Association of ApoE and LRP mRNA levels with dementia and AD neuropathology

Abstract

Inheritance of the ε4 allele of apolipoprotein E (ApoE) is the only confirmed and consistently replicated risk factor for late onset Alzheimer's disease (AD). ApoE is also a key ligand for low-density lipoprotein (LDL) receptor-related protein (LRP), a major neuronal low-density lipoprotein receptor. Despite the considerable converging evidence that implicates ApoE and LRP in the pathogenesis of AD, the precise mechanism by which ApoE and LRP modulate the risk for AD remains elusive. Moreover, studies investigating expression of ApoE and LRP in AD brain have reported variable and contradictory results. To overcome these inconsistencies, we studied the mRNA expression of ApoE and LRP in the postmortem brain of persons who died at different stages of dementia and AD-associated neuropathology relative to controls by quantitative polymerase chain reaction (qPCR) and Western blotting analyses. Clinical dementia rating scores were used as a measure of dementia severity, whereas, Braak neuropathological staging and neuritic plaque density were used as indexes of the neuropathological progression of AD. ApoE and LRP mRNA expression was significantly elevated in the postmortem inferior temporal gyrus (area 20) and the hippocampus from individuals with dementia compared with those with intact cognition. In addition to their strong association with the progression of cognitive dysfunction, LRP and ApoE mRNA levels were also positively correlated with increasing neuropathological hallmarks of AD. Additionally, Western blot analysis of ApoE protein expression in the hippocampus showed that the differential expression observed at the transcriptional level is also reflected at the protein level. Given the critical role played by LRP and ApoE in amyloid beta (Aβ) and cholesterol trafficking, increased expression of LRP and ApoE may not only disrupt cholesterol homeostasis but may also contribute to some of the neurobiological features of AD, including plaque deposition.

Figure 1

LRP (Black bars) and ApoE mRNA (white bars) expression in individuals with and without dementia or AD-associated neuropathology in area 20. Mean values ± standard error of the mean (SEM) are shown. *, p < 0.05; **, p < 0.01; ****, p < 0.0001. Number within the parentheses indicates the individuals within each group.

Figure 2

Normalized LRP (Black bars) and ApoE mRNA (white bars) expression in area 20 plotted against CDR scores, Braak neuropathological stages and NP density groups. ANCOVA was used to compare gene expression in individuals with varying degree of dementia (CDR 0.5–5) and AD associated neuropathology (Braak stage I-VI, NP density 2–5) relative to the control group. Mean values ± SEM are shown. *, p < 0.05; **, p < 0.01; ***, p < 0.001; ****, p < 0.0001. Number within the parentheses indicates the individuals within each group.

Figure 3

LRP (Black bars) and ApoE mRNA (white bars) expression in individuals with and without dementia or AD associated neuropathology in the hippocampus. Mean values ± SEM are shown. *, p < 0.05;**, p < 0.01; ***, p < 0.001; ****, p < 0.0001. Number within the parentheses indicates the individuals within each group.

Figure 4

Normalized LRP (black bars) and ApoE mRNA (white bars) expression in the hippocampus plotted against CDR scores, Braak neuropathological stages and NP density groups. ANCOVA was used to compare gene expression in individuals with varying degree of dementia (CDR 0.5–5) and AD associated neuropathology (Braak stage I–VI, NP density 2–5) relative to the control group in the hippocampus. Mean values ± SEM are shown. *, p < 0.05; **, p < 0.01. Number within the parentheses indicates the individuals within each disease severity group.

Figure 5

Western blot analysis of ApoE in the hippocampus of cognitively intact controls and individuals with varying severity of dementia. A, Representative immunoblots of ApoE protein expression are shown. Total tissue homogenates were separated by reducing SDS-PAGE and probed with mouse anti-ApoE and mouse anti-VCP antibodies. Tissue lysate from each individual were loaded in triplicate and pooled tissue lysate (last 4 lanes) were run in quadruplicates. B, Protein quantification was done by assessing the ratio of APOE and VCP signal. Mean values ± SEM are shown. *, p < 0.05; **, p < 0.01. Number within the parentheses indicates the individuals within each group.

Figure 6

Association between ApoE and LRP gene expression. A. Partial correlation of LRP and ApoE gene expression controlling for age and RIN in area 20 is (r = 0.768, df = 84, p < 0.0001). B. Partial correlation of LRP and ApoE gene expression controlling for age and pH in hippocampus is (r = 0.681, df = 69, p < 0.0001).