Increased expression of cholesterol transporter ABCA1 is highly correlated with severity of dementia in AD hippocampus

Abstract

To gain insight into ATP-binding cassette transporter A1 (ABCA1) function and its potential role in AD pathology, we analyzed the expression of the cholesterol transporter ABCA1 in postmortem hippocampus from persons at different stages of dementia and AD associated neuropathology relative to cognitively intact normal donors by quantitative polymerase chain reaction (qPCR) and Western blot. In this study clinical dementia rating (CDR) scores were used as a measure of dementia severity, whereas, Braak neuropathological staging and neuritic plaque density were used as an index of the neuropathological progression of AD. Correlation analysis showed that ABCA1 mRNA expression was significantly elevated at the earliest recognizable stage of dementia compared to persons with intact cognition. ABCA1 mRNA was also positively correlated with Braak neuropathological stages and neuritic plaque density counts. Additionally, ABCA1 mRNA levels showed robust correlation with dementia severity even after controlling for the confounding contribution of accompanying neuropathological parameters to ABCA1 mRNA expression. Western blot analyses showed that the differential expression observed at the transcriptional level is also reflected at the protein level. Thus, our study provides transcriptional and translational evidence that the expression of ABCA1, a key modulator of cholesterol transport across the plasma membrane, is dysregulated in the AD brain and that this dysregulation is associated with increasing severity of AD, whether measured functionally as dementia severity or neuropathologically as increased neuritic plaque and neurofibrillary tangle density.

Figure 1

Normalized ABCA1 mRNA expression plotted against CDR scores, Braak neuropathological stages and plaque density groups. A, ANCOVA was used to determine ABCA1 gene expression in different CDR groups. Age and RIN were used as covariates. CDR groups are significantly different relative to control group (F_5,67 = 7.16, p < 0.0001). B, ABCA1 gene expression plotted against different Braak neuropathological stages also revealed significant differences between different stages (F_6,66 = 3.28, p < 0.01). C, ABCA1 gene expression plotted against plaque density groups revealed significant differences between different groups (F_3,69 = 3.42, p < 0.05). Average values ± SEM are shown. * = p < 0.05, ** = p < 0.01, *** = p < 0.001 and **** = p < 0.0001.

Figure 2

Western blot analysis of ABCA1 in the hippocampus of cognitively intact controls and subjects with varying severity of dementia and NFT pathology. Representative immunoblot of ABCA1 protein expression is shown. Total tissue homogenates were separated by reducing SDS-PAGE and probed with mouse anti-ABCA1 and mouse anti-VCP antibodies. Second row shows VCP signal detected by Odyssey IR imaging system. The positions of marker proteins are indicated. Tissue lysate from each subject and pooled tissue lysate (last 3 lanes) were loaded in triplicate.

Figure 3

Association between mRNA levels and protein levels of ABCA1. Increase in ABCA1 mRNA was highly correlated (r = 0.80, p < 0.0001) with protein levels of ABCA1 in a subset of cases used in PCR analysis.