Altered clearance of beta-amyloid from the cerebrospinal fluid following subchronic lead exposure in rats: Roles of RAGE and LRP1 in the choroid plexus

Abstract

Formation of amyloid plaques is the hallmark of Alzheimer's disease. Our early studies show that lead (Pb) exposure in PDAPP transgenic mice increases β-amyloid (Aβ) levels in the cerebrospinal fluid (CSF) and hippocampus, leading to the formation of amyloid plaques in mouse brain. Aβ in the CSF is regulated by the blood-CSF barrier (BCB) in the choroid plexus. However, the questions as to whether and how Pb exposure affected the influx and efflux of Aβ in BCB remained unknown. This study was conducted to investigate whether Pb exposure altered the Aβ efflux in the choroid plexus from the CSF to blood, and how Pb may affect the expression and subcellular translocation of two major Aβ transporters, i.e., the receptor for advanced glycation end-products (RAGE) and the low density lipoprotein receptor protein-1 (LRP1) in the choroid plexus. Sprague-Dawley rats received daily oral gavage at doses of 0, 14 (low-dose), and 27 (high-dose) mg Pb/kg as Pb acetate, 5 d/wk, for 4 or 8 wks. At the end of Pb exposure, a solution containing Aβ40 (2.5 μg/mL) was infused to rat brain via a cannulated internal carotid artery. Subchronic Pb exposure at both dose levels significantly increased Aβ levels in the CSF and choroid plexus (p < 0.05) by ELISA. Confocal data showed that 4-wk Pb exposures prompted subcellular translocation of RAGE from the choroidal cytoplasm toward apical microvilli. Furthermore, it increased the RAGE expression in the choroid plexus by 34.1 % and 25.1 % over the controls (p < 0.05) in the low- and high- dose groups, respectfully. Subchronic Pb exposure did not significantly affect the expression of LRP1; yet the high-dose group showed LRP1 concentrated along the basal lamina. The data from the ventriculo-cisternal perfusion revealed a significantly decreased efflux of Aβ40 from the CSF to blood via the blood-CSF barrier. Incubation of freshly dissected plexus tissues with Pb in artificial CSF supported a Pb effect on increased RAGE expression. Taken together, these data suggest that Pb accumulation in the choroid plexus after subchronic exposure reduces the clearance of Aβ from the CSF to blood by the choroid plexus, which, in turn, leads to an increase of Aβ in the CSF. Interaction of Pb with RAGE and LRP1 in choroidal epithelial cells may contribute to the altered Aβ transport by the blood-CSF barrier in brain ventricles.