Role of microglia in inflammation-mediated degeneration of dopaminergic neurons: neuroprotective effect of interleukin 10

Inflammation in the brain has been recognized to play an increasingly important role in the pathogenesis of several neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease. Inflammation-mediated neurodegeneration involves activation of the brain's resident immune cells, the microglia, which produce proinflammatory and neurotoxic factors including cytokines, reactive oxygen species (ROS), nitric oxide, and eicosanoids that directly or indirectly cause neurodegeneration. In this study, we report that IL-10, an immunosuppressive cytokine, reduced the inflammation-mediated degeneration of dopaminergic (DA) neurons through the inhibition of microglial activation. Pretreatment of rat mesencephalic neuronglia cultures with IL-10 significantly attenuated the lipopolysaccharide (LPS) induced DA neuronal degeneration. The neuroprotective effect of IL-10 was attributed to inhibition of LPS-stimulated microglial activation. IL-10 significantly inhibited the microglial production of tumor necrosis factor alpha (TNF-alpha), nitric oxide, ROS and superoxide free radicals after LPS stimulation.     

Transient receptor potential vanilloid subtype 1 contributes to mesencephalic dopaminergic neuronal survival by inhibiting microglia-originated oxidative stress

The present study examined whether capsaicin (CAP), an agonist of transient receptor potential vanilloid subtype 1 (TRPV1) can prevent 1-methyl-4-phenylpyridinium (MPP(+))-induced dopaminergic (DA) neuronal death in the substantia nigra (SN). Unilateral injection of MPP(+) into the median forebrain bundle of rat brain resulted in a significant loss of nigral DA neurons, assessed by tyrosine hydroxylase (TH) immunostaining. In parallel, activation of microglia, visualized by OX-42 and OX-6 immunostaining were also observed in the SN, where degeneration of nigral neurons was found. By contrast, MPP(+) neurotoxicity was partially inhibited by co-treatment with MPP(+) and CAP. Interestingly, CAP significantly decreased not only immunoreactivity of OX-42 and OX-6 but also production of microglia-derived reactive oxygen species (ROS) in the SN of MPP(+)-treated rats. In experiments designed to further verify effectiveness of CAP against microglia-derived neurotoxicity, CAP inhibited ROS production and blocked MPP(+)-induced death of DA neurons in co-cultures of mesencephalic neurons and microglia, but not in microglia-free, neuron-enriched mesencephalic cultures. This beneficial effect was reversed by capsazepine, an antagonist of TRPV1, expressed in microglia, indicating TRPV1 involvement. Our data demonstrate for the first time that CAP may inhibit microglial activation-mediated oxidative stress via TRPV1, suggesting that CAP and its analogs may have therapeutic value by inhibiting microglial activation and/or ROS generation that occurs in Parkinson's disease.     

Neuromelanin Activates Microglia and Induces Degeneration of Dopaminergic Neurons: Implications for Progression of Parkinson’s Disease

In Parkinson’s disease (PD), there is a progressive loss of neuromelanin (NM)-containing dopamine neurons in substantia nigra (SN) which is associated with microgliosis and presence of extracellular NM. Herein, we have investigated the interplay between microglia and human NM on the degeneration of SN dopaminergic neurons. Although NM particles are phagocytized and degraded by microglia within minutes in vitro, extracellular NM particles induce microglial activation and ensuing production of superoxide, nitric oxide, hydrogen peroxide (H₂O₂), and pro-inflammatory factors. Furthermore, NM produces, in a microglia-depended manner, neurodegeneration in primary ventral midbrain cultures. Neurodegeneration was effectively attenuated with microglia derived from mice deficient in macrophage antigen complex-1, a microglial integrin receptor involved in the initiation of phagocytosis. Neuronal loss was also attenuated with microglia derived from mice deficient in phagocytic oxidase, a subunit of NADPH oxidase, that is responsible for superoxide and H₂O₂ production, or apocynin, an NADPH oxidase inhibitor. In vivo, NM injected into rat SN produces microgliosis and a loss of tyrosine hydroxylase neurons. Thus, these results show that extracellular NM can activate microglia, which in turn may induce dopaminergic neurodegeneration in PD. Our study may have far-reaching implications, both pathogenic and therapeutic.     

