Clusterin levels are increased in Alzheimer's disease and influence the regional distribution of Aβ

Abstract

Clusterin, also known as apoJ, is a lipoprotein abundantly expressed within the CNS. It regulates Aβ fibril formation and toxicity and facilitates amyloid-β (Aβ) transport across the blood-brain barrier. Genome-wide association studies have shown variations in the clusterin gene (CLU) to influence the risk of developing sporadic Alzheimer's disease (AD). To explore whether clusterin modulates the regional deposition of Aβ, we measured levels of soluble (NP40-extracted) and insoluble (guanidine-HCl-extracted) clusterin, Aβ40 and Aβ42 by sandwich ELISA in brain regions with a predilection for amyloid pathology-mid-frontal cortex (MF), cingulate cortex (CC), parahippocampal cortex (PH), and regions with little or no pathology-thalamus (TH) and white matter (WM). Clusterin level was highest in regions with plaque pathology (MF, CC, PH and PC), approximately mirroring the regional distribution of Aβ. It was significantly higher in AD than controls, and correlated positively with Aβ42 and insoluble Aβ40. Soluble clusterin level rose significantly with severity of cerebral amyloid angiopathy, and in MF and PC regions was highest in APOE ɛ4 homozygotes. In the TH and WM (areas with little amyloid pathology) clusterin was unaltered in AD and did not correlate with Aβ level. There was a significant positive correlation between the concentration of clusterin and the regional levels of insoluble Aβ42; however, the molar ratio of clusterin : Aβ42 declined with insoluble Aβ42 level in a region-dependent manner, being lowest in regions with predilection for Aβ plaque pathology. Under physiological conditions, clusterin reduces aggregation and promotes clearance of Aβ. Our findings indicate that in AD, clusterin increases, particularly in regions with most abundant Aβ, but because the increase does not match the rising level of Aβ42, the molar ratio of clusterin : Aβ42 in those regions falls, probably contributing to Aβ deposition within the tissue.

Keywords: Alzheimer's disease; amyloid-β; amyloid-β clearance; apoJ; cerebral amyloid angiopathy; clusterin; plaque.

Figure 1

Bar charts showing regional levels of Aβ level in control (black bars) and AD brains (red bars), in MF, CC, PH, medial parietal cortex (PC), thalamus (TH) and parietal WM. Bars indicate the mean and SEM. *P < 0.05 **P < 0.01 ***P < 0.001 ****P < 0.0001. Lines indicate significant differences between regions, in the controls (black lines) and AD groups (red lines); the thickness of the line indicates the significance of the difference between the two regions, ranging from P < 0.01 to P < 0.0001.

Figure 2

Bar chart showing regional levels of clusterin level in control (black bars) and AD brains (red bars), in the soluble and insoluble brain tissue fractions. Bars indicate the mean and SEM. *P < 0.05 **P < 0.01 ***P < 0.001. Lines indicate significant differences between regions, in the controls (black lines) and AD groups (red lines); the thickness of the solid lines indicates the significance of the difference between the two regions, ranging from P < 0.01 to P < 0.0001. The interrupted horizontal lines indicate differences significant at the P < 0.5 level.

Figure 3

Regional association between clusterin and insoluble Aβ42 levels. The concentration of clusterin in soluble (A, B) and insoluble (C, D) brain tissue fractions was plotted against insoluble Aβ42 level in each region in controls and AD cases. The solid circles and thin bars indicate the mean values and SEM for clusterin (horizontal bars) and Aβ42 (vertical bars). The thick solid and dotted lines indicate the best‐fit linear regression and 95% confidence intervals.

Figure 4

The ratio of clusterin : insoluble Aβ42 was lowest in regions with a predilection for Aβ42 deposition. The solid circles and bars indicate the mean values and SEM.

Figure 5

Bar charts showing clusterin level in relation to APOE genotype (A–C) and severity of CAA. Bars indicate the mean and SEM. *P < 0.05 **P < 0.01. Clusterin level was highest in APOE ɛ4 homozygotes in MF and PC and increased with severity of CAA.