Abstract

Acetylcholinesterase (AChE), an enzyme involved in the hydrolysis of the neurotransmitter acetylcholine, consistently colocalizes with the amyloid deposits characteristic of Alzheimer's disease and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. Ln order to identify the structural domains of the amyloid-beta-peptide (A beta) involved in the aggregation induced by AChE, we have studied the effect of this cholinergic enzyme on A beta peptide fragments of different sizes. AChE enhanced the aggregation of the A beta(12-28) and A beta(25-35) peptides but not of the A beta(1-16) fragment. The inductive effect of AChE on the aggregation of A beta(12-28) was abolished by the presence of either A beta(1-16) or A beta(9-21). The effect of the enzyme was also analysed using two different mutant fragments, possessing a low and the other a high capacity for fibrillogenesis. The fragments used were A beta(12-28)(Val18-->Ala) and A beta(12-28)(Glu22-->Gln), respectively. AChE was able to promote the aggregation of these fragments in a very specific way and both mutant peptides were able to form amyloid fibrils, as revealed by negative staining under the electron microscope. Binding assays indicated that AChE was bound to A beta(12-28), as well as to the A beta(1-16) peptide. AChE was seen to form strong complexes with the A beta(12-28) fibrils as such complexes stained positively for both thioflavine-T and AChE activity, were resistant to high ionic strength treatment, and were partially sensitive to detergents, suggesting that hydrophobic interactions may play a role in the stabilization of the AChE-A beta complex. Our results suggest that such amyloid-AChE complexes are formed when AChE interacts with the growing amyloid fibrils and accelerates the assembly of A beta peptides. This is consistent with the fact that AChE is known to be present within A beta deposits including the pre-amyloid diffuse and mature senile plaques found in Alzheimer's brain. (C) 1997 Academic Press Limited.