Involvement of microglia-neuron interactions in the tumor necrosis factor-alpha release,microglial activation,and neurodegeneration induced by trimethyltin

Trimethyltin (TMT) is a neurotoxicant known to induce early microglial activation. The present study was undertaken to investigate the role played by these microglial cells in the TMT-induced neurotoxicity. The effects of TMT were investigated in monolayer cultures of isolated microglia or in neuron-enriched cultures and in neuron-microglia and astrocyte-microglia cocultures. The end points used were morphological criteria; evaluation of cell death and cell proliferation; and measurements of tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and nitric oxide (NO) release in culture supernatant. The results showed that, in cultures of microglia, TMT (10(-6) M) caused, after a 5-day treatment, an increased release of TNF-alpha, without affecting microglial shape or cell viability. When microglia were cocultured with astrocytes, TNF-alpha release was decreased to undetectable levels. In contrast, in neuron-microglia cocultures, TNF-alpha levels were found to increase at lower concentrations of TMT (i.e., 10(-8) M). Moreover, at 10(-6) M of TMT, microglia displayed further morphological activation, as suggested by process retraction and by decrease in cell size. No morphological activation was observed in cultures of isolated microglial cells and in astrocyte-microglia cocultures. With regard to neurons, 10(-6) M of TMT induced about 30% of cell death, when applied to neuron-enriched cultures, whereas close to 100% of neuronal death was observed in neuron-microglia cocultures. In conclusion, whereas astrocytes may rather dampen the microglial activation by decreasing microglial TNF-alpha production, neuronal-microglial interactions lead to enhanced microglial activation. This microglial activation, in turn, exacerbates the neurotoxic effects of TMT. TNF-alpha may play a major role in such cell-cell communications.     

Manganese toxicity in serumless dissociated mesencephalic and striatal primary culture

Exposure to elevated levels of Manganese (Mn) can result in an irreversible brain disease characterized by extrapyramidal signs and symptoms resembling Parkinson's disease. To identify the neuronal target of Mn neurotoxicity, MnCl₂ was added to serumless dissociated mesencephalic-striatal cultures from rat embryo on day 4 in vitro. High affinity ³H-dopamine (DA) and ¹⁴C-GABA uptakes were assessed as specific functional markers of DAergic and GABAergic cell viability, respectively. After 60-min exposure, MnCl₂ at 0–200 μM did not modify the morphologic appearance of the cultures, specific DA and GABA uptakes, or the number of DA neurons visualized by immunocytochemical staining with tyrosine hydroxylase. In contrast, culture exposure to 20 μM MnCl₂ for 24 h selectively reduced specific GABA uptake without affecting specific DA uptake or the number of DA neurons. The exposure to a higher MnCl₂ concentration was accompanied by signs of general toxicity. Striatal GABA neurons seemed to be more susceptible to Mn toxicity than mesencephalic GABA neurons. Overall, our data suggest that striatal neurons rather than mesencephalic DA neurons may be the main target of Mn neurotoxicity.     

Thrombin-activated microglia contribute to death of dopaminergic neurons in rat mesencephalic cultures: dual roles of mitogen-activated protein kinase signaling pathways

This study evaluated the role of thrombin-activated microglia in the neurodegeneration of mesencephalic cultures. Immunocytochemical and biochemical evidence indicated that in co-cultures consisting of rat cortical microglia and mesencephalic neurons, thrombin led to nonselective loss of mesencephalic neurons. Accompanying neurodegeneration, microglial activation was obvious, evidenced by expression of tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, IL-1beta, inducible nitric oxide synthase (iNOS), and cyclooxygenase-2 (COX-2) and by increasing production of TNF-alpha and nitric oxide (NO). In mesencephalic neurons treated with conditioned media (CM) taken from thrombin-activated microglia, the number of dopaminergic neurons was significantly attenuated. The neurotoxicity of the CM was diminished when it was derived from microglia co-treated with thrombin and either an extracellular signal-regulated kinase 1/2 (ERK1/2) pathway inhibitor (PD98059) or a p38-mitogen-activated protein kinase (p38-MAPK) inhibitor (SB203580). Moreover, jun N-terminal kinase (JNK) and p38-MAPK were activated in mesencephalic neurons treated with CM of thrombin-activated microglia. Inhibition of JNK and p38-MAPK rescued the dopaminergic neurons. Collectively, these results indicate that thrombin-activated microglia induce neurodegeneration in cultured mesencephalic neurons and that the MAPKs actively participate in both microglial activation and neurodegeneration. The present data carefully suggest that microglial activation triggered by thrombin may be involved in the neuropathological processes of dopaminergic neuronal cell death that occur in Parkinson's disease.     

