Sequences from the Low Density Lipoprotein Receptor-related Protein (LRP) Cytoplasmic Domain Enhance Amyloid β Protein Production via the β-Secretase Pathway without Altering Amyloid Precursor Protein/LRP Nuclear Signaling

Abstract

Increasing evidence suggests that the low density lipoprotein receptor-related protein (LRP) affects the processing of amyloid precursor protein (APP) and amyloid ␤ (A␤) protein production as well as mediates the clearance of A␤ from the brain. Recent studies indicate that the cytoplasmic domain of LRP is critical for this modulation of APP processing requiring perhaps a complex between APP, the adaptor protein FE65, and LRP. In this study, we expressed a small LRP domain consisting of the C-terminal 97 amino acids of the cytoplasmic domain, or LRP-soluble tail (LRP-ST), in CHO cells to test the hypothesis that the APP⅐LRP complex can be disrupted. We anticipated that LRP-ST would inhibit the normal interaction between LRP and APP and therefore perturb APP processing to resemble a LRPdeficient state. Surprisingly, CHO cells expressing LRP-ST demonstrated an increase in both sAPP secretion and A␤ production compared with control CHO cells in a manner reminiscent of the cellular effects of the APP "Swedish mutation." The increase in sAPP secretion consisted mainly of sAPP␤, consistent with the increase in A␤ release. Further, this effect is LRP-independent, as the same alterations remained when LRP-ST was expressed in LRP-deficient cells but not when the construct was membrane-anchored. Finally, deletion experiments suggested that the last 50 amino acid residues of LRP-ST contain the important domain for altering APP processing and A␤ production. These observations indicate that there are cellular pathways that may suppress A␤ generation but that can be altered to facilitate A␤ production.The low density lipoprotein receptor-related protein (LRP) 1 is a large type I transmembrane protein that functions as a multifunctional endocytosis receptor for a diverse array of extracellular ligands (1, 2). LRP is synthesized as a 600-kDa precursor protein that is subsequently cleaved in the transGolgi compartment by furin to generate a large 515-kDa ␣-chain and a smaller 85-kDa membrane-associated ␤-chain that remain non-covalently linked (3, 4). The cytoplasmic tail contains two NPXY endocytosis motifs and binds a number of cytoplasmic adaptor and scaffold proteins, such as FE65, Mint2, Disabled-1 (Dab1), Shc, and JIP-1 and -2, probably through the second NPXY motif (5-7). In addition, LRP itself undergoes a presenilin-dependent ␥-secretase intramembrane proteolysis, releasing a potentially transcriptionally active fragment in a manner similar to APP and Notch (8,9).Recent studies have implicated a role for the LRP pathway in Alzheimer disease pathogenesis. LRP and three of its key ligands, apoE, ␣2-macroglobulin, and APP, are genetically associated with Alzheimer disease and are found in senile plaques in the brains of Alzheimer disease patients (10 -13). LRP also mediates the binding and clearance of amyloid ␤ peptide (A␤) complexes bound to apoE or ␣2-macroglobulin in cultured cells and in the brain (14 -16). In addition, LRP may play a crucial role in brain efflux of A␤ isoforms at the blood-brain barrier (17, 18). Thes...

γ-Secretase-mediated Release of the Low Density Lipoprotein Receptor-related Protein 1B Intracellular Domain Suppresses Anchorage-independent Growth of Neuroglioma Cells

“…Interestingly, our results also indicate that the LRP1 ICD, which does not contain the basic cluster found in LRP1B, does not localize to the nucleus. This result is in contrast with a previous study (44) but in agreement with the studies of Yoon et al (45).…”

Novel Role of RanBP9 in BACE1 Processing of Amyloid Precursor Protein and Amyloid β Peptide Generation

“…The polyclonal antibody CT15 (against C-terminal 15 residues of APP) and B279 (against BACE1) and the monoclonal antibodies 1G7 (against APP ectodomain), 26D6 (against 1-16 of A␤), and Ab9 (against 1-16 of A␤) have been described previously (5,6). Monoclonal antibodies 9E10 (against Myc; Calbiochem), Ab-1 (against the ␤-tubulin; Calbiochem) M2 (anti-FLAG; Sigma), flotillin-1 (against flotillin-1; Santa Cruz Biotechnology), 82E1 (against N terminus of A␤; IBL, Japan), 6E10 (against 1-17 of A␤; Covance Research), anti-biotin (Jackson ImmunoResearch), anti-sAPP␤ (IBL-America), and anti-Swe-sAPP␤ (clone 6A1, IBL-America) were purchased from the indicated vendors.…”

“…Specifically, LRP-CT lacking the extracellular ligand binding regions but containing the transmembrane domain and the cytoplasmic tail is capable of rescuing amyloidogenic processing of APP and A␤ release in LRP deficient cells (3). Moreover, the LRP soluble tail (LRP-ST) lacking the transmembrane domain and only containing the cytoplasmic tail of LRP is sufficient to enhance A␤ secretion (5). This activity of LRP-ST is achieved by promoting APP/BACE1 interaction (6), although the precise mechanism is unknown.…”

APP at a glance

“…In the case of Fe65 and Dab1, the effects are cell type dependent, possibly owing to the existing complement of competing cellular APP-binding proteins in each cell type (Hoe et al, 2006a;Hoe et al, 2006b;Parisiadou and Efthimiopoulos, 2007). In addition, tripartite complexes involving APP-binding proteins, APP and other molecules, such as alcadein-X11L-APP and LRP-Fe65-APP, have been shown to influence A␤ generation (Araki et al, 2004;Yoon et al, 2005).…”