Inhibition of β2-Microglobulin Amyloid Fibril Formation by α2-Macroglobulin

Abstract

The relationship between various amyloidoses and chaperones is gathering attention. In patients with dialysis-related amyloidosis, ␣ 2 -macroglobulin (␣2M), an extracellular chaperone, forms a complex with ␤ 2 -microglobulin (␤2-m), a major component of amyloid fibrils, but the molecular mechanisms and biological implications of the complex formation remain unclear. Here, we found that ␣2M substoichiometrically inhibited the ␤2-m fibril formation at a neutral pH in the presence of SDS, a model for anionic lipids. Binding analysis showed that the binding affinity between ␣2M and ␤2-m in the presence of SDS was higher than that in the absence of SDS. Importantly, SDS dissociated tetrameric ␣2M into dimers with increased surface hydrophobicity. Western blot analysis revealed that both tetrameric and dimeric ␣2M interacted with SDS-denatured ␤2-m. At a physiologically relevant acidic pH and in the presence of heparin, ␣2M was also dissociated into dimers, and both tetrameric and dimeric ␣2M interacted with ␤2-m, resulting in the inhibition of fibril growth reaction. These results suggest that under conditions where native ␤2-m is denatured, tetrameric ␣2M is also converted to dimeric form with exposed hydrophobic surfaces to favor the hydrophobic interaction with denatured ␤2-m, thus dimeric ␣2M as well as tetrameric ␣2M may play an important role in controlling ␤2-m amyloid fibril formation.A number of proteins and peptides can misfold into ␤-sheetrich structures, called amyloid fibrils (1). The deposition of amyloid fibrils in intra-and extracellular spaces is responsible for more than 40 serious diseases, including Alzheimer and Parkinson diseases and dialysis-related amyloidosis (DRA) 2 (1). ␤ 2 -Microglobulin (␤2-m) is a major structural component of amyloid fibrils in DRA, a common and serious complication in long term hemodialysis patients (2). The formation of ␤2-m amyloid fibrils is thought to be induced by partial unfolding of ␤2-m (3, 4). Several groups have established conditions under which ␤2-m amyloid fibril formation occurs at a neutral pH (5-11). We found that ␤2-m amyloid fibrils are formed at a neutral pH in the presence of SDS (11). SDS below its critical micelle concentration unfolds the compact structure of ␤2-m to an amyloidogenic conformer and stabilizes the extended amyloid fibrils. SDS is an anionic detergent that mimics some characteristics of biological membranes and is considered to be a good model for anionic lipids. We recently reported that some lysophospholipids, especially lysophosphatidic acid as well as nonesterified fatty acids induce the extension of ␤2-m amyloid fibrils at a neutral pH by partially unfolding the compact structure of ␤2-m to an amyloidogenic conformer as well as by stabilizing the extended fibrils (6, 8). Although many groups have proposed the mechanisms by which ␤2-m amyloid fibrils are formed under physiological conditions, the biological machineries to inhibit the formation and deposition of ␤2-m amyloid fibrils are poorly understood (12).␣ 2 -Macroglobulin (...

Extracellular Chaperones and Proteostasis

“…As for the other ECs, α 2 M appears to suppress amyloid formation by interacting with prefibrillar species that occur early in the aggregation process (86). A recent study showed that mildly acidic pH or 0.5 mM sodium dodecyl sulphate (which induce dissociation α 2 M tetramers into dimers), increased the binding of α 2 M to β 2 -microglobulin (114). This report proposed that dimeric α 2 M may be more chaperoneactive than the tetramer, however, currently it is unknown whether α 2 M dimers are generated in humans in vivo.…”

Ultrasonication: An Efficient Agitation for Accelerating the Supersaturation-Limited Amyloid Fibrillation of Proteins

“…Recently, the mechanism of amyloid fibrillation under physiological pH conditions has been focused on. 18,[25][26][27][28] Amyloid fibrillation consists of nucleation and growth. 16,24,29,30) The nucleation process, in which a number of monomers associate to form a minimal fibril unit, does not readily occur.…”

Hypochlorite-induced structural modifications enhance the chaperone activity of human α2-macroglobulin

“…Although our understanding of extracellular proteostasis has increased in recent years, it is still limited compared with our extensive knowledge of the mechanisms comprising intracellular proteostasis (12). Several in vitro studies have shown that α 2 M can inhibit both fibril formation by amyloidogenic proteins and peptides (13)(14)(15)(16) and stress-induced amorphous aggregation of other systems (17). In addition, recent studies have shown that α 2 M can protect neurons from toxic oligomers of the protein HypF-N and the Alzheimer's diseaserelevant amyloid β-peptide (Aβ ) by preventing them from binding to the cell surface (18,19).…”