Neuroprotective effects of the anti-inflammatory compound triflusal on ischemia-like neurodegeneration in mouse hippocampal slice cultures occur independent of microglia

Microglial cells, known to play key roles in neuroinflammation, can be immunotoxically eliminated from hippocampal slice cultures by treatment with saporin coupled to the microglial receptor Mac1. Considering microglial cells as a target for anti-inflammatory treatment we studied the effects of microglial depletion on anti-inflammatory treatment of mouse hippocampal slice cultures subjected to ischemia-like neurodegeneration, induced by oxygen-glucose deprivation (OGD).Hippocampal slice cultures, derived from 7-day-old mice and grown for 2 weeks, were divided into 8 groups: (1) control cultures; (2) sham-OGD cultures; (3) OGD cultures; (4) OGD cultures treated with triflusal during OGD; (5) microglia-depleted control cultures; (6) microglia-depleted sham-OGD cultures; (7) microglia-depleted OGD cultures; and (8) microglia-depleted OGD cultures treated with triflusal during OGD. The resulting neurodegeneration was quantified by densitometric measurements of cellular uptake of propidium iodide (PI), with focus on the hippocampal CA1 subfield.Subjection of regular cultures to OGD for 30 min induced a significant increase in PI uptake in the CA1 pyramidal cell layer, compared to control cultures. The presence of 100 μM triflusal during OGD protected against OGD-induced neurodegeneration, and reduced the number of OGD-induced NFkB positive-cells correspondingly. Cultures treated with the Mac1-saporin complex for 7 days displayed an almost total loss of microglial cells. When subjected to OGD after microglial depletion, these cultures displayed a significant increase in OGD-induced PI uptake compared to non-depleted cultures. The presence of triflusal during OGD of these cultures reduced neurodegeneration of the irrespective absence of microglia. In accordance with that, the presence of triflusal during OGD significantly inhibited the increase in the number of reactive microglia and proliferative cells in the CA1 pyramidal and dentate granule cell layers.We conclude that immunotoxic microglia depletion significantly increases the susceptibility of CA1 pyramidal cells to neurodegeneration and that the anti-inflammatory drug triflusal still can exert its neuroprotective role following depletion of microglia.     

Aspirin Protects Dopaminergic Neurons Against Lipopolysaccharide-Induced Neurotoxicity in Primary Midbrain Cultures

Aspirin (ASA) is one of the most widely used nonsteroidal anti-inflammatory drugs. ASA has primarily been used to treat headaches, rheumatic pain, and inflammation, but its therapeutic effects have recently been demonstrated on a range of disorders, including those of the central nervous system. In this study, we investigated whether ASA is neuroprotective in inflammation-mediated neurodegenerative diseases. Pretreatment with ASA reduced the lipopolysaccharide (LPS)-induced degeneration of dopaminergic (DA) neurons in mesencephalic neuron–glia cultures in a dose-dependent manner. The neuroprotective effect of ASA was attributed to the inhibition of microglial activation because of its observed inhibitory effects on LPS-stimulated nitric oxide, tumor necrosis factor-α, and superoxide production by microglial cells. Moreover, ASA increased the production of the anti-inflammatory cytokines transforming growth factor beta-1 and interleukin-10 in neuron–glia cultures after stimulation with LPS. Mechanistic studies revealed that the neuroprotective effects of ASA were mediated through the inhibition of nicotinamide adenine dinucleotide phosphate oxidase (PHOX), a key enzyme for superoxide production in microglia. These results suggest that ASA protects DA neurodegeneration by inhibiting the microglial-mediated oxidative stress/inflammatory response and by regulating the production of anti-inflammatory cytokines.     

