ABSTRACT
Proteolytic processing of the amyloid-ß precursor protein (APP) and generation of amyloid-ß peptide (Aß) are key events in Alzheimer's disease (AD) pathogenesis. Cell biological and genetic evidence has implicated the low-density lipoprotein and sorting receptor LR11/SorLA in AD through mechanisms related to APP and Aß production. Defining the cellular pathway(s) by which LR11 modulates Aß production is critical to understanding how changes in LR11 expression affect the development of Aß pathology in AD progression. We report that the LR11 ectodomain is required for LR11-mediated reduction of Aß and that mutagenesis of the LR11 Golgi-localizing, γ-adaptin ear homology domain, ADP-ribosylation factor (GGA)-binding motif affects the endosomal distribution of LR11, as well as LR11's effects on APP traffic and Aß production. Targeted small interfering RNA (siRNA) knockdown studies of GGA1, GGA2, and GGA3 indicate a surprising degree of specificity toward GGA1, suggesting that GGA1 is a candidate regulator of LR11 traffic. Additional siRNA knockdown experiments reveal that GGA1 is necessary for both LR11 and ß-site APP-cleaving enzyme-1 (BACE1) modulation of APP processing to Aß. Mutagenesis of BACE1 serine 498 to alanine enhances BACE1 targeting to LR11-positive compartments and nullifies LR11-mediated reduction of Aß. On basis of these results, we propose that GGA1 facilitates LR11 endocytic traffic and that LR11 modulates Aß levels by promoting APP traffic to the endocytic recycling compartment.
Subject(s)
Adaptor Proteins, Vesicular Transport/metabolism , Alzheimer Disease/metabolism , Amyloid beta-Peptides/biosynthesis , Amyloid beta-Protein Precursor/metabolism , LDL-Receptor Related Proteins/metabolism , Membrane Transport Proteins/metabolism , Adaptor Proteins, Vesicular Transport/genetics , Alzheimer Disease/genetics , Alzheimer Disease/pathology , Amyloid Precursor Protein Secretases/genetics , Amyloid Precursor Protein Secretases/metabolism , Amyloid beta-Protein Precursor/genetics , Aspartic Acid Endopeptidases/genetics , Aspartic Acid Endopeptidases/metabolism , Brain/metabolism , Brain/pathology , Cell Line , Endocytosis , Endosomes/metabolism , HEK293 Cells , Humans , Mutagenesis , RNA Interference , RNA, Small InterferingABSTRACT
Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive cognitive decline and neuropathological changes, including the deposition of amyloid beta (Abeta) in senile plaques. The mechanisms causing the disease and Abeta accumulation are not well understood, but important genetic associations with apolipoprotein E genotype and involvement of lipoprotein receptors have become apparent. LR11 (also known as SorLA), a member of the low-density lipoprotein receptor family, has been identified previously as an altered transcript in microarray analyses of samples from human AD cases. Here, we show neuronal expression of the lipoprotein receptor LR11 in control brain in regions vulnerable to AD neuropathology and marked reduction of LR11 expression in these regions in AD brains before cell death. Overexpression of LR11 drastically reduces levels of extracellular Abeta and also lowers levels of total cellular amyloid precursor protein (APP). LR11 colocalizes with APP and regulates its trafficking in endocytic compartments, which are important intracellular sites for APP processing and Abeta generation. Endogenous LR11 localizes to neuronal multivesicular bodies in both rat and human brain. The robust correlation between reduced LR11 expression and AD neuropathology and its potent effects on extracellular Abeta levels suggest that this neuronal lipoprotein receptor could play an important role in AD pathogenesis.
Subject(s)
Alzheimer Disease/metabolism , Amyloid beta-Peptides/biosynthesis , Amyloid beta-Protein Precursor/metabolism , Endosomes/metabolism , Membrane Transport Proteins/physiology , Receptors, LDL/physiology , Alzheimer Disease/pathology , Amyloid beta-Protein Precursor/analysis , Animals , Brain/cytology , Cell Line , Endosomes/chemistry , Humans , LDL-Receptor Related Proteins , Membrane Transport Proteins/analysis , Neurons/chemistry , Protein Transport , Rats , Receptors, LDL/analysisABSTRACT
BACKGROUND: Genetic, epidemiologic, and biochemical evidence suggests that apolipoprotein E, low-density lipoprotein receptors, and lipid metabolism play important roles in sporadic Alzheimer disease (AD). OBJECTIVE: To identify novel candidate genes associated with sporadic AD. DESIGN: We performed an unbiased microarray screen for genes differentially expressed in lymphoblasts of patients with sporadic AD and prioritized 1 gene product for further characterization in AD brain. SETTING: Emory University, Atlanta, Ga. SUBJECTS: Cell lines were used from 14 patients with AD and 9 normal human control subjects. RESULTS: Six genes were differentially expressed in lymphoblasts of 2 independent groups of patients with probable AD and autopsy-proven AD. We hypothesized that 1 of the genes, termed low-density lipoprotein receptor relative with 11 binding repeats (LR11) (reduced 1.8- and 2.5-fold in AD lymphoblasts vs controls), might be associated with sporadic AD on the basis of its function as neuronal apolipoprotein E receptor. We found dramatic and consistent loss of immunocytochemical staining for LR11 in histologically normal-appearing neurons in AD brains. This reduction of LR11 protein was confirmed by quantitative Western blotting (P =.01). CONCLUSIONS: There is loss of the microarray-derived candidate, LR11, in neurons of AD brains. This study shows that microarray analysis of widely available lymphoblasts derived from patients with AD holds promise as a primary screen for candidate genes associated with AD.
