Metal binding sheds light on mechanisms of amyloid assembly

Abstract

β-2 microglobulin (β2m) is the protein responsible for amyloid deposition in Dialysis-Related Amyloidosis (DRA). Aggregation can be induced by various solution conditions including exposure to divalent metal, incubation at acidic pH, and limited proteolysis. Using Cu 2+ as a trigger, we have trapped, isolated, and crystallized a stable oligomer of β2m that is populated under amyloidogenic solution conditions (Calabrese et al. Nat Struct Mol Biol 2008; 15:965-71). This structure reveals that Cu 2+ -binding is associated with dramatic conformational rearrangements. This has allowed us to postulate a set of structural changes common to all β2m aggregation pathways. Cu 2+ serves as a potential trigger in other aggregation systems such as Aβ, α-synuclein, and mammalian Prion (PrP). A comparison of Cu 2+ binding to β2m and PrP reveals common features. Therefore, in addition to providing insight into DRA, induction of structure by Cu 2+ binding appears to be a recurring structural motif for pathological changes in conformation. Backgroundβ2m is the 99 residue, 12 kDa non-covalent light chain of the Class I Major Histocompatibility Complex (MHC). Its function is to ensure proper folding and cell-surface expression of the MHC. 1 In the course of normal turnover, β2m is released to the serum and catabolized by the kidneys. In patients suffering from endstage renal disease who are treated with hemodialysis therapy, β2m levels rise and amyloid plaques develop throughout the joints and connective tissue. 2 Although an elevated serum level of β2m may be necessary for aggregation, it is not sufficient as β2m remains stably folded at > 1 mM concentration at 37 °C. 3,4 Furthermore, it can be reversibly folded in vitro 5-7 and amyloid is not reported in other diseases with elevated levels in serum. 8,9 Therefore, aggregation must be triggered by features unique to hemodialysis therapy.As β2m exists as a stable monomer under near physiological conditions, 3,10 much effort has been focused on understanding the molecular mechanism by which self association is triggered. Many approaches have been used to induce β2m aggregation. These include limited proteolysis, 11 incubation at acidic pH, 12 and exposure to detergents or organic solvents. 13,14 In 2001, we discovered that β2m is a Cu 2+ -binding protein. 4 Whereas β2m apo is not amyloidogenic, β2m holo is aggregation-prone. Cu 2+ Mediated OligomerizationWe are interested in Cu 2+ -dependent aggregation of β2m for several reasons. First, hemodialysis patients may be exposed to elevated levels of divalent cation as a consequence of therapy. This is apparent in the historical use of Cu 2+ in the preparation of dialysis membranes, and in water quality standards which permit as much as 1.6 μM Cu 2+ in hemodialysate. 15 This is comparable to the ~3 μM affinity of β2m measured in vitro. 4 Second, early experiments revealed that although Cu 2+ is necessary to initiate aggregation, mature fibers remain stable in the presence of a metal chelate. 10 This allowed the possibility that Cu 2+ acti...

Zinc Modulates Self-Assembly of Bacillus thermocatenulatus Lipase

“…Such changes in the intermolecular interactions of the late aggregates, observed for other fibrillar aggregates, 26 might be used to support the hypothesis stating that the late aggregates form fibrillar structures.…”

“…Such distinct biochemical features might indicate different aggregate forms representing different stages of the aggregation process. 26 The kinetic analysis revealed that aggregation was much faster at 50 μM zinc than it was at 10 μM zinc (Figure 4 and Figure S3 of the Supporting Information), suggesting that using a fixed incubation time, the aggregates formed at low zinc supplies would represent the early stages of the process while those formed at high zinc would represent the late stages. In this respect, we considered the aggregates formed in 2 h at low zinc concentrations (≤10 μM) as the early aggregates and those formed at high concentrations (≥50 μM) as the late aggregates.…”

No Association of LOXL1 Gene Polymorphisms with Alzheimer’s Disease

“…or Zn 2? (Calabrese and Miranker 2009), binding to the extracellular matrix or that polymerization of Ab might depend on lysine cross-linking (Boros et al 2004).…”

Increased β-Sheet Dynamics and D–E Loop Repositioning Are Necessary for Cu(II)-Induced Amyloid Formation by β-2-Microglobulin

“…6,7 Other aspects of the dialysis treatment also play a role in the converting soluble monomeric β2m into insoluble fibrils, but the exact causes are still subject to debate. Research has shown that acidic conditions, 8,9 certain mutations, 10,11 cleavage of the six N-terminal amino acids, 12,13 limited proteolysis, 14 stoichiometric concentrations of Cu(II), 15–20 and other conditions 21,22 can induce the β2m amyloidosis in vitro. 8,23 Our group has been interested in the potential role that Cu(II) could play in inducing the β2m amyloid formation.…”

“…20,27 The oligomers, particularly the dimer 28,29 and tetramer 29,30 maintain a native-like structure. 27,29,31,32 Cu(II) has also been shown to play a catalytic role in the formation of β2m fibrils as it is necessary for oligomer formation but is released before the final fibrils are formed.…”