Chronic restraint stress triggers dopaminergic and noradrenergic neurodegeneration: Possible role of chronic stress in the onset of Parkinson’s disease

Parkinson’s disease (PD) is a neurodegenerative disease characterized by the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and, to a lesser extent, in the noradrenergic neurons of the locus coeruleus (LC). Most cases of PD are idiopathic and sporadic and are believed to be the result of both environmental and genetic factors. Here, to the best of our knowledge, we report the first evidence that chronic restraint stress (8 h/day, 5 days/week) substantially reduces nigral DA and LC noradrenergic neuronal cell numbers in rats. Loss of DA neurons in the SNpc was evident after 2 weeks of stress and progressed in a time-dependent manner, reaching up to 61% at 16 weeks. This reduction was accompanied by robust microglial activation and oxidative stress and was marked by nitrotyrosine in the SNpc and LC of the midbrain. These results indicate that chronic stress triggers DA and noradrenergic neurodegeneration by increasing oxidative stress, and that activated microglia in the substantia nigra and LC may play an important role in modulating the neurotoxic effects of oxidative stress. Taken together, these data suggest that exposure to chronic stress triggers DA and noradrenergic neurodegeneration, which is a cause of PD.     

Redox regulation of NF‐κB p50 and M1 polarization in microglia

Redox‐signaling is implicated in deleterious microglial activation underlying CNS disease, but how ROS program aberrant microglial function is unknown. Here, the oxidation of NF‐κB p50 to a free radical intermediate is identified as a marker of dysfunctional M1 (pro‐inflammatory) polarization in microglia. Microglia exposed to steady fluxes of H₂O₂ showed altered NF‐κB p50 protein–protein interactions, decreased NF‐κB p50 DNA binding, and augmented late‐stage TNFα expression, indicating that H₂O₂ impairs NF‐κB p50 function and prolongs amplified M1 activation. NF‐κB p50^?/? mice and cultures exhibited a disrupted M2 (alternative) response and impaired resolution of the M1 response. Persistent neuroinflammation continued 1 week after LPS (1 mg/kg, IP) administration in the NF‐κB p50^?/? mice. However, peripheral inflammation had already resolved in both strains of mice. Treatment with the spin‐trap DMPO mildly reduced LPS‐induced 22 h TNFα in the brain in NF‐κB p50^+/+ mice. Interestingly, DMPO failed to reduce and strongly augmented brain TNFα production in NF‐κB p50^?/? mice, implicating a fundamental role for NF‐κB p50 in the regulation of chronic neuroinflammation by free radicals. These data identify NF‐κB p50 as a key redox‐signaling mechanism regulating the M1/M2 balance in microglia, where loss of function leads to a CNS‐specific vulnerability to chronic inflammation. GLIA 2015;63:423–440     

