Manganese potentiates nuclear factor‐κB‐dependent expression of nitric oxide synthase 2 in astrocytes by activating soluble guanylate cyclase and extracellular responsive kinase signaling pathways

Abstract

Inflammatory activation of glial cells is associated with neuronal injury in several degenerative movement disorders of the basal ganglia, including manganese neurotoxicity. Manganese (Mn) potentiates the effects of inflammatory cytokines on nuclear factor-κB (NF-κB)-dependent expression of nitric oxide synthase 2 (NOS2) in astrocytes, but the signaling mechanisms underlying this effect have remained elusive. It was postulated in the present studies that direct stimulation of cGMP synthesis and activation of mitogen-activated protein (MAP) kinase signaling pathways underlies the capacity of Mn to augment NF-κB-dependent gene expression in astrocytes. Exposure of primary cortical astrocytes to a low concentration of Mn (10 µM) potentiated expression of NOS2 mRNA and protein along with production of NO in response to interferon-γ (IFNγ) and tumor necrosis factor-α (TNFα), which was prevented by overexpression of dominant negative IκBα. Mn also potentiated IFNγ- and TNFα-induced phosphorylation of extracellular response kinase (ERK), p38, and JNK, as well as cytokine-induced activation of a fluorescent NF-κB reporter construct in transgenic astrocytes. Activation of ERK preceded that of NF-κB and was required for maximal activation of NO synthesis. Independently of IFNγ/TNFα, Mn-stimulated synthesis of cGMP in astrocytes and inhibition of soluble guanylate cyclase (sGC) abolished the potentiating effect of Mn on MAP kinase phosphorylation, NF-κB activation, and production of NO. These data indicate that near-physiological concentrations of Mn potentiate cytokine-induced expression of NOS2 and production of NO in astrocytes via activation of sGC, which promotes ERK-dependent enhancement of NF-κB signaling.

Microglia amplify inflammatory activation of astrocytes in manganese neurotoxicity

“…In the absence of microglia, Mn treatment alone was insufficient to increase expression of inflammatory genes in astrocytes, except for Nos2 , even at the highest dose examined (100 μM). These results may help to explain why previous studies in our laboratory and others that examined astrocyte inflammatory responses to Mn reported that cytokines, presumably from microglia, were required to activate astrocytes in the presence of low micromolar concentrations of Mn [7, 64]. The results of the present study also corroborate earlier findings that microglia were required for Mn-induced neuronal death in mixed glial-neuronal cultures [55].…”

Improvement of Cerebellum Redox States and Cholinergic Functions Contribute to the Beneficial Effects of Silymarin Against Manganese-Induced Neurotoxicity

“…However, the direct involvement of ACHE and iNOS associated with cerebellum impairments has not been elucidated in manganese induced neurotoxicity. Recently, interesting evidences suggest that manganese neurotoxicity involves activation of microglia BV2 [11] and/or astrocytes [12] with an increase in nitric oxide (NO) production, leading to the damage of the adjacent neurons. These high levels of NO may make cerebellum more susceptible to oxidative and nitrosative stress and induces the formation of reactive nitrogen species (RNS) [13].…”

Manganese and Neuroinflammation

“…Other studies have now built upon these initial findings, revealing that Mn can exacerbate LPS and cytokine-induced activation of both microglia and astrocytes, resulting in increased levels of TNFa, IL-1b, ROS, and NOS2 expression. [111][112][113][114][115] Increased levels of the aforementioned inflammatory genes has also been measured in both rodent 29,116 and non-human primate 117 studies with deletion or inhibition of these pathways showing neuroprotection. 18,116,118 To decipher the complex signaling mechanisms likely to influence development of a neuroinflammatory phenotype in Mn neurotoxicity, primary astrocytes and microglia can be isolated from neonatal mouse and the relative contributions of each cell type defined by using immunopurification-based methods to establish pure astroglial and microglial cultures from mixed isolates (Figure 12.1).…”