Concentration-Dependent Inhibitory Effects of Apolipoprotein E on Alzheimer's β-Amyloid Fibril Formation in Vitro

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Abstract
Recently, many research groups have examined the effect of apolipoprotein E (apoE) on β-amyloid fibril (βAf) formation in vitro. However, their data were somewhat controversial and no exact kinetic assessment of the role of apoE has thus far been available. We examined the effect of human apoE on βAf formation in vitro, starting with various concentrations of freshly prepared β-amyloid(1−40) (β1−40) and using fluorescence spectroscopy with thioflavine T. When 50 μM of β1−40 was incubated with a 1:1000 to 1:100 molar ratio of apoE, a dose-dependent inhibitory effect of apoE was observed. Both the nucleation and extension phases of βAf formation in vitro were inhibited by apoE. On the other hand, when 300 μM of β1−40 was incubated with a 1:100 molar ratio of apoE, the inhibitory effect of apoE was completely abolished. We then focused our study on the kinetics of the inhibitory effect of apoE on the extension phase of βAf formation in vitro, utilizing the recently established first-order kinetic model of βAf extension in vitro [Naiki, H., & Nakakuki, K. (1996) Lab. Invest. 74, 374−383]. The mathematical treatment of the data suggests that apoE inhibits the extension of βAf in vitro, by making a complex with β1−40, thus eliminating free β1−40 from the reaction mixture. The equilibrium association constant with β1−40 was practically the same among the three major recombinant apoE isoforms. These results indicate that the effects of apoE on βAf formation in vitro is differential and could settle some of the controversy about β-amyloid-apoE interaction in vitro.

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