铁选择性损伤多巴胺能神经元的神经免疫炎症机制

目的 在模拟人中脑黑质的多种原代细胞培养体系中,研究铁对多巴胺能神经元的损伤及其神经免疫炎症机制.方法 采用5、25和100μmol/L的Fecl2(Fe2+):(1)处理中脑原代神经元-小胶质细胞-星形胶质细胞混合培养体系,7 d后计数酪氨酸羟化酶(TH)(+)多巴胺能神经元及抗神经特异性核蛋白抗体(Neu-N)(+)的全部神经元的数量,观察神经元形态,研究Fe2+对多巴胺能神经元的选择性损伤;(2)同时处理中脑原代神经元-小胶质细胞-星形胶质细胞混合培养体系和神经元-星形胶质细胞培养体系,7 d后计数TH(+)多巴胺能神经元的数量,研究小胶质细胞在Fe2+选择性损伤多巴胺能神经元中的作用;(3)处理原代小胶质细胞培养体系,测量细胞外超氧化物(O2·-)及细胞内活性氧类物质(iROS),研究Fe2+激活小胶质细胞导致的功能变化;(4)处理中脑原代神经元-小胶质细胞-星形胶质细胞混合培养体系,7 d后计数抗补体受体-3(OX-42)(+)小胶质细胞的数量,观察细胞形态,研究Fe2+激活小胶质细胞导致的形态学变化.结果 (1)5、25和100μmol/L Fe2+处理组TH(+)多巴胺能神经元的数量分别为对照组的89%、70%和55%,其中25、100 μmol/L Fe2+处理组与对照组比较差异具有统计学意义(F=12.047,P＜0.01);多巴胺能神经元胞体皱缩,胞质淡染,神经突起数目减少;(2)5、25和100 μmol/L Fe2+处理组Neu-N(+)全部神经元的数量分别为对照组的100%、104%和101%,与对照组比较差异均无统计学意义;5、25和100μmol/L Fe2+处理组Neu-N(+)全部神经元的数量与TH(+)多巴胺能神经元数量的差值分别为11%、34%和46%,其中25和100 μmol/L Fe2+处理组两种神经元数量的差异具有统计学意义(分别为t=8.098,P＜0.05;t=11.218,P＜0.05);(3)在中脑原代神经元-小胶质细胞-星形胶质细胞混合培养体系和神经元-星形胶质细胞培养体系中,25μmoL/L Fe2+处理组TH(+)多巴胺能神经元的数量分别为对照组的70%和98%,差值为28%,差异具有统计学意义(t=8.061,P＜0.05);100 μmoL/LFe2+处理组TH(+)多巴胺能神经元的数量分别为对照组的55%和75%,差值为20%,差异具有统计学意义(t=9.025,P＜0.05);(4)5、25和100μmoL/L Fe2+处理组产生的细胞外O2·-分别为对照组的100%、127%和163%,其中100 μmol/L Fe2+处理组与对照组比较差异具有统计学意义(t=9.015,P＜0.005);5、25和100μmol/L Fe2+处理组产生的iROS分别为对照组的147%、172%和231%,其中25和100μmol/L Fe2+处理组与对照组比较差异具有统计学意义(F=3.091,P＜0.05);(5)5、25和100 μmol/L Fe2+处理组OX-42(+)小胶质细胞数量分别为对照组的183%、190%和240%,其中25和100μmol/L Fe2+处理组与对照组比较差异具有统计学意义(F=6.101,P＜0.01);小胶质细胞激活后胞体变大,形状不规则.结论 Fe2+激活小胶质细胞,使其形态发生显著变化,并产生大量神经毒性因子,选择性损伤多巴胺能神经元;为抑制铁过度激活小胶质细胞成为治疗PD的新策略提供依据.

Abstract：

Objective To investigate the role and neuroinflammatory mechanism of iron on dopamine ( DA) neurons in multiple primary midbrain cultures that mimic human substantia nigra pars compacta.Methods Ferrous chloride ( Fe2+ ) with the desired concentrations of 5,25 and 100 μmol/L was used to ( 1 ) treat primary midbrain neuron-microglia-astroglia cultures for 7 days and the numbers of DA neurons and total neurons were counted after tyrosine hydroxylase (TH) and neuron-specific neuclear protein neurons in 5,25 and 100 μmol/L Fe2 + -treated groups were 89%,70% and 55% of control group,and 25,100 μmol/L Fe2+ significantly decreased DA neuronal numbers compared with control group ( F = 12.047,P ＜0.01);DA neuronal bodies were shrunk and smaller,cytoplasmic stainings were reduced,neuronal dendrites were decreased;(2) The numbers of Neu-N ( + ) total neurons in 5,25 and 100 μmol/L Fe2+-treated groups were 100%,104% and 101% of control group and Fe2+ did not decrease DA neuronal numbers compared with control group (t =4.458,P ＞ 0.05 );5,25 and 100 μmol/L Fe2+-induced difference between total neurons and DA neurons were 11%,34% and 46%,and 25 and 100 (Amol/L Fe2+ produced significant difference(t =8.098,P ＜0.05;t = 11.218,P＜0.05);(3) In primary midbrain neuron-microglia-astroglia and neuron-astroglia cultures,the numbers of DA neurons in 25 μmol/L Fe+-treated group were 70% and 98% of control group,respectively.The difference between two groups was 28%,which was statistically significant (t =8.061,P＜0.05);The numbers of DA neurons in 100 μmol/L groups were 183%,190 % and 240% of control group,and 25 and 100 μmol/L Fe2 + significantly increased microglial numbers compared with control group ( F = 6.101,P ＜ 0.01 );dramatic changes of microglial morphology were indicated by the enlarged cell bodies and irregular shape.Conclusions Fe2 + provokes selective DA neuronal damage and microglia are the mediators of the neurotoxic effect,which may be due to microglial over-activation featured by the significant production of neurotoxic factors and morphological changes of microglia.This investigation cast a new light for PD therapy by inhibiting Fe2+ -induced neuroinflammation characterized by the microglial over-activation.     

