Ribosome dysfunction is an early event in Alzheimer's disease

Abstract

Alzheimer's disease (AD) is a progressive and devastating disorder that is often preceded by mild cognitive impairment (MCI). In the present study, we report that in multiple cortical areas of MCI and AD subjects, there is a significant impairment in ribosome function that is not observed in the cerebellum of the same subjects. The impairment in ribosome function is associated with a decreased rate and capacity for protein synthesis, decreased ribosomal RNA and tRNA levels, and increased RNA oxidation. No alteration in the level of initiation factors was observed in the brain regions exhibiting impairments in protein synthesis. Together, these data indicate for the first time that impairments in protein synthesis may be one of the earliest neurochemical alterations in AD and directly demonstrate that the polyribosome complex is adversely affected early in the development of AD. These data have important implications for AD studies involving proteomics and studies analyzing proteolysis in AD, indicate that oxidative damage may contribute to decreased protein synthesis, and suggest a role for alterations in protein synthesis as a novel contributor to the onset and development of AD.

Figure 1.

Ribosome function is altered in MCI and AD. A, The polyribosome content was measured in the SMTG, IP, and cerebellum. Analysis was conducted using 1 mg of brain tissue from each brain region in control, MCI, and AD subjects. B, The rate of protein synthesis (specific activity) was measured in each of the brain regions used for polysome content, using 100 μg of polysome. The rates of protein synthesis were calculated as the number of counts in precipitate per minute and then expressed as percentage control. C, The total protein synthesis capability for 1 mg of brain tissue (polyribosome-specific activity × amount of polyribosome in 1 mg of tissue) was calculated for each brain region. The data are presented as the mean and SEM. ^*p < 0.03 compared with control; ^**p < 0.05 compared with either control or MCI. CEREB, Cerebellum.

Figure 2.

Alterations in rRNA and tRNA molecules, and elevated RNA oxidation, is observed in MCI and AD. A, B, The levels of individual rRNA species (45S, 35S, 28S, 18S, 5.8S, 5S, and pre-5S) were analyzed by reverse transcription-PCR in the cerebellum (A) and inferior parietal lobule (B). Analysis was conducted in control, MCI, and AD subjects. The levels of individual tRNA species were also calculated in the cerebellum (C) and inferior parietal lobule (D) of these same subjects. The expression of several initiation factors and their regulators (FRAP, phosphorylated p70S6 kinase, PKR, and eIF2) were analyzed in the cerebellum (E) and inferior parietal lobule (F) of these same subjects. G, An antibody against 8-OHG was used to detect oxidized RNA in total RNA pool from the cerebellum or inferior parietal lobule of control, MCI, and AD subjects. The inserted photo is a representative result for control (Cont), MCI, and early AD (from left to right). The dark arrow indicates oxidized RNA at the 28S rRNA position, with the white arrow representing the 18S rRNA bands in the gel before transfer. Error bars represent SEM. ^*p < 0.01 compared with control.