Interaction between Aβ(1−42) and Aβ(1−40) in Alzheimer's β-Amyloid Fibril Formation in Vitro

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Abstract
We analyzed the interaction of two kinds of amyloid β-peptides (Aβ), i.e., Aβ(1−42) and Aβ(1−40), in the kinetics of β-amyloid fibril (fAβ) formation in vitro, based on a nucleation-dependent polymerization model using fluorescence spectroscopy with thioflavin T. When 25 μM Aβ(1−42) was incubated with increasing concentrations of amyloidogenic Aβ(1−40), the time to proceed to equilibrium was extended dose-dependently. A similar inhibitory effect was observed when 45 μM Aβ(1−40) was incubated with increasing concentrations of Aβ(1−42). On the other hand, when 50 μM of nonamyloidogenic Aβ(1−40) was incubated with Aβ(1−42) at a molar ratio of 10:1 or 5:1, Aβ(1−42) initiated fAβ formation from Aβ(1−40). The lag time of the reaction shortened in a concentration-dependent manner, with Aβ(1−42). We next examined the seeding effect of fAβ formed from Aβ(1−42) (fAβ(1−42)) on nonamyloidogenic Aβ(1−40). When 50 μM of nonamyloidogenic Aβ(1−40) was incubated with 10 or 20 μg/mL (2.2 or 4.4 μM) of fAβ(1−42), the fluorescence showed a sigmoidal increase. The lag time of the reaction was shortened by fAβ(1−42) in a concentration-dependent manner. However, the time to proceed to equilibrium was much longer than when an equal concentration of fAβ formed from Aβ(1−40) (fAβ(1−40)) was added to Aβ(1−40). The fluorescence increased hyperbolically without a lag phase when 25 μM Aβ(1−42) was incubated with 10 or 20 μg/mL (2.3 or 4.6 μM) of fAβ(1−40), and proceeded to equilibrium more rapidly than without fAβ(1−40). An electron microscopic study indicated that the morphology of fAβ formed is governed by the major component of fresh Aβ peptides in the reaction mixture, not by the morphology of preexisting fibrils. These results may indicate the central role of Aβ(1−42) for fAβ deposition in vivo, among the different coexisting Aβ species.