The nigro-striatal dopamine system in the basal ganglia is highly sensitive to damage from environmental neurotoxins. Exposure to elevated levels of the essential element manganese (Mn) causes neuronal injury to this brain region, as well as cortical and subcortical structures. The results of this neurotoxicity represent a continuum of neurological effects ranging from cognitive and behavioral impairment in children exposed to Mn in drinking water, to Mn-induced parkinsonism (manganism) from high-dose occupational exposure in adults. However, it is not clear how early life exposures to Mn might increase susceptibility to other neurotoxic challenges throughout life. Pesticides such as rotenone that affect mitochondrial function are amongst the environmental neurotoxins thought to amplify the effects of heavy metals such as Mn to increase risk for neurodegenerative disease. The capacity of Mn to sensitize neural tissue to damage from pesticide exposure may involve persistent inflammatory changes in glial cells. Mn- induced expression of neuroinflammatory genes in glial cells is regulated by the transcription factor, Nuclear Factor Kappa B (NF-?B), which promotes neuronal injury. However, the signaling mechanisms between microglia and astrocytes that regulate this damaging glial phenotype are not well understood. Lack of this information hinders scientific and medical progress in understanding key signaling pathways that may render individuals more susceptible to neurological disease following combined exposures to environmental neurotoxins, including Mn and pesticides. To address this question, we postulate that Mn exposure during development stimulates NF-?B-dependent inflammatory signaling between microglia and astrocytes, resulting in persistent glial activation that enhances susceptibility to neurotoxic injury during aging. This hypothesis will tested in three Specific Aims that will 1) Determine how manganese exposure during juvenile development promotes inflammatory activation of glial cells and modulates the effects of rotenone on neuronal injury during aging, 2) Identify critical inflammatory signaling pathways in microglia that modulate the effects of manganese and rotenone on neurological injury, and 3) Characterize the intercellular signaling factors between microglia and astrocytes that mediate neuronal injury during exposure to manganese and rotenone. To accomplish these Specific Aims, we will use unique microglia-specific NF-?B knockout mice generated in our laboratory in a `two-hit' model to determine how juvenile exposure to Mn alters glial activation and susceptibility to neurotoxic injury during aging following exposure to the environmental pesticide, rotenone, a systemic mitochondrial complex I inhibitor. It is our expectation that use of this powerful transgenic model will enable the determination of specific molecular signaling events underlying NF-?B-dependent activation of neuroinflammatory genes in glial cells in response to developmental exposure to Mn that may increase susceptibility to neurotoxic insults during aging.
Statement of Relevance to Public Health Children in the US and around the world are exposed to excessive levels of heavy metals such as manganese (Mn) and are also increasingly subjected to the risk of co-exposure to neurotoxic pesticides. Research in the Tjalkens laboratory has examined how developmental exposure to Mn can stimulate a persistent neuroinflammatory phenotype leading to neuronal injury and neurological dysfunction during development. The current proposal extends this work in an important new direction by investigating how exposure to Mn during juvenile development may sensitize glial cells to respond to later exposure to neurotoxic pesiticides during adulthood with a more robust inflammatory response that damages neurons. Understanding such mechanisms would add significantly to our knowledge of gene- environment interactions that could predispose susceptible individuals to neurological diseases.
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