A Protective Role of the Low Density Lipoprotein Receptor-related Protein against Amyloid β-Protein Toxicity

Abstract

In order to delineate the neuroprotective role of the low density lipoprotein receptor-related protein (LRP) against amyloid ␤-protein toxicity, studies were performed in C6 cells challenged with amyloid ␤-protein in the presence or absence of activated ␣ 2 -macroglobulin. Toxicity was assessed via two cell viability assays. We found that this endocytic receptor conferred protection against amyloid ␤-protein toxicity in the presence of activated ␣ 2 -macroglobulin and its down-regulation via inhibition by receptor-associated protein or transfection of cells with presenilin 1, increased susceptibility to amyloid ␤-protein toxicity. Increased surface LRP immunoreactivity in response to amyloid ␤-protein challenge was associated with increased translocation of LRP from the endoplasmic reticulum to the surface, rather than from increased mRNA or protein expression. Furthermore, this translocation of LRP to the surface was mediated by a calcium/calmodulin protein kinase II-dependent signaling pathway. These studies provide evidence for a protective role of LRP against amyloid ␤-protein toxicity and may explain the aggressive nature of presenilin-1 mutation in familial Alzheimer's disease. Low density lipoprotein (LDL)1 receptor-related protein (LRP) is a multifunctional endocytic receptor that binds a variety of ligands including apolipoprotein E (apoE)-containing lipoproteins (1, 2), activated ␣ 2 -macroglobulin (*␣ 2 m) complexes, and proteins containing Kunitz-type protease inhibitor domains such as secreted amyloid precursor protein (3). Recent studies have suggested that altered LRP regulation may be an integral component of Alzheimer's disease (AD) pathogenesis (4, 5). Importantly, LRP and its ligands, APP, apoE, and ␣ 2 m, have all been genetically linked to AD (6 -9), and colocalize in senile plaques, a hallmark of AD (4, 10). In addition, patients with probable AD have increased serum levels of LRP ligands including apoE, ␣ 1 -antichymotrypsin, plasmin, and urokinase (11-13), suggesting that LRP expression (or function) may be deficient in these patients. Furthermore, we have previously shown that overexpression of or mutations in the presenilin 1 (PS1) gene, which has been closely linked to the majority of early onset familial AD cases, results in significant down-regulation of LRP (5). Taken together, these data indicate that LRP plays a central role in AD pathogenesis.Although the mechanisms through which LRP might be involved in AD remain unclear, a recent study has suggested a role for LRP in clearance of amyloid ␤-protein (A␤) (14). Specifically, A␤ is internalized by LRP when bound to apoE or *␣ 2 m (14 -18). Whether this effect is toxic or protective is not completely known. Thus, the main objective of the present study was to determine if addition of an A␤-binding LRP ligand prevents A␤ toxicity. The results suggest that LRP mediates protection against A␤ toxicity in the presence of *␣ 2 m and that LRP surface expression is preferentially up-regulated in response to A␤ challenge via a calcium/calmod...

Increased soluble amyloid-β peptide and memory deficits in amyloid model mice overexpressing the low-density lipoprotein receptor-related protein

“…A putative role for LRP in AD is supported by in vitro studies showing that apoE and ␣ 2 M can form stable complexes with A␤ and promote its clearance via cell-surface LRP (5)(6)(7)(8)(9)(10). Furthermore, LRP appears to influence APP endocytic trafficking and cellular distribution such that processing to A␤ and its extracellular release are enhanced (11,12).…”

“…The low-density lipoprotein (LDL) receptor-related protein (LRP) has been genetically linked to AD (3,4) and has been shown to influence A␤ metabolism in vitro (5)(6)(7)(8)(9)(10)(11)(12). LRP is an Ϸ600-kDa cell-surface endocytic receptor member of the LDL receptor family (13).…”

Increased Extracellular Amyloid Deposition and Neurodegeneration in Human Amyloid Precursor Protein Transgenic Mice Deficient in Receptor-Associated Protein

“…In addition, previous studies have shown that, in the presence of ␣2M, LRP protects neural cell lines against the neurotoxic effects of A␤ (Fabrizi et al, 1999;Van Uden et al, 1999b. These effects were blocked by exogenous administration of RAP and are dependent on calcium/calmodulin-dependent kinase 2 signaling (Van Uden et al, 2000). In hAPP tg mice, previous studies have shown that A␤ is synaptotoxic (Mucke et al, 2000b) and disrupts synaptic transmission independently of plaque formation (Hsia et al, 1999).…”

α-Synuclein Protects against Oxidative Stress via Inactivation of the c-Jun N-terminal Kinase Stress-signaling Pathway in Neuronal Cells

“…After the addition of 0.4% trypan blue for 2 min (Sigma), the percent of dead cells (trypan bluestained) was recorded within 10 min and visualized with an Axiovert inverted microscope. In addition, a DNA fragmentation assay was performed as previously described (38). Briefly, cells (1 ϫ 10 6 ) were washed and collected by centrifugation.…”