Low concentrations of sodium dodecyl sulfate induce the extension of beta 2-microglobulin-related amyloid fibrils at a neutral pH

Abstract

In beta(2)-microglobulin-related (Abeta2M) amyloidosis, partial unfolding of beta(2)-microglobulin (beta2-m) is believed to be prerequisite to its assembly into Abeta2M amyloid fibrils in vivo. Although low pH or 2,2,2-trifluoroethanol at a low concentration has been reported to induce partial unfolding of beta2-m and subsequent amyloid fibril formation in vitro, factors that induce them under near physiological conditions have not been determined. Using fluorescence spectroscopy with thioflavin T, circular dichroism spectroscopy, and electron microscopy, we here show that at low concentrations, sodium dodecyl sulfate (SDS) converts natively folded beta2-m monomers into partially folded, alpha-helix-containing conformers. Surprisingly, this results in the extension of Abeta2M amyloid fibrils at neutral pH, which could be explained basically by a first-order kinetic model. At low concentrations, SDS also stabilized the fibrils at neutral pH. These SDS effects were concentration-dependent and maximal at approximately 0.5 mM, around the critical micelle concentration of SDS (0.67 mM). As the concentration of SDS was increased above 1 mM, the alpha-helix content of beta2-m rose to approximately 10%, while the beta-sheet content decreased to approximately 20%, a change paralleled by a complete cessation of fibril extension and the destabilization of the fibrils. Detergents of other classes had no significant effect on the extension of fibrils. These findings are consistent with the hypothesis that in vivo, specific factors (e.g., phospholipids) that affect the conformation and stability of beta2-m and amyloid fibrils will have significant effects on the kinetics of Abeta2M fibril formation.