Acetylcholinesterase Involvement in Apoptosis

Acetylcholinesterase (AChE), an important component of cholinergic synapses, colocalizes with amyloid-beta peptide (A beta) deposits of Alzheimer's brain. We report here that bovine brain AChE, as well as the human and mouse recombinant enzyme, accelerates amyloid formation from wild-type A beta and a mutant A beta peptide, which alone produces few amyloid-like fibrils. The action of AChE was independent of the subunit array of the enzyme, was not affected by edrophonium, an active site inhibitor, but it was affected by propidium, a peripheral anionic binding site ligand. Butyrylcholinesterase, an enzyme that lacks the peripheral site, did not affect amyloid formation. Furthermore, AChE is a potent amyloid-promoting factor when compared with other A beta-associated proteins. Thus, in addition to its role in cholinergic synapses, AChE may function by accelerating A beta formation and could play a role during amyloid deposition in Alzheimer's brain.

The discovery of the first neurotransmitter--acetylcholine--was soon followed by the discovery of its hydrolysing enzyme, acetylcholinesterase. The role of acetylcholinesterase in terminating acetylcholine-mediated neurotransmission made it the focus of intense research for much of the past century. But the complexity of acetylcholinesterase gene regulation and recent evidence for some of the long-suspected 'non-classical' actions of this enzyme have more recently driven a profound revolution in acetylcholinesterase research. Although our understanding of the additional roles of acetylcholinesterase is incomplete, the time is ripe to summarize the evidence on a remarkable diversity of acetylcholinesterase functions.

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. In order to identify the structural domains of the amyloid-beta-peptide (Abeta) involved in the aggregation induced by AChE, we have studied the effect of this cholinergic enzyme on Abeta peptide fragments of different sizes. AChE enhanced the aggregation of the Abeta(12-28) and Abeta(25-35) peptides but not of the Abeta(1-16) fragment. The inductive effect of AChE on the aggregation of Abeta(12-28) was abolished by the presence of either Abeta(1-16) or Abeta(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 Abeta(12-28)Val18-->Ala and Abeta(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 Abeta(12-28), as well as to the Abeta(1-16) peptide. AChE was seen to form strong complexes with the Abeta(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-Abeta complex. Our results suggest that such amyloid-AChE complexes are formed when AChE interacts with the growing amyloid fibrils and accelerates the assembly of Abeta peptides. This is consistent with the fact that AChE is known to be present within Abeta deposits including the pre-amyloid diffuse and mature senile plaques found in Alzheimer's brain. Copyright 1997 Academic Press Limited.