β2 Adrenergic receptor activation induces microglial NADPH oxidase activation and dopaminergic neurotoxicity through an ERK‐dependent/protein kinase A‐independent pathway

Activation of the β2 adrenergic receptor (β2AR) on immune cells has been reported to possess anti‐inflammatory properties, however, the pro‐inflammatory properties of β2AR activation remain unclear. In this study, using rat primary mesencephalic neuron‐glia cultures, we report that salmeterol, a long‐acting β2AR agonist, selectively induces dopaminergic (DA) neurotoxicity through its ability to activate microglia. Salmeterol selectively increased the production of reactive oxygen species (ROS) by NADPH oxidase (PHOX), the major superoxide‐producing enzyme in microglia. A key role of PHOX in mediating salmeterol‐induced neurotoxicity was demonstrated by the inhibition of DA neurotoxicity in cultures pretreated with diphenylene‐iodonium (DPI), an inhibitor of PHOX activity. Mechanistic studies revealed the activation of microglia by salmeterol results in the selective phosphorylation of ERK, a signaling pathway required for the translocation of the PHOX cytosolic subunit p47^phox to the cell membrane. Furthermore, we found ERK inhibition, but not protein kinase A (PKA) inhibition, significantly abolished salmeterol‐induced superoxide production, p47^phox translocation, and its ability to mediate neurotoxicity. Together, these findings indicate that β2AR activation induces microglial PHOX activation and DA neurotoxicity through an ERK‐dependent/PKA‐independent pathway. © 2009 Wiley‐Liss, Inc.     

Lipopolysaccharide-Induced Microglia Activation Promotes the Survival of Midbrain Dopaminergic Neurons In Vitro

Microglia are the resident immune cells of the central nervous system (CNS) and respond to a variety of endogenous and exogenous stimuli in order to restore cell and tissue homeostasis. Lipopolysaccharide (LPS) is one of these exogenous stimuli, constitutes a major component of the outer membrane of Gram-negative bacteria, and binds to the microglial pattern recognition receptor Toll-like receptor 4 (TLR4). LPS-induced microglia activation is believed to promote neurodegeneration by release of neurotoxic factors such as interleukin-1β, tumor necrosis factor α, or nitric oxide. In the present study, we investigated whether the physical presence of microglia is required to promote neurotoxicity and whether microglia-derived factors are essential. Interestingly, we observed that dopaminergic (mDA) neuron survival was only affected in mixed neuron-glia cultures containing microglia but not in neuron-enriched cultures. Moreover, we clearly demonstrate that microglia-conditioned medium (MCM) after LPS treatment increased mDA neuron survival, process numbers as well as process length. The observed protective effects of MCM was rather caused by microglia-derived factors and only partially dependent on the increase in reactive astrocytes. These results indicate that LPS-induced microglia activation does not necessarily have detrimental effects on mDA neurons and further support the hypothesis that activated microglia support neuron survival by release of neurotrophic and neuroprotective factors.     

