Reduced hippocampal insulin-degrading enzyme in late-onset Alzheimer's disease is associated with the apolipoprotein E-epsilon4 allele

Abstract

Abeta is the major component of amyloid plaques characterizing Alzheimer's disease (AD). Abeta accumulation can be affected by numerous factors including increased rates of production and/or impaired clearance. Insulin-degrading enzyme (IDE) has been implicated as a candidate enzyme responsible for the degradation and clearance of Abeta in the brain. We have previously shown that AD patients exhibit abnormalities in insulin metabolism that are associated with apoliprotein E (APOE) status. The possible association of IDE with AD, as well as the link between APOE status and insulin metabolism, led us to examine the expression of IDE in AD. We report that hippocampal IDE protein is reduced by approximately 50% in epsilon4+ AD patients compared to epsilon4- patients and controls. The allele-specific decrease of IDE in epsilon4+ AD patients is not associated with neuronal loss since neuron-specific enolase levels were comparable between the AD groups, regardless of APOE status. Hippocampal IDE mRNA levels were also reduced in AD patients with the epsilon4 allele compared to AD and normal subjects without the epsilon4 allele. These findings show that reduced IDE expression is associated with a significant risk factor for AD and suggest that IDE may interact with APOE status to affect Abeta metabolism.

Figure 1.

Quantitative Western blot analysis of hippocampal IDE (A) and NSE (B) protein expression. Representative immunoblots contain 100 μg (IDE) or 4 μg (NSE) of protein per lane from three normal and/or three Alzheimer’s (AD) subjects without (ε4−) and with (ε4+) the APOε4 allele. Graphs represent quantitative differences in expression of IDE protein in normal and AD subjects (A) and of NSE protein in AD subjects (B) with and without the ε4 allele. Each bar represents the mean +/− SEM. IDE protein is reduced in AD patients with an APOE-ε4 allele compared to patients without the ε4 allele (***, P = 0.0008), to normal adults with the ε4 allele (***, P = 0.0004), and to normal adults without the ε4 allele (**, P = 0.0011). No difference in NSE levels were observed for AD groups (P = 0.89).

Figure 2.

Pattern of IDE protein (A–D) and mRNA (E–F) expression in the hippocampus of a normal adult. A: Representative immunostained section of whole hippocampus (magnification, × 1) depicting high levels of IDE protein in the dentate granular layer (DGL), hilus, and CA-2/3, and low levels in the CA-1. No immunostaining is detected when the primary IDE antibody is omitted (B). Increased magnification (× 20) depicting high neuronal expression in the hilus (C) compared to the CA-1 (D). Representative film autoradiogram of in situ hybridization with the IDE antisense RNA probe (E) demonstrates that the pattern of IDE mRNA expression is similar to that of protein. No hybridization signal is observed with the IDE sense RNA probe (F).

Figure 3.

Quantitative in situ hybridization analysis of hippocampal IDE mRNA expression in normal and LOAD subjects. A: Representative film autoradiograms depicting the differences in expression of hippocampal IDE mRNA in normal and Alzheimer’s (AD) subjects with (ε4+) and without (ε4−) the APOE-ε4 allele. B: Graphic representation of the quantitative differences in IDE mRNA expression. Each bar represents the mean +/− SEM. IDE mRNA is reduced in LOAD subjects with the ε4 allele (gray bar) compared to LOAD and normal subjects without the ε4 allele (white bar) in the dentate granular layer (DGL), hilus, and CA-2/3. In addition, IDE mRNA is decreased in the hilus of normal ε4+ subjects compared to normal ε4− subjects. (*, P < 0.05, **, P < 0.01, ***, P < 0.001, ^†, P = 0.082).