Microglia enhance β‐amyloid peptide‐induced toxicity in cortical and mesencephalic neurons by producing reactive oxygen species

Abstract

The purpose of this study was to assess and compare the toxicity of b-amyloid (Ab) on primary cortical and mesencephalic neurons cultured with and without microglia in order to determine the mechanism underlying microglia-mediated Ab-induced neurotoxicity. Incubation of cortical or mesencephalic neuron-enriched and mixed neuron-glia cultures with Ab(1-42) over the concentration range 0.1-6.0 lM caused concentration-dependent neurotoxicity. High concentrations of Ab (6.0 lM for cortex and 1.5-2.0 lM for mesencephalon) directly injured neurons in neuron-enriched cultures. In contrast, lower concentrations of Ab (1.0-3.0 lM for cortex and 0.25-1.0 lM for mesencephalon) caused significant neurotoxicity in mixed neuron-glia cultures, but not in neuron-enriched cultures. Several lines of evidence indicated that microglia mediated the potentiated neurotoxicity of Ab, including the observations that low concentrations of Ab activated microglia morphologically in neuron-glia cultures and that addition of microglia to cortical neuron-glia cultures enhanced Ab-induced neurotoxicity. To search for the mechanism underlying the microglia-mediated effects, several proinflammatory factors were examined in neuron-glia cultures. Low doses of Ab significantly increased the production of superoxide anions, but not of tumor necrosis factor-a, interleukin-1b or nitric oxide. Catalase and superoxide dismutase significantly protected neurons from Ab toxicity in the presence of microglia. Inhibition of NADPH oxidase activity by diphenyleneiodonium also prevented Ab-induced neurotoxicity in neuron-glia mixed cultures. The role of NADPH oxidase-generated superoxide in mediating Ab-induced neurotoxicity was further substantiated by a study which showed that Ab caused less of a decrease in dopamine uptake in mesencephalic neuron-glia cultures from NADPH oxidase-deficient mutant mice than in that from wild-type controls. This study demonstrates that one of the mechanisms by which microglia can enhance the neurotoxicity of Ab is via the production of reactive oxygen species.

β-Amyloid Peptides Induce Mitochondrial Dysfunction and Oxidative Stress in Astrocytes and Death of Neurons through Activation of NADPH Oxidase

“…Nevertheless, the dramatic protection afforded by DPI, at concentrations that do not affect other potential targets (mostly mitochondrial dehydrogenases), and by apocynin, an agent that binds at a separate site on the target enzyme, strongly suggests that this enzyme represents a major source of radical species in response to ␤A and that the selective expression of the enzyme might account in some measure for the selective effects of ␤A on astrocyte, and not neuronal, mitochondria. The importance of the NADPH oxidase in ␤A toxicity has already been suggested but with reference to the expression of the enzyme in microglia (Bianca et al, 1999;Qin et al, 2002). In the present study, however, astrocytic ROS generation seems to play a critical role in ␤A neurotoxicity, suggesting that the astrocyte may have been overlooked as a source of ROS.…”

NADPH Oxidase Mediates Lipopolysaccharide-induced Neurotoxicity and Proinflammatory Gene Expression in Activated Microglia

“…These results not only support the role of microglia in LPS-induced neurodegeneration, but they also emphasize the critical role of microglia in other models of dopaminergic neurodegeneration and Parkinson's disease. Specifically, NADPH oxidase has been implicated in the neurotoxic mechanisms of several other dopaminergic neurotoxic compounds such as rotenone (41), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (42), and ␤-amyloid (17). Further, increased expression of NADPH oxidase has been reported in the post-mortem brains of Parkinson's disease patients (43).…”

“…First, microglia respond to toxic agents, such as the bacterial endotoxin, lipopolysaccharide (LPS), human immunodeficiency virus, type 1 coat protein gp120, and ␤-amyloid peptides, and release ROS, which in turn induce neuronal degeneration (6,(15)(16)(17). Specifically, both in vitro and in vivo studies support the possibility that microgliamediated neurotoxicity is influenced by the release of superoxide and other ROS (3,9,17,18). Alternatively, there is increasing evidence that ROS can also function as second messengers to regulate several downstream signaling molecules, including protein kinase C, mitogen-activated protein kinase, and NF-B (19 -21).…”

Neuroprotective properties of the innate immune system and bone marrow stem cells in Alzheimer's disease

“…75,76 As discussed earlier, this has been linked to cytokine production and neuronal death in cell cultures. [76][77][78][79][80] There are multiple b-amyloid isoforms present in the CNS, though it seems that neurotoxic properties observed in vitro are mostly attributed to the bAmyloid 1-42 isoform. 78,80 Some indicate that microglial activation is induced by aggregated b-amyloid, 81 whereas others report that diffuse forms may also be responsible for the immune response seen in this disease.…”

“…[76][77][78][79][80] There are multiple b-amyloid isoforms present in the CNS, though it seems that neurotoxic properties observed in vitro are mostly attributed to the bAmyloid 1-42 isoform. 78,80 Some indicate that microglial activation is induced by aggregated b-amyloid, 81 whereas others report that diffuse forms may also be responsible for the immune response seen in this disease. 82,83 Regardless, since amyloid plaques are predominantly composed of b-amyloid 1-42 , 84 this theory to explain neuronal loss in AD patients has gained much support in the field.…”