Neurons Induce the Activation of Microglial Cellsin Vitro

Although microglial cells are well known to become activated in the pathological brain, mechanisms underlying the microglial activation are not fully understood. In the present study, with an aim to elucidate whether neurons are involved in the microglial activation, we compared the morphology and the superoxide anion (O₂⁻)-generating activity of rat microglial cells in pure culture with those of cells cocultured with rat primary cortical neurons. Microglial cells in pure culture in serum-free Eagle's minimum essential medium on poly- -lysine-coated coverslips displayed ramified morphology and suppressed activity of O₂⁻generation. In contrast, microglial cells in neuron–microglia coculture under the same conditions as those for the pure culture displayed ameboid shape and upregulated activity of O₂⁻generation. Electron microscopic observation revealed that microglial cells in coculture were more abundant in Golgi apparatus and secretory granules than those in pure culture and that some of microglial cells in the vicinity of neurites exhibited membrane specialization reminiscent of a junctional apparatus with high electron density between a microglial soma and a neurite. Microglial cells in coculture tended to tie neurites in bundles by extending processes. Medium conditioned by neurons significantly enhanced O₂⁻generation by microglia, but microglial cells in contact with or in close apposition to cocultured neurons were much more intensely activated than those remote from the neurons. Furthermore, the membrane fraction of cortical neurons activated microglial cells, and this effect was abolished by treating the neuronal membrane with trypsin or neuraminidase. In conclusion, neuronal–microglial contact may be necessary to mediate microglial activation. The present findings suggest that the contact of microglia with damaged neurons in the brain is a plausible cause to activate microglia in the neuropathological pro cesses.     

Region-specific activation of microglial cells in the rat septal complex following fimbria-fornix transection

Studies of postlesional microglial activation may gain insight into microglia/neuronal interactions in processes of neurodegeneration. We compared the microglial response after axotomy of septohippocampal projection neurons with that seen after selective immunolesioning of cholinergic septohippocampal neurons with the immunotoxin 192 IgG-saporin. Using the microglial marker isolectin B₄ from Griffonia simplicifolia (GSA I-B₄), we found striking differences in the microglial response between these two lesion paradigms. Following axotomy of septohippocampal neurons by fimbria-fornix transection (ff-t), there was only a moderate and short-lasting microglial reaction in the medial septum (MS) in the early postlesion period. Prelabeling of septohippocampal neurons with Fluoro-Gold (FG) prior to axotomy revealed the survival of most neurons, and only very rarely were microglial cells observed that had phagocytosed FG-labeled debris. In the lateral septum (LS) containing the degenerating terminals of hippocamposeptal fibers transected by ff-t, a heavy reaction of lectin-labeled activated microglial cells associated with high phagocytotic activity was noticed. Unexpectedly, after a long survival time (6 months) following ff-t, we observed an increase in microglial GSA I-B₄ labeling in the MS. In contrast, an inverse pattern of the microglial response, i.e., a strong initial reaction in the MS and very little microglial activation in the LS, was observed after immunolesioning. Our results indicate that the microglial reaction in the MS following ff-t differs substantially from that seen in other models of axotomy. J. Comp. Neurol. 390:481–496, 1998. © 1998 Wiley-Liss, Inc.     

NADPH oxidase and aging drive microglial activation,oxidative stress,and dopaminergic neurodegeneration following systemic LPS administration

Parkinson's disease is characterized by a progressive degeneration of substantia nigra (SN) dopaminergic neurons with age. We previously found that a single systemic lipopolysaccharide (LPS, 5 mg/kg, i.p.) injection caused a slow progressive loss of tyrosine hydroxylase immunoreactive (TH+IR) neurons in SN associated with increasing motor dysfunction. In this study, we investigated the role of NADPH oxidase (NOX) in inflammation‐mediated SN neurotoxicity. A comparison of control (NOX2^+/+) mice with NOX subunit gp91^phox‐deficient (NOX2^?/?) mice 10 months after LPS administration (5 mg/kg, i.p.) resulted in a 39% (P < 0.01) loss of TH+IR neurons in NOX2^+/+ mice, whereas NOX2^?/? mice did not show a significant decrease. Microglia (Iba1+IR) showed morphological activation in NOX2^+/+ mice, but not in NOX2^?/? mice at 1 hr. Treatment of NOX2^+/+ mice with LPS resulted in a 12‐fold increase in NOX2 mRNA in midbrain and 5.5–6.5‐fold increases in NOX2 protein (+IR) in SN compared with the saline controls. Brain reactive oxygen species (ROS), determined using diphenyliodonium histochemistry, was increased by LPS in SN between 1 hr and 20 months. Diphenyliodonium (DPI), an NOX inhibitor, blocked LPS‐induced activation of microglia and production of ROS, TNFα, IL‐1β, and MCP‐1. Although LPS increased microglial activation and ROS at all ages studied, saline control NOX2^+/+ mice showed age‐related increases in microglial activation, NOX, and ROS levels at 12 and 22 months of age. Together, these results suggest that NOX contributes to persistent microglial activation, ROS production, and dopaminergic neurodegeneration that persist and continue to increase with age. © 147.     

