α₂‐Macroglobulin Complexes with and Mediates the Endocytosis of β‐Amyloid Peptide via Cell Surface Low‐Density Lipoprotein Receptor‐Related Protein

Abstract

A primary histopathological feature of Alzheimer's disease is the accumulation of β‐amyloid (Aβ) in the brain of afflicted individuals. However, Aβ is produced continuously as a soluble protein in healthy individuals where it is detected in serum and CSF, suggesting the existence of cellular clearance mechanisms that normally prevent its accumulation and aggregation. Here, we demonstrate that Aβ forms stable complexes with activated α2‐macroglobulin (α2M⋆), a physiological ligand for the low‐density lipoprotein receptor‐related protein (LRP) that is abundantly expressed in the CNS. These α2M⋆/125I‐Aβ complexes are immunoreactive with both anti‐Aβ and anti‐α2M IgG and are stable under various pH conditions, sodium dodecyl sulfate, reducing agents, and boiling. We demonstrate that α2M⋆/125I‐Aβ complexes can be degraded by glioblastoma cells and fibroblasts via LRP, because degradation is partially inhibited by receptor‐associated protein (RAP), an antagonist of ligand interactions with LRP. In contrast, the degradation of free 125I‐Aβ is not inhibited by RAP and thus must be mediated via an LRP‐independent pathway. These results suggest that LRP can function as a clearance receptor for Aβ via a physiological ligand.

Extracellular Chaperones and Amyloids

“…In support of this notion, clusterin and α 2 M (activated and non activated) seem to preferentially bind intermediate protein species formed during amyloid-related aggregation but do not bind to native structures of the same proteins (Narita et al, 1997;Lauer et al, 2001;Hatters et al, 2002;Mettenburg et al, 2002;Kumita et al, 2007;Yerbury et al, 2007). Five times more clusterin-Aβ complex was formed when clusterin was incubated for 18 h at 37 o C with Aβ monomer than was the case when it was incubated under the same conditions with pre-aggregated Aβ (Matsubara et al, 1995).…”

“…Five times more clusterin-Aβ complex was formed when clusterin was incubated for 18 h at 37 o C with Aβ monomer than was the case when it was incubated under the same conditions with pre-aggregated Aβ (Matsubara et al, 1995). Similarly, α 2 M did not initially form complexes with Aβ but under conditions that promote amyloid formation stable α 2 M-Aβ complexes were formed after 2 hours (Narita et al, 1997). This pattern of binding may be explained by the different levels of hydrophobicity exposed on the various protein conformations produced at different stages of the amyloid-forming pathway.…”

“…Aside from its interactions with proteases, α 2 M binds to Aβ peptide and β 2 -microglobulin, which are associated with Alzheimer's disease (Narita et al, 1997) and dialysis related amyloidosis (Motomiya et al, 2003), respectively, to cytokines and growth factors (Mettenburg et al, 2002), and to a range of hydrophobic molecules including endotoxin, phenyl-Sepharose, and liposomes (Barrett 1981). The binding to hydrophobic molecules does not inhibit the trapping of proteases and is not known to be associated with any conformational changes (Barrett 1981).…”

“…However, there are conflicting reports of the particular α 2 M conformations that bind to Aβ. There are several reports claiming that Aβ binds to active α 2 M but not to native α 2 M (Narita et al, 1997;Lauer et al, 2001;Mettenburg et al, 2002). In contrast, others claim that native α 2 M binds to Aβ (Du et al, 1997;Hughes et al, 1998).…”

Expression of Panza, an α2-macroglobulin, in a restricted dorsal domain of the primitive gut in Xenopus laevis

“…Activated α 2 M binds to β-amyloid (Aβ) and the α 2 M-Aβ complex is cleared and degraded by LRP-mediated internalization (Narita et al, 1997). Consistent with a role in regulating Aβ function, α 2 M reduced Aβ aggregation and fibril formation, as well as the resulting neurotoxicity in cortical neurons (Du et al, 1998;Hughes et al, 1998).…”