive inflammation and the resulting progression of microglia-mediated neurodegeneration in the CNS.

Nitric oxide regulates antagonistically phagocytic and neurite outgrowth inhibiting capacities of microglia

Traumatic injury or the pathogenesis of some neurological disorders is accompanied by inflammatory cellular mechanisms, mainly resulting from the activation of central nervous system (CNS) resident microglia. Under inflammatory conditions, microglia up-regulate the inducible isoform of NOS (iNOS), leading to the production of high concentrations of the radical molecule nitric oxide (NO). At the onset of inflammation, high levels of microglial-derived NO may serve as a cellular defense mechanism helping to clear the damaged tissue and combat infection of the CNS by invading pathogens. However, the excessive overproduction of NO by activated microglia has been suggested to govern the inflammation-mediated neuronal loss causing eventually complete neurodegeneration. Here, we investigated how NO influences phagocytosis of neuronal debris by BV-2 microglia, and how neurite outgrowth of human NT2 model neurons is affected by microglial-derived NO. The presence of NO greatly increased microglial phagocytic capacity in a model of acute inflammation comprising lipopolysaccharide (LPS)-activated microglia and apoptotic neurons. Chemical manipulations suggested that NO up-regulates phagocytosis independently of the sGC/cGMP pathway. Using a transwell system, we showed that reactive microglia inhibit neurite outgrowth of human neurons via the generation of large amounts of NO over effective distances in the millimeter range. Application of a NOS blocker prevented the LPS-induced NO production, totally reversed the inhibitory effect of microglia on neurite outgrowth, but reduced the engulfment of neuronal debris. Our results indicate that a rather simple notion of treating excessive inflammation in the CNS by NO synthesis blocking agents has to consider functionally antagonistic microglial cell responses during pharmaceutic therapy.