Inhibition of UDP/P2Y6 purinergic signaling prevents phagocytosis of viable neurons by activated microglia in vitro and in vivo

Microglia activated through Toll‐like receptor (TLR)‐2 or ‐4 can cause neuronal death by phagocytosing otherwise‐viable neurons—a form of cell death called “phagoptosis.” UDP release from neurons has been shown to provoke microglial phagocytosis of neurons via microglial P2Y₆ receptors, but whether inhibition of this process affects neuronal survival is unknown. We tested here whether inhibition of P2Y₆ signaling could prevent neuronal death in inflammatory conditions, and whether UDP signaling can induce phagoptosis of stressed but viable neurons. We find that delayed neuronal loss and death in mixed neuronal/glial cultures induced by the TLR ligands lipopolysaccharide (LPS) or lipoteichoic acid was prevented by: apyrase (to degrade nucleotides), Reactive Blue 2 (to inhibit purinergic signaling), or MRS2578 (to specifically block P2Y₆ receptors). In each case, inflammatory activation of microglia was not affected, and the rescued neurons remained viable for at least 7 days. Blocking P2Y₆ receptors with MRS2578 also prevented phagoptosis of neurons induced by 250 nM amyloid beta 1–42, 5 μM peroxynitrite, or 50 μM 3‐morpholinosydnonimine (which releases reactive oxygen and nitrogen species). Furthermore, the P2Y₆ receptor agonist UDP by itself was sufficient to stimulate microglial phagocytosis and to induce rapid neuronal loss that was prevented by eliminating microglia or inhibiting phagocytosis. In vivo, injection of LPS into rat striatum induced microglial activation and delayed neuronal loss and blocking P2Y₆ receptors with MRS2578 prevented this neuronal loss. Thus, blocking UDP/P2Y₆ signaling is sufficient to prevent neuronal loss and death induced by a wide range of stimuli that activate microglial phagocytosis of neurons. GLIA 2014;62:1463–1475     

MAC1 mediates LPS-induced production of superoxide by microglia: the role of pattern recognition receptors in dopaminergic neurotoxicity

Microglia-derived superoxide is critical for the inflammation-induced selective loss of dopaminergic (DA) neurons, but the underlying mechanisms of microglial activation remain poorly defined. Using neuron-glia and microglia-enriched cultures from mice deficient in the MAC1 receptor (MAC1-/-), we demonstrate that lipopolysaccharide (LPS) treatment results in lower TNFalpha response, attenuated loss of DA neurons, and absence of extracellular superoxide production in MAC1-/- cultures. Microglia accumulated fluorescently labeled LPS in punctate compartments associated with the plasma membrane, intracellular vesicles, and the Golgi apparatus. Cytochalasin D (CD), an inhibitor of phagocytosis, blocked LPS internalization. However, microglia derived from Toll-like receptor 4 deficient mice and MAC1-/- mice failed to show a significant decrease in intracellular accumulation of labeled LPS, when compared with controls. Pretreatment with the scavenger receptor inhibitor, fucoidan, inhibited 79% of LPS accumulation in microglia without affecting superoxide, indicating that LPS internalization and superoxide production are mediated by separate phagocytosis receptors. Together, these data demonstrate that MAC1 is essential for LPS-induced superoxide from microglia, implicating MAC1 as a critical trigger of microglial-derived oxidative stress during inflammation-mediated neurodegeneration.