LR11/SorLA expression is reduced in sporadic Alzheimer disease but not in familial Alzheimer disease

Abstract

LR11 is an ApoE receptor that is enriched in the brain. We have shown that LR11 is markedly downregulated in patients with sporadic Alzheimer disease (AD). This finding led us to explore whether reduced LR11 expression reflects a primary mechanism of disease or merely a secondary consequence of other AD-associated changes. Therefore, LR11 expression was assessed in a transgenic mouse model of AD and familial AD (FAD) brains. Immunohistochemistry and immunoblotting of LR11 in PS1/APP transgenic and wild-type mice indicated that LR11 levels are not affected by genotype or accumulation of amyloid pathology. LR11 expression was also evaluated based on immunoblotting and LR11 immunostaining intensity in human frontal cortex in controls, sporadic AD, and FAD, including cases with presenilin-1 (PS1) and presenilin-2 (PS2) mutations. Although LR11 was reduced in sporadic AD, there was no difference in protein level or staining intensity between control and FAD cases. The finding that LR11 expression is unaffected in both a mouse model of AD and autosomal-dominant forms of AD suggests that LR11 is not regulated by amyloid accumulation or other AD neuropathologic changes. We hypothesize that LR11 loss may be specific to sporadic AD and influence amyloid pathology through mechanisms independent of substrate-enzyme interactions regulated by FAD mutations.

FIGURE 1

Amyloid pathology in brain. (A) Coronal sections from 3-, 6-, 9-, and 18-month-old PS1/APP mice. Thioflavine-S stain reveals increasing accumulation of amyloid plaques with age in cortex (Ctx) and hippocampus (Hipp). (B) Coronal sections of frontal cortex from non-diseased control (CTL) and sporadic Alzheimer disease (AD) human brain. Thioflavine-S staining reveals no plaque pathology in control brain and extensive plaque accumulation in human AD brain. Original magnification: 5×.

FIGURE 2

LR11 immunofluorescence in presenilin-1/amyloid precursor protein (PS1/APP) mouse brain. Representative coronal sections from one-month-old (left) and 18-month-old (right) wild-type (WT) and PS1/APP mice were processed for LR11 immunostaining. (Top panels) Low-magnification view of LR11 staining in cortex (ctx) and CA1 region of hippocampus (ca1). Corpus callosum is also labeled (cc). Scale bar = 100 μm. (Middle panels) High-magnification view of LR11 staining in cortical pyramidal neurons. Scale bar = 10 μm. (Bottom panels) High-magnification view of LR11 staining in CA1 hippocampal pyramidal neurons. Scale bar = 10 μm.

FIGURE 3

Immunoblots of LR11 protein from PS1/APP mouse brain. Cortical tissue from wild-type (WT) and PS1/APP mice at 3, 6, 9, and 18 months of age were immunoblotted for LR11 and EF1α protein expression to establish loading consistency. Each band is representative of one animal. Densitometric quantitation of LR11 expression is graphed on the right side (relative intensity). LR11 expression in PS1/APP mice was not significantly different from control at any of the time points tested.

FIGURE 4

LR11 immunohistochemistry in human brain. Paraffin sections from frontal cortex of cognitively normal (control), sporadic Alzheimer disease, presenilin (PS) -1 variants of familial Alzheimer disease (FAD) (PS1), and PS2 variants of FAD (PS2) were stained for LR11 expression and visualized with DAB (brown deposits). Low-magnification images captured at 20× and high-magnification insets were captured at 100×. Cell bodies were counterstained with hematoxylin (blue).

FIGURE 5

Results from semiquantitative LR11 staining intensity measures in human brain. LR11-immunolabeled paraffin sections were evaluated on a 5-point scale for intensity of staining. Scale definition: 1 = little to no staining; 2 = light staining; 3 = moderate staining; 4 = moderate–strong staining; and 5 = strong staining. **, Mean LR11 immunostaining intensity was significantly less in sporadic Alzheimer disease than both control and familial Alzheimer disease (FAD). The FAD group consists of presenilin (PS) -1 and PS2 cases grouped together for statistical comparison. Refer to the Table for staining means and standard error of mean. Each point represents the intensity score of a single case. The mean staining intensity of each group is plotted as a horizontal line.

FIGURE 6

Representative immunoblots for LR11 protein expression in human brain. Immunoblots were probed with an LR11 C-terminus antibody and N-terminus β-actin antibody as a loading control. Graphic representation of LR11 protein is plotted as percent of control protein level. LR11 protein expression is decreased by approximately 30% in sporadic Alzheimer disease (AD) cases (p = 0.058) compared with control. Familial AD cases have equivalent LR11 protein compared with controls (p = 0.699). Error bars represent standard error of the mean.