Impaired hepatic amyloid-beta degradation in Alzheimer's disease

Abstract

Extensive research strongly suggests that amyloid beta (Aβ) aggregates in the brain have a central role in Alzheimer's disease (AD) pathogenesis. Pathological Aβ deposition is likely due to an altered balance between overproduction and elimination. Rodent studies have suggested that the liver has a major role in Aβ degradation. It is possible alterations of liver function could affect brain Aβ levels through changes in blood Aβ concentration. In this study, we hypothesized hepatic Aβ degradation to be impaired in AD subjects. To test our hypothesis, an Aβ degradation assay was developed using synthetic fluorescein-labeled Aβ40 and Aβ42 spiked into human liver homogenates. Aβ degradation rates were lower in AD-derived homogenates as compared with those from non-demented (ND) control subjects, even after accounting for such covariates as age, sex, and APOE genotype. The protein expression of potential Aβ-degrading enzymes were also examined. Neprilysin levels were not different in AD liver samples, while cathepsin D and insulin-degrading enzyme were significantly altered in AD subjects. The results support the possibility that impaired hepatic Aβ degradation could be a factor contributing to increased brain Aβ accumulation and AD.

Fig 1. Representative western blot showing faster Aβ degradation in the liver of non-demented controls subjects when compared to the AD group.

Lyophilized fluorescein-labeled Aβ40 (A) or Aβ42 (B) peptides were added to liver homogenates to quantify its degradation. Sucrose lysis buffer containing complete protease inhibitor cocktail was added to stop the reaction and Western blot performed to visualized Aβ degradation (molecular weight ~7kDA).

Fig 2. ELISA measurements of Aβ40 and Aβ42 degradation in liver homogenates from non-demented controls (NDC) and Alzheimer’s disease (AD) subjects.

Homogenates from NDC degraded Aβ faster than AD subjects. Aβ40 degraded slower (A) than Aβ42 (B) and even though both peptides degraded faster in the NDC group.

Fig 3. Protein expression of proteolyic degradation enzymes, IDE, CatD and neprilysin in the liver of non-demented controls (NDC) and Alzheimer’s disease (AD) subjects.

Liver protein expression of the proteolytic degradation enzyme insulin degrading enzyme (IDE) was lower in AD subjects (A); while cathepsin (CatD) levels (B) were higher. Neprilysin was not statistically significantly between the two groups (C). GAPDH was used as a total protein loading control.