Alpha-2-Macroglobulin in Inflammation, Immunity and Infections

Abstract

Alpha-2-macroglobulin is an extracellular macromolecule mainly known for its role as a broad-spectrum protease inhibitor. By presenting itself as an optimal substrate for endopeptidases of all catalytic types, alpha-2-macroglobulin lures active proteases into its molecular cage and subsequently 'flags' their complex for elimination. In addition to its role as a regulator of extracellular proteolysis, alpha-2-macroglobulin also has other functions such as switching proteolysis towards small substrates, facilitating cell migration and the binding of cytokines, growth factors and damaged extracellular proteins. These functions appear particularly important in the context of immune-cell function. In this review manuscript, we provide an overview of all functions of alpha-2-macroglobulin and place these in the context of inflammation, immunity and infections.

Keywords: alpha-2-macroglobulin; immunity; infections; inflammation; macrophages; neutrophils; proteolysis.

Figure 1

Illustration of the interaction between the A2M tetramer and active endopeptidases. Functional A2M is formed by a non-covalent interaction between two covalently linked dimers. Each monomer contains a protease bait region (red lines) and a buried receptor binding domain (RBDs, gray triangles). Active proteases can cleave the bait region, resulting in A2M activation (A2M*) and conformational rearrangement. This process results in physical trapping of the protease and the exposure of a reactive thioester bond which may result in the formation of a covalent bond between A2M and protease lysine residues (red hooks). Simultaneously, the receptor binding domains are exposed to the protein surface, thereby enabling A2M* to bind its cell-surface receptors.

Figure 2

The complement system and A2M. (A) Overview of the classical, lectin and alternative complement pathways. MAC, membrane attack complex; MASP, mannose-binding protein-associated serine protease; MBL, mannose-binding lectin. (B) Overview of the domain organization of a selection of human proteins belonging to the A2M family (MEROPS protease inhibitor family I39). A2ML1, A2M-like-1; ANA, anaphylatoxin domain; BRD, bait-region domain; C345C, C-terminal extension of C3-5; CUB, complement C1r/C1s, Uegf, Bmp1; GPI, glycosyl-phosphatidyl-inositol linker; MG, macroglobulin-like domains; TED, thioester domain. (C) comparison of the crystal structures of the A2M monomer (PDB structure: 4ACQ) (2) and C3 (PDB structure: 2A73) (20). Domain colors are based on the colors used in Panel (B) For A2M, part of the bait region and the MG8/RBD are not shown.

Figure 3

LRP-1, a multifunctional receptor for A2M*. The LRP-1 receptor (green) is composed out of a 515-kDa extracellular domain (α-chain) and an 85 kDa transmembrane domain. The α-chain has four regions with cysteine-rich complement-type repeat (CR) clusters (clusters I-IV) which interact with extracellular ligands such as A2M*, tissue plasminogen activator (tPA) and apolipoprotein E (apoE). The intracellular part of the β-chain initiates endocytosis and interacts with adaptor molecules to initiate signaling pathways. Whereas endocytosis is mediated through the YxxL motif and dileucine repeats, the initiation of cell signaling pathways relies on the presence and phosphorylation of NPxY motifs and recruitment of adaptor and scaffolding proteins containing phosphotyrosine-binding (PTB) domains [e.g. Disabled homolog 1 (DAB1), C-Jun-amino-terminal kinase-interacting protein 1 & 2 (JIP1-2)], Src homology 2 (Sh2) domains [e.g. Sh2-containing protein tyrosine phosphatase 2 (SHP2)] and PDZ (post synaptic density protein (PSD95), drosophila disc large tumor suppressor and zonula occludens-1 protein) domains [e.g. PSD-95, protein carboxy-terminal PDZ ligand of nNOS (CAPON)]. Hence, LRP-1 can activate different signaling pathways, thereby affecting processes such as cytoskeletal reorganization, cell proliferation and cell adhesion. In addition, several co-receptor relationships have been proposed, for example, with the N-methyl-D-aspartate (NMDA) receptor, the tyrosine receptor kinase (trk) receptor and glucose-regulated protein (GRP)-78.

Figure 4

A2M and neutrophil function. A2M* aids neutrophils through stimulation of their capacity to bind to endothelial cells (1), to migrate (2) and to phagocytose and kill pathogens (3). For example, the chemotactic potential of neutrophils is increased in the presence of A2M* by reducing LPS-induced G-protein-coupled receptor kinase 2 (GRK2) expression, which subsequently results in increased C-X-C motif chemokine receptor 2 (CXCR2) expression and increased neutrophil migration towards IL-8 (2). A2M* also induces CD11b in neutrophils challenged with LPS, thereby increasing the interaction with ICAM-1 and aiding neutrophil adhesion to the vascular endothelium (1). A2M* augments phagocytosis and bacterial killing by neutrophils, possibly through interaction with LRP-1 (4). Finally, A2M can inhibit all major proteases secreted by neutrophils and reduce their proteolytic activity against large substrates (5). However, in the presence of reactive oxygen species (ROS), such as hypochlorite, A2M dissociates thereby switching its activity from protease inhibition to an extracellular chaperone and cytokine carrier (6).

Figure 5

A2M and macrophage function. Macrophages produce A2M and express both A2M receptors. Hence, several functions for A2M have been proposed in macrophage biology. A2M promotes bacterial phagocytosis, killing and the production of ROS (1) and this effect might be mediated through interaction with LRP-1. Triggering of LRP-1 and/or GRP78-associated signaling pathways leads to several cellular effects. A first effect is increased cell growth, which is mediated through activation of MAPK pathways, phosphorylation of cAMP response element-binding (CREB) protein and activation of protein kinase C (PKC) (2). A second effect is increased cell migration, which might occur through increased phosphorylation of cofilin, activation of actin polymerization and the formation of enlarged cellular protrusions containing MT1-MMP (3). Binding to LRP-1 and/or GRP78 also results in a rapid increase in intracellular calcium which is thought to contribute to the release of mediators of inflammation including platelet activating factor (PAF), prostaglandin E2 (PGE2) and matrix metalloproteinase-9 (MMP-9) (4). Finally, A2M also promotes antigen presentation by macrophages where both MHC-I and MHC-II presentation have been reported (5).