Apolipoprotein E inhibits the depolymerization of β2-microglobulin-related amyloid fibrils at a neutral pH

beta2-Microglobulin-related (A beta 2M) amyloidosis is a common and serious complication in patients on long-term hemodialysis, and beta2-microglobulin (beta2-m) is a major structural component of A beta 2M amyloid fibrils. Fluorescence spectroscopic analysis with thioflavin T and electron microscopic study revealed that A beta 2M amyloid fibrils readily depolymerize into monomeric beta2-m at a neutral to basic pH. Circular dichroism analysis revealed that soon after the initiation of the depolymerization reaction at pH 7.5, the characteristic spectrum of beta2-m in A beta 2M amyloid fibrils changes to resemble that of monomeric beta2-m at pH 7.5. Apolipoprotein E (apoE), a representative amyloid-associated protein, formed a stable complex with A beta 2M amyloid fibrils and inhibited the depolymerization of A beta 2M amyloid fibrils dose-dependently in a range of 0-10 muM. These results showed that apoE could enhance the deposition of amyloid fibrils in vivo, possibly by binding directly to the surface of the fibrils and stabilizing the conformation of beta2-m in the fibrils.