The recent addition of a manganese (Mn)-containing antiknock compound methylcyclo-pentadienyl Mn tricarbonyl (MMT) to the US gasoline supply has raised a great concern about the health risks associated with a potential increase in the environmental levels of Mn. Both environmental and occupational exposures to (Mn) result in neurodegenerative symptoms resembling Parkinson's disease. However, the mechanisms underlying Mn neurotoxicity remain unknown. Our recent results show that in vivo and in vitro exposures to Mn alter both systemic and subcellular Fe status. While the former facilitates influx of Fe from the blood circulation to the cerebral spinal fluid (CSF), the latter promotes cellular Fe overload. We also found that accumulation of Mn in the choroid plexus, a tissue where blood-CSF barrier resides, enhances the density of transferrin receptor (TfR) mRNA, which has 3' binding IRE (iron responsive element) loops receptive to [4Fe-4S] cluster-containing iron regulatory protein-1 (IRP-1, or cytosolic aconitase). These findings have led us to propose that the mechanism by which Mn causes abnormal Fe metabolism is likely via its interaction with IRP-1 and the subsequent overexpression of TfR. The events, in turn, expedite Fe transport at the brain barrier systems and aggravate Fe accumulation in neuronal cells. Thus, we hypothesize in this proposal that accumulation of Mn in the choroid plexus alters Fe regulatory mechanisms in the blood-CSF barrier and thereby disturbs Fe homeostasis in the CSF, which may contribute to Mn-induced neurodegenerative Parkinsonism. Our research goals are to better understand the mechanism of Mn-induced Parkinsonism and in so doing identify and prevent environmental causes of neurodegenerative diseases.
Our specific aims are (1) to test the working hypothesis that Mn exposure alters the Fe regulatory mechanism in the choroid plexus, leading to a distorted Fe status in the CSF. We will define the dose and time response relationship of Mn exposure and Fe in CSF, blood, and choroid plexus, examine the activity of IRP-1, and determine the expression of TfR in blood-CSF barrier and selected brain areas; (2) to test the working hypothesis that Mn-facilitated transport of Fe at the blood-CSF barrier is directed toward the CSF and quantitatively is more significant than transport by the blood-brain barrier. We will use in vitro transport models to determine the direction and magnitude of Fe fluxes at two major brain barriers and to investigate if blocking of cellular trafficking of TfR antagonizes Mn-augmented unidirectional transport of Fe; and (3) to test the working hypothesis that alteration by Mn of cellular Fe regulation takes place at the level of mRNA expression, but not at the level of transcriptional modulation of genomic DNA. We will deterrnine the effect of Mn on RNA binding capability of IRP-1 and pertinent expression of TfR, examine if Mn inhibits degradation of TfR mRNA, and study cellular ferritin status in both barriers. In addition, we will study the effect of Mn on the RNA binding capability of a newly discovered IRP which regulates the synthesis of an [Fe-S] subunit of Complex-I.
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