Involvement of insulin-degrading enzyme in the clearance of beta-amyloid at the blood-CSF barrier: Consequences of lead exposure.
BACKGROUND
Alzheimer's disease (AD) is characterized by the deposition of beta-amyloid (Abeta) peptides in the brain extracellular matrix, resulting in pathological changes and neurobehavioral deficits. Previous work from this laboratory demonstrated that the choroid plexus (CP) possesses the capacity to remove Abeta from the cerebrospinal fluid (CSF), and exposure to lead (Pb) compromises this function. Since metalloendopeptidase insulin-degrading enzyme (IDE), has been implicated in the metabolism of Abeta, we sought to investigate whether accumulation of Abeta following Pb exposure was due to the effect of Pb on IDE.


METHODS
Rats were injected with a single dose of Pb acetate or an equivalent concentration of Na-acetate; CP tissues were processed to detect the location of IDE by immunohistochemistry. For in vitro studies, choroidal epithelial Z310 cells were treated with Pb for 24 h in the presence or absence of a known IDE inhibitor, N-ethylmaleimide (NEM) to assess IDE enzymatic activity and subsequent metabolic clearance of Abeta. Additionally, the expression of IDE mRNA and protein were determined using real time PCR and western blots respectively.


RESULTS
Immunohistochemistry and confocal imaging revealed the presence of IDE towards the apical surface of the CP tissue with no visible alteration in either its intensity or location following Pb exposure. There was no significant difference in the expressions of either IDE mRNA or protein following Pb exposure compared to controls either in CP tissues or in Z310 cells. However, our findings revealed a significant decrease in the IDE activity following Pb exposure; this inhibition was similar to that seen in the cells treated with NEM alone. Interestingly, treatment with Pb or NEM alone significantly increased the levels of intracellular Abeta, and a greater accumulation of Abeta was seen when the cells were exposed to a combination of both.


CONCLUSIONS
These data suggest that Pb exposure inhibits IDE activity but does not affect its expression in the CP. This, in turn, leads to a disrupted metabolism of Abeta resulting in its accumulation at the blood-CSF barrier.
Behl
Mamta
M
School of Health Sciences, Purdue University, West Lafayette, IN 47907, USA. wzheng@purdue.edu.
Zhang
Yanshu
Y
Zheng
Wei
W