Alpha-2-Macroglobulin, a Hypochlorite-Regulated Chaperone and Immune System Modulator

Abstract

Alpha-macroglobulins are ancient proteins that include monomeric, dimeric, and tetrameric family members. In humans, and many other mammals, the predominant alpha-macroglobulin is alpha-2-macroglobulin (α ₂M), a tetrameric protein that is constitutively abundant in biological fluids (e.g., blood plasma, cerebral spinal fluid, synovial fluid, ocular fluid, and interstitial fluid). α ₂M is best known for its remarkable ability to inhibit a broad spectrum of proteases, but the full gamut of its activities affects diverse biological processes. For example, α ₂M can stabilise and facilitate the clearance of the Alzheimer's disease-associated amyloid beta (Aβ) peptide. Additionally, α ₂M can influence the signalling of cytokines and growth factors including neurotrophins. The results of several studies support the idea that the functions of α ₂M are uniquely regulated by hypochlorite, an oxidant that is generated during inflammation, which induces the native α ₂M tetramer to dissociate into dimers. This review will discuss the evidence for hypochlorite-induced regulation of α ₂M and the possible implications of this in neuroinflammation and neurodegeneration.

Figure 1

Schematic diagram showing the function consequences of hypochlorite-induced modification of α₂M. (a) Native α₂M, a tetramer (shown in green), is constitutively present in biological fluids and covalently binds to a broad range of proteases. Binding to proteases results in a conformational change that exposes the binding site on α₂M for LRP1, which is responsible for the clearance of the protease-transformed α₂M complex (shown in dark blue). α₂M also binds to a large number of noncovalent ligands including cytokines and misfolded proteins. In many cases, noncovalent binding of ligands occurs preferentially to the protease-transformed conformation (not shown). In the instance that native α₂M binds noncovalently to a nonprotease substrate, protease interaction is required to enable clearance of the complex via LRP1. (b) Reaction with hypochlorite induces the dissociation of the native α₂M tetramer into dimers. This process abolishes the protease-trapping activity of α₂M; however, the binding to some cytokines (i.e., TNF-α, IL-2, and IL-6) and misfolded proteins is enhanced. On the other hand, the binding of α₂M to other noncovalent ligands (i.e., β-NGF, PDGF-BB, TGF-β1, and TGF-β2) is reduced. The dissociation of the native α₂M tetramer into dimers reveals the binding site on α₂M for LRP1. Therefore, α₂M dimers can facilitate the clearance of substrates in a protease-independent manner. N.B.: Inflammatory processes potentially elevate levels of protease-transformed α₂M and hypochlorite-modified α₂M dimers, concomitantly.

Figure 2

Theoretical model showing the binding sites for monomeric Aβ on native α₂M and PZP. (a) The binding sites for monomeric Aβ (magenta; centred at amino acids 1314–1365 according to [21]) are normally concealed at the noncovalent interface of the (i) native α₂M tetramer. (ii) Binding to proteases (yellow triangles) results in the partial opening of the noncovalent interface between α₂M dimers and exposes the binding sites for monomeric Aβ on each subunit of transformed α₂M. (iii) The binding sites for monomeric Aβ are also exposed by hypochlorite-induced dissociation of the native α₂M tetramer into dimers. (iv) Native PZP (a disulfide-linked dimer) shares 82.7% sequence identity with α₂M in the Aβ binding region (magenta). The dimeric quaternary structure of native PZP results in surface exposure of the binding sites for monomeric Aβ. Although the binding sites for other misfolded proteins are not known, intuitively, they are also located at the normally buried hydrophobic interface of noncovalently associated α₂M dimers. (b) Image of the crystal structure of the transformed α₂M tetramer from PBD 4ACQ [3] with the binding sites for monomeric Aβ shown in magenta, which is comparable to the model shown in (a (ii)). The crystal structures of native α₂M or hypochlorite-modified α₂M dimers have not been solved.