Heating during agitation of ?2-microglobulin reveals that supersaturation breakdown is required for amyloid fibril formation at neutral pH

Amyloidosis-associated amyloid fibrils are formed by denatured proteins when supersaturation of denatured proteins is broken. ?(2)-Microglobulin (?2m) forms amyloid fibrils and causes dialysis-related amyloidosis in patients receiving long-term hemodialysis. Although amyloid fibrils of ?2m in patients are observed at neutral pH, formation of ?2m amyloids in vitro has been difficult to discern at neutral pH because of the amyloid-resistant native structure. Here, to further understand the mechanism underlying in vivo amyloid formation, we investigated the relationship between protein folding/unfolding and misfolding leading to amyloid formation. Using thioflavin T assays, CD spectroscopy, and transmission EM analyses, we found that ?2m efficiently forms amyloid fibrils even at neutral pH by heating with agitation at high-salt conditions. We constructed temperature- and NaCl concentration?dependent conformational phase diagrams in the presence or absence of agitation, revealing how amyloid formation under neutral pH conditions is related to thermal unfolding and breakdown of supersaturation. Of note, after supersaturation breakdown and following the law of mass action, the ?2m monomer equilibrium shifted to the unfolded state, destabilizing the native state and thereby enabling amyloid formation even under physiological conditions with a low amount of unfolded precursor. The amyloid fibrils depolymerized at both lower and higher temperatures, resembling cold- or heat-induced denaturation of globular proteins. Our results suggest an important role for heating in the onset of dialysis-related amyloidosis and related